Talk:Alternation of generations

Biology history

Does someone know the biology history about "alternation of generations".

Cleanup needed

This article has many typos and run-on sentences. Someone should proofread the article. Also, the article needs to have the current information about hypotheses on the origin of alternation of generations. The situation has evolved several times independently (each time multicellularity evolved). Protists in particular exhibit many different forms of alternation of generations (hey, even the green algae display several major variations). There's a LOT that an expert could add to this article. -- EncycloPetey 11:00, 12 November 2005 (UTC)

Actually, the problem is much deeper. In the intro it states that alternation-of-generations involves "two free-living phases" (a life-cycle or one generation includes both phases), then the article goes on to discuss flowering plants in which there are not two "free-living" phases. In other words, the article is confusing several related concepts - Marshman 21:41, 15 November 2005 (UTC)

Yes, it mixes together several concepts. At the core of them lies the fact that alternation of generations if the result of several cases of independent elaboration of some stage of the sexual reprocutive process. However, it is correct to say that all land plants have an alternation of generations. The alternation does not require the stages be free living. The term alternation of generations is usually restricted to those life cycles where there is both a multicellular haploid stage and a multicellular diploid stage. Thus, the term does not apply to animals or to those protists which never develop a multicellular stage. -- EncycloPetey 02:04, 16 November 2005 (UTC)

No, I do not agree. Since all sexual organisms "alternate" in that way, it is even worse to call that "alternation of generation". A life cycle is one generation, so the term is really wrong if you apply it to organisms that are not even independent in each phase (the only possible way one could see them as separate "generations"). That would mean each of us is the alternate generation of a previous generation of eggs and sperm. The concept you are pushing is called sexual reproduction, meiosis, or some such. Without that inclusion of "free living" the concept becomes trite, and is the reason this article is so confusing. - Marshman 03:03, 16 November 2005 (UTC)

But not all sexual organisms do. Humans do not have an alternation of generations because there is no multicellular haploid stage. I know of only a very few animal species with a multicellular haploid stage in the lifecycle, and in those cases, the haploid stage is sterile. Also, no botanist I know would call a single generation in a plant a life cycle. A life cycle by definition is a return to the starting point, and in plants that always means passing through two generations. That is one term that has a very, very different usage between botany and zoology. Except, of course, among parasitologists where the parasite life cycle typically involves several stages.

The problem is whether one defines the other. I agree a life cycle is just that, from egg to egg, or seed to seed. But I would not view an egg or seed as another generatioon separate from the adult it will grow into. I cannot figure out what your botanisrt friends are saying (only what they are not saying), so it is hard to respond to that. - Marshman

Neither do botanists consider a seed to be a separate generation. The seed is a container for an immature sporophyte. It is not my "botanist friends" that are saying this, it is all botanists. If you don't understand, try D. H. Campbell's (1918) Mosses and Ferns p.562-575. Some of the language and ideas are a bit outdated now, but it is an entire chapter on the subject, and one of the key sources still referenced today. -- EncycloPetey 08:10, 18 November 2005 (UTC)

If you force "free living" onto the definition of alternation of generations, then the only plants with alternation of generations would be the pteridophytes, which is not at all what a botanist means by the term. See any standard botany textbook to see support for my definition...

All I see is confusion over the term "generation", which is defined as one of two phases of a life-cycle. Indeed, the text I picked up says this: "The word generation is used here in a different sense from the usual one, which refers to the time interval between the birth of the parents and that of the progeny." How silly: redefine generation to confuse yet another generation of young botanists. I hold science in a little higher regard than that. I have no problem including your sense of the term "alternation of generation" as long as it is put in a proper context. To just go on redefining terms to suit a thought may be ok for a tetbook (the authors control the "unverse" of the text), but not a dynamic source of information like Wikipedia. Here, definitions need to be internally consistent or there better be some text explaining why not. It should not be our mission to simply regurgitate confusing, inconsistent, and sometimes outdated pronouncements from textbooks or "botanists we know" because they say so. Question and clarify, or this source of information is not worth consulting. - Marshman 18:58, 16 November 2005 (UTC)

The term "generation" is not merely a time interval, but a cohort of organisms produced by a parental group. Even in casual English this dual definition exists, hence "generation-X", which is a group of people, and not a time interval. It is thus not my sense of the word, but the scientific communitity's sense. If you have reason to disagree, why not provide a source instead of a personal opinion? By the way, you do know who the authors of the various texts I quoted are, don't you? Peter H. Raven is director of the Missouri Botanical Garden, and one of the world's most respected botanists. Gifford & Foster are two of the world's most respected paleobotanists, and their text is one of two standards used in college courses in English speaking countries. Again, if you truly believe that their ideas are inconsistent and outdated, please provide some reason why you believe this. Is it a personal belief, or is it foudned in some reason? -- EncycloPetey 08:10, 18 November 2005 (UTC)

"Multicellular haploid organisms that appear in alternation with diploid forms are found in plants; in some brown, red, and green algae; and in two closely related genera of chytrids and one or more other groups of protists not discussed in this book. Such organisms exhibit the phenomenon known as alternation of generations." p.236 Raven, Evert, & Eichhorn (2005) Biology of Plants, (7th ed.).

Statement's only problem is the oft expressed (but not by me) idea that algae are not plants. But nothing in that text says "free-living" should not be part of the definition; and in fact if you look up "organism" you will see they are saying exactly that ("haploid organisms that appear in alternation with diploid forms" = the n plant gives rise to the 2n plant gives rise to the n plant, ad infinitim) - Marshman

Nothing in that particular statement forbids "free-living" from being part of the definition, but if you look at the section of their book where the quote appears, you'll see that it cannot be logically included. The diagrams which accompany the discussion make it clear that a sporophyte "diploid individual" alternated with a gametophyte "haploid individual", and that this alternation applies equally to bryophytes (where the sporophyte individual remains attached to the gametophyte), pteridophytes (where the two generations live independently), and to seed plants (where the gametophyte individual remains attached to and is nutritionally dependent upon the sporophyte). -- EncycloPetey 08:10, 18 November 2005 (UTC)

"Alternation of generations, or phases in the life cycle is a consistent feature of all groups of vascular plants and hence represents the basic pattern of reproduction in these dominant plants of the modern world." p.15 Gifford & Foster (1988) Morphology and Evolution of Vascular Plants, (3rd ed.).

Only true to the extent that the pattern can be traced up through all the higher plant groups, but there is significant (I think) shift in what results in the conifers and angiosperms where the gametophyte phase ceases to be independent in any sense (i.e., cannot be regarded as a "generation"); and note again the use of "phases", recognition by these authors that "generation" is not necessarily the correct term - Marshman

So, logically, you would argue that a fetus is neither an individual nor a generation because it is dependent upon and attached to its mother? In both cases (fetus and angiosperm GPT), the dependent generation is genetically distinct from its parent. Generation is the correct term, since it develops as a multicellular organism from a single cell. (see below) -- EncycloPetey 08:10, 18 November 2005 (UTC)

"Sexual reproduction in most plants involves an alternation of dramatically different generations, whereas in animals reproduction involves the direct production of sperm and eggs as a result of meiosis in the germ cell line. The life cycle of a flowering plant is shown in Fig. 1.14. Plants have two alternating generations: the dominant sporophytic generation and a much reduced, parasitic gametophytic generation." p.27 Fosket (1994) Plant Growth and Development: A Molecular Approach.

I consider the above not a good statement, including the caveate about "most" plants and the lack of clear understanding of a term like generation. It emphasizes a point-of-view that even uses the idea that the gametophyte is a "parasite" to justify the somewhat fog-bound thinking that there are still really two generations in the higher plants. You don't think it is pushing things more than a bit to regard the multicellular gametophyte in a flower as a "parasite"? - Marshman

In the strictest sense of what a "parasite" is, yes. A parasite is an organism that derives its nutrition from another organism. That is exactly what is going on in this situation. Johannes Proskauer did significant experimental research on the relative independence of generations in hornworts, and found that they were not quite as dependent as is often cited. The sporophyte, though usually regarded as a strict parasite, was able to survive on its own. In any event, if total independence of existence is required to define an individual organism, then most forest trees do not qualify. Approximately 80-90% of all forest tree species rely on symbiotic fungi to survive. -- EncycloPetey 08:10, 18 November 2005 (UTC)

As you can see from these quotes (from the first three books I had at hand), (1) all plants have an alternation of generations, (2) the two generations are part of a single life-cycle, (3) animals do not have an alternation of generations, and (4) the alternating generations do not have to be free-living. You may not agree with this definition, but it is the definition that everyone else is using.

-- EncycloPetey 05:08, 16 November 2005 (UTC)

I think read more into those than is stated. Only the last really supports your contention about what is the definition, and I agree it certainly is one point of view. I think we need to do better here. - Marshman 18:58, 16 November 2005 (UTC)

What is the alternative point of view? Does anyone but you support it? -- EncycloPetey 08:10, 18 November 2005 (UTC)

What to do?

Rather than expend a lot of time and energy pushing forth ideas that are really not that far apart, let's admit that it is the use of confuising terminology that constitutes the real muddle here. I understand the evolutionary significance for the higher plants of the concept of "alternation of generations" and even the point that Fosket (1994) is making; but he dug himself a hole and is simply digging deeper to avoid admitting his confusion of terminolgy.

I'm not clear what you mean by this. His caveats are intended to cover those cases where plant species are known to reproduce only asexually, without meiosis and syngamy. There are many plant species that propogate asexually and no other way, so it is quite natural for someone who is technically minded to use the caveats when talking about plant reproduction. Among sexually reproducing plants, all species undergo an alternation of gametophyte and sporophyte generations. In some groups (e.g. seed plants) one generation is reduced but that makes it no less of a generation. You do not have an issue with Fosket, you have an issue with the evolution of development in plants.

Let's write this article so it clarifies the confusing aspects and not assume the reader is a "botanist" (wink, wink). The flowering plants can (and should) be brought in, but in the context of the evolutionary importance, not a blanket "Sexual reproduction in most plants involves an alternation of dramatically different generations..." which is silly for the use of "most" and "dramatically" and frankly, pablum for non-thinkers. I like the Gifford & Foster (1988) approach: "...phases in the life cycle is a consistent feature of all groups of vascular plants" – stated that way makes sense and is a good lead-in to what you want to include in this article - Marshman 19:19, 16 November 2005 (UTC)

I certainly agree that the concept is a confusing one. I have taught introductory botany for years (at Duke and UC Berkeley among other places). You expect that the students at such institutions are of top caliber in biology, but even they might take three or four weeks of digesting the idea before its meaning would sink in.

I can agree to some sort of "middle ground" as long as two critically important points are maintained. First, it must be clear that an alternation of generations is a life cycle that is split between two multicellular organisms that are genetically different, each being the offspring of the other. When the morphologist William Hoffmeister discovered this feature was common to (and homologous across) all land plants, the idea revolutioned botany. It is a core concept holding together plant life-cycles and plant evolution. Second, we should not attempt to redfine alternation of generations into something that it does not mean. The phrase is used quite consistently in the literature, unless you are familiar with a publication that I am not. The purpose of an encylcopedia is to make the information accessible, but is not the place to redfine words or formulate a personal position. -- EncycloPetey 04:33, 18 November 2005 (UTC)

I agree with what you are saying here. I'm not trying to "redefine" anything. If there is no conflict (all biologists accept that generation means a certain thing; perhaps as you describe below) then lets go with it. But I don't think that is the case. And again, I fully accept that the concept is important in understanding evolution in the plant kingdom, so I'm not trying to argue against including that; it should be the ultimate outcome that a complete article leads to here. - Marshman 18:19, 18 November 2005 (UTC)

Life-cycle vs generation

I've been reading and re-reading your suggested text changes, and think I've come across one specific issue that could be clarified. You are assuming that the terms "generation" and "completion of a life cycle" are necessarily one and the same. They are not. A life-cycle is a complete turn through the stages of organismal reproduction from one point back to that same point. I think we agree on that as a definition, at least. A generation, however, is the production of a multicellular organism from a single cell. This happens twice in organisms with a haplo-dipontic or a diplo-haplontic life cycle, since both the zygote and the spore undergo multiple mitotic divisions, to produce the diploid sporophyte and haploid gametophyte, respectively. The two terms are indeed synonymous among animals (and all organisms with a haplontic or diplontic life-cycle), but only because the life cycle includes a single multicellular stage. -- EncycloPetey 04:43, 18 November 2005 (UTC)

I just do not think (at this point) that your definition of generations is necessarily the generally accepted one (although I do see your point). I've run across the ideas, multiple times, that in "alternations of generations" generations is defined differently (quote I gave above) and that it should be alternation of phases within a generation to be correct. Obviously, the "problem" does revolve around that definition, which seems to be not generally fixed among biologists. But even your sources (you quoted above) used "phases" (in addition to generation in one case, presumably to satisfy both "camps"). While I agree, the term usually used is "alternation of generations", to call the gametophyte of the higher plants another generation becomes a real stretch (another phase within the generation is a lot cleaner). I think it is an idea some botanists like to promote (Fosket, 1994 for example) because it connjurs up an interesting picture of the gametophyte being "captured" by the sporophyte over evolutionary time; but that is just a metaphore. One can also view it as simply the reduction of importance of the gametophyte phase to its basic functions, paralleling animal evolution. Obviously with such confusions between botanists and everyone else and within botany itself, it is safest to avoid the term "generation"; but we cannot do that, because it is in the article title (for good reason). Therefore I suggest the "explanatory" approach I have been e3diting towards. That includes pointing out the conflicting definitions of "generation" used by biologists. It might help to first get the life cycle article tuned up. - Marshman 18:12, 18 November 2005 (UTC)

More Diagrams?

I think that having more diagrams to illustrate the various types of alternation of generations (ie: moss, gymnosperms, angiosperms) would make this article more readily understood. Alex Ng 04:23, 4 May 2006 (UTC)

Be BOLD and add images that are already located in the various sub-articles on groups of plants. In particular, LadyofHats has done some very nice diagrams, and might be asked to do a few more. --EncycloPetey 09:05, 4 May 2006 (UTC)

More examples would be nice

The real problem is that there are not enough examples. Experts in the subject have a bunch of examples in their heads that keep all these ideas straight; regular folk scratch their heads and don't see how it all fits together. For example, in plain language how do the apple trees in my yard go through this process? What are the different stages?? How do the flowers fit into this system? How does moss pass through this process? It just isn't clear to non experts. The article may be accurate but it is completely worthless to a high school freshman trying to read her biology text.

I agree that it would be nice to have more direct examples that could help those who have a good general biology understanding but a small plant reproduction understanding keep their ideas straight. However, finding examples that have been cited as correct in literature could be a harder task than one might expect. 19345beta (talk) 02:12, 20 September 2020 (UTC)

All or some vascular plants

Alternation of generations is a reproductive cycle of certain vascular plants, fungi, and protists.

Should it not be "all vascular plants" ? I do not know exceptions but the introduction suggests that it is rare. From what I have read there is always alternation of generations, the timespan/length of the haploid and diploid stages however vary. [1] Shyamal 03:59, 4 December 2006 (UTC)

Oops, looks like this has been discussed in the threads above. But the final introduction is still not satisfactory as per the earlier discussion. Shyamal 12:10, 4 December 2006 (UTC)

This has still not been cleared up. The article refers to "certain plants", and then below in the section "vascular plants", only "ferns and their allies" are mentioned. This implies that other vascular plants do not exhibit alternation of generations, contrary to what I thought was the case, although I'm not a botanist and I'm not sure. I suggest that a clear statement should be made one way or the other, either "most other vascular plants exhibit alternation of generations also", or "most other vascular plants do not exhibit alternation of generations". Bayle Shanks 03:34, 14 July 2007 (UTC)

Strangely, I too noticed that this has not been fixed just now and was further surprised by the GA status. I would think this article needs rework on citations, layout and wording. Unfortunately my botany is a little weak and rusty for this. Shyamal 03:40, 14 July 2007 (UTC)

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Cnidaria

Reference in the text to this phenomena in the Cnidaria (medusa to polyp forms) needs to be made Goldfinger820 05:38, 13 July 2007 (UTC)

Please elaborate on it, then. Speciate 06:03, 13 July 2007 (UTC)

Cnidarians do not undergo alternation of generations. They do have more than one life-cycle phase, but this is not the same as alternation of generations, which involves stages with different ploidy of tissues. In Cnidarians, all the stages are diploid, and the medusa stage is genetically identical to the stage from which it buds asexually. In alternation of generations, one stage is produced by meiosis and the other is produced by syngamy. --EncycloPetey 02:30, 14 July 2007 (UTC)

fair comment - however the term (although a misnomer) is still used in Cnidarian textbooks to describe the change between medusa and polyp phases. this should be made clear here, with a note that it is not true alternation of generations as alluded to above. Goldfinger820 03:57, 14 July 2007 (UTC)

If you will provide references to these textbooks, I can probably acquire them and add the information. The invertebrate texts I have available do not use this terminology. --EncycloPetey 20:16, 15 July 2007 (UTC)

although not a invertebrate only text, "Zoology Miller S.A and Harley, J.B. (1999) 4th ed IBSN 0-697-34555-6" has a reference to Cnidarian AoG on page 269. it is used here as a subtitle to describe the change in body form (but as you correctly point out -not in ploidy). will try to find some other refs if I get some time cheers Goldfinger820 04:44, 14 August 2007 (UTC)

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Dependent on the gametophyte?

"Indeed it is a defining characteristic of the land plants, or embryophytes (and hence the name), that a developing multicellular sporophyte is, for at least the first stages of its development, nurtured by the gametophyte, as can be seen most clearly in the bryophytes."

I know this is true for Monilophytes and the lower tracheophytes, but I was under the impression that a seed was diploid and thus a sporophyte (with the exception of triploid endosperm). This would seem to show that the above quoted statement from the article is only true for Monilophytes and lower plants.

Can someone with more botanical knowledge than I verify this?
Mastercampbell (talk) 04:38, 13 June 2009 (UTC)

The seed is not always all diploid, but actually contains material from as many as three different generations. The seed coat derives from diploid tissue of the parent sporophyte. In some seeds, especially gymnosperms, a portion of the haploid female gametophyte remains present in the seed. The embryo is the new diploid sporophyte, and it is also present within the seed. Additionally, in plants with double fertilization (especially angiosperms), a nutritive endosperm tissue develops, which is triploid. So, the seed does not contain a uniform genetic makeup, and may contain tissues or structures with more than one ploidy level and from more than one generation.

In gymnosperms and angiosperms, the new sporophyte embryo begins development within the female gametophyte. It is therefore accurate to say that it is nurtured by that gametophyte"at least the first stages of its development". However, the time it spends being nurtured by the gametophyte is greatly shortened in angiosperms as the embryo quickly outgrows its tiny parent gametophyte. --EncycloPetey (talk) 22:51, 23 August 2009 (UTC)

Animals

I believe this applies to animals too -- if not nearly all eukaryotes. Animals simply have a very reduced form of alternation of generations, after all. 128.143.102.189 (talk) 05:11, 13 October 2010 (UTC)

This is discussed at some length in the article (especially the "Distinctions" section). If you think that section is incorrect, please supply some references. Kingdon (talk) 11:25, 17 October 2010 (UTC)

It could only be a "reduced" form if the ancestor of animals had such a life cycle. Current scientific opinion is that alternation of generations has arisen independently multiple times, but that it was not present in the common ancestor of animals. --EncycloPetey (talk) 05:11, 18 October 2010 (UTC)

On different note - the article states in few places that "all animals are diploid" wihich is drastically incorect. While animals do not have haploid/diploid alternate generations, there are speicies of animals with haploid/diploid sex determination, with fully adult haploid males 159.205.250.124 (talk) 02:26, 17 May 2017 (UTC)

This is a very good point, and I have edited the article to try to account for this. As the IP editor notes, haploid males and diploid females is not alternation of generations because males are always haploid and females diploid, but it does mean that it can't be said that "all animals are diploid". Peter coxhead (talk) 16:11, 17 May 2017 (UTC)

Misleading diagram

The first diagram is surely misleading. It implies that a single sporocyte gives rise to BOTH microspores and megaspores. This is inconsistent with the discussion later under "Vascular Plants". I'm new to this article (which I want to link to from an article I'm working on); I have to say that just reading the article, I found it confusing (and I believe I do understand the biology involved!). Looking at the talk page, it seems clear that the odd wording of the article may have to do with the differences of opinion among editors in the past. However illogical or irrational terminology in an area seems to be to us as editors, our job is to explain clearly the terminology actually used. Peter coxhead (talk) 14:17, 28 December 2010 (UTC)

This is a generalized diagram, and shows the process of 4 spores from a single sporocyte. The situation you describe, with megaspores and microspores, is a special case of this situation called heterospory. Some heterosporous groups do indeed produce both kinds of spores from a single tetrad, although you are correct that seed plants do not do this. That is because seed plants also are heterosporophyllous, with separate sporophylls bearing different sporangia. This is actually unusual among the various groups of organisms that have alternation of generations. Consider that the diagram is generalized to apply also the mosses, green algae, kelps, and red algae, and the details of spore production differ in those groups from those in seed plants. Likewise, the diagram shows separate gametes from separate gametophytes from separate spores. That doesn't always happen. Some organisms have gametophytes that are monoicous and therefore a single haploid gametophyte produces both egg and sperm. That never happens in seed plants. So the diagram is fine for what it does, which is to show the sequence of major structures and events in a generalized way. Any general diagram will always have details that do not fit specific situations, but that's because it's generalized. And if it were altered to fit a specific case, the generality of the diagram would be lost. --EncycloPetey (talk) 18:22, 28 December 2010 (UTC)

Yes, I understand all this biology. The difficulty I have is in trying to understand what someone who doesn't will make of the article. It looked to me (but maybe not to that person) as though a point was made by the way that it's presented. If the idea is just to demonstrate the alternation of haploid/diploid, then it seems to me that it would be better to have just one gametophyte and one sporophyte; you then only have to show one of the spores generating a gametophyte. Um... See my comment below! Peter coxhead (talk) 22:48, 28 December 2010 (UTC)

The catch there is that the diagram would then imply self-fertilization, which is far from the norm in most species. --EncycloPetey (talk) 22:56, 28 December 2010 (UTC)

I was thinking about something like this (which I've very quickly edited): Revised diagram. It seems to represent the absolute minimum you need to grasp FIRST, i.e. before being exposed to the complexities of archegonia/antheridia, monoecious/dioecious, micro- & mega-sporangia/spores, monoicous/dioicous, etc. I'd like to know what you think before I do any more re-drawing. Peter coxhead (talk) 09:58, 29 December 2010 (UTC)

I know this may re-open old arguments (yes, I have read all of the talk page!), but the resolution of the 'alternation of generations' / 'alternation of phases' issue is surely this:

• The archetype or prototype of alternation of generations is a life-cycle in which both the sporophyte and the gametophyte are distinct free-living organisms. In this case, by any normal use of the term 'generation' (INCLUDING the wiki-linked article of this title), there is indeed alternation of generations. As the "generation" article says clearly, generation is linked to the creation of off-spring. The sporophyte is the off-spring of the gametophyte; the gametophyte is the off-spring of the sporophyte.
• When it was discovered that seed plants have a life cycle which is a heavily compressed form of the archetype described above, then it was natural to use the same terminology. However, for seed plants it is indeed rather odd to call the gametophyte a 'generation' in the normal sense of the word, and 'phase' would be a better word.
• However, it would not be right to use 'alternation of generations' for the first case and 'alternation of phases' for the second case, since this would disguise the underlying continuity. So we're left with the standard usage, which is clearly "alternation of generations". A crude, but revealing test, is that Google produces 169,000 pages with the exact phrase "alternation of generations", but only 12,600 pages with the exact phrase "alternation of phases".

So I conclude that the article needs to have the term "phase" demoted in importance to reflect NPOV. Peter coxhead (talk) 16:23, 28 December 2010 (UTC)

The "alternation of phases" is in there because of a pushy editor who understood neither WP:OWN nor the diversity of the alternation process among the many groups of plants and algae. The article needs many improvements, and the beauty of Wikipedia is that you are free to make those improvements without waiting for someone else. --EncycloPetey (talk) 18:22, 28 December 2010 (UTC)

Ok, well when I've finished some other stuff, maybe I'll come back to this... Peter coxhead (talk) 22:48, 28 December 2010 (UTC)

This has also been on my to-do list for some time. However, I've been spending my limited time on Wikipedia with other pursuits. If you do take a serious stab at this in the coming months, I may be able to offer suggestions and references if you ask. Part of the problem is that I've recently moved and haven't yet found (or unpacked) all my biological library) and part of the problem is that I've started a new time-demanding job with a longer commute. This doesn't leave much time for wiki work.

I do have extensive notes (somewhere) started for the revision of the gametophyte article, which will be my first stab at helping the situation. I just have to find those notes at some point. --EncycloPetey (talk) 22:56, 28 December 2010 (UTC)

Major re-write

I have undertaken a major re-write and expansion of what seems to me to be an important article which previously had some serious faults, as noted above. Some more material is needed, particularly in the section on the life-cycles of different plant groups. Comments and edits welcome! (At least copy-editing is doubtless needed.) Peter coxhead (talk) 20:48, 1 January 2011 (UTC)

Pictures

Can someone upload the two pictures, one of the gametophyte and the other of the sporophyte, of the same species - preferably a common one - for a better understanding of the subject?

Thanks in advance

Hariya1234 (talk) 11:37, 21 July 2011 (UTC)

This isn't quite as simple a request as it seems, because of the huge variation in the relative importance of the two (gametophyte and sporophyte) among the different groups. Actually the pictures are already there, although perhaps not brought out as clearly as possible.

• For the "bryophytes" (liverworts, mosses and hornworts), where the gametophyte is the dominant generation, the fourth photo down, of Pellia epiphylla actually shows both the gametophyte (the flattened green thallus) and the sporophyte (the pale green vertical stems with darker heads).
• For ferns, all the gametophytes will look very similar to that of Onoclea sensibilis (in the first photo). There are photos of the gametophyte and sporophyte of Dicksonia in the section "Life cycles of different plant groups".
• For seed plants, the gametophyte is microscopic, so a photo wouldn't show any more than the Angiosperm life cycle diagram does (in the section "Life cycles of different plant groups").

"Common" plants are probably mostly flowering plants, which have microscopic gametophytes. Peter coxhead (talk) 14:17, 21 July 2011 (UTC)

Unhappy and contradictory development of terminology.

I found Alternation of generations while looking for iwsisters for a planned article on svwp (the Swedish wikipedia) with a similar title. I was a bit upset with what I found, and more by by glancing through the article history. The article seems to have reached a reasonably stable state, accompanied by the usual consensus-achieving discussion. This is all well and good; but the achieved consensus seems rather loop-sided, in several respects.

Essentially, this appears as a page by botanists and for botanists. The older and general zoological meaning of the term alternation of generations has disappeared or been relegated to an erroneous usage, with references to concepts asuch as heterogamy, which in its turn just mentions "the alternation of differently organized generations", without referring to any useful article. Formally, the "new consensus wp definition" of the term covers all kinds of organism; but it focuses genetically different phases in the life cycle of an organism, instead of reproductory different phases. This leads to several inconsistencies:

• The present article contradicts older English usage of the term alternation of generations, as found in older and newer encyclopediae.
• The present article contradicts the definition given in some of the interwiki sisters; notably the German sister de:Generationswechsel (literary: "Generation alternation"). Likewise, it contradicts the traditional meaning given to the corresponding terms in Swedish (Generationsväxling) and Norwegian (Generasjonsveksel).
• The present article occupies a name, which otherwise could have been used for an important zoological concept, using the traditional terminology. I have found no other article covering this sublect, with or without a reference to disparate terminology.

Nothing of this means that the article necessarily "is in the wrong". I notice that several botanical experts seem to have cooperated in the article; which means that the definition given here might be the appropriate one, at least for modern botanists. However, even if this is the case, the older meaning should be mentioned, and the iw should be corrected. Moreover, I'm still slightly suspicious of the outcome; I note some strange points in the development of the article, and also a clear absence of sources which supports the rejection of the older meaning.

As I said, the traditional definition focusses on reproductory differences. A typical example is the alternation between the polyp generation (with asexual reproduction) and the medusa generation (with sexual reproduction) in jellyfishes. Both generations are diploid.

In the sequel, I elaborate the details of my points supra.

• The on line (present edition) Encyclopædia Britannica gives the following definition of alternation of generations:

"alternation of generations, also called Metagenesis, or Heterogenesis, in biology, the alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically, and sometimes chromosomally, distinct."

The EB article then continues with a brief description of this phenomenon for "algae, fungi, mosses, ferns, and seed plants", which is in reasonably good accordance with our article. However, it is concluded by an even briefer description of the zoology case:

"Among animals, many invertebrates have an alternation of sexual and asexual generations (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown."

(Citations copyrighted by the Encyclopædia Britannica; quoted only for illustrating terminological usage; the formulations may not be used freely in our articles.)

• The new universal encyclopedia has a reasonably long article Alternation of generations, starting

"Alternation of Generation. Process of reproduction. ... Among both animals and plants other patterns of reproduction appear, some of which are grouped together under the term metagenesis or the alteration of generations.
The essential characteristic of all such alternations is that successive generations reproduce in different ways, for example sexually and asexually.

The NUE article continues with fairly thorough descriptions of alternation of generations among animals, with an illustration depicting the life cycle of "the bell-shaped jelly fish common in most seas". It notes that such "animal alternations" not necessarily involves changes in the chromosome numbers, but then continues with plants, where

"alternation of generations may mean differences not only in the form of the individuals and in their mode of reproduction, but also in the intimate details of their cytological structure".

Here, there is a new illustration, listing the various elements in the plant life cycle, and marking them as diploid or haploid, resectively. This part about "plants" (including fungi; this is by an older systematic) covers about two third of the article.

(Citations copyrighted by the Caxton Publishing Company Limited; quoted only for illustrating terminological usage; the formulations may not be used freely in our articles.)

• Our German sister article, de:Generationswechsel, starts

Als Generationswechsel bezeichnet man in der Biologie eine Form der Fortpflanzung, bei der die beiden Varianten der Fortpflanzung – die geschlechtliche und die ungeschlechtliche – von Generation zu Generation abwechselnd auftreten. Generationswechsel tritt bei vielen Protisten, bei Rotalgen, Moosen, Farnen und Samenpflanzen, aber auch bei Nesseltieren und Tunicaten auf. Er kombiniert die Vorteile beider Fortpflanzungsarten: Die sich ungeschlechtlich vermehrende Generation kann eine große Anzahl von Nachkommen erzeugen, ohne dafür einen Sexualpartner zu benötigen. Die sich geschlechtlich vermehrende Generation sorgt für eine Durchmischung des Genpools.

which roughly translates as

In Biology, Alternation of generations denotes a form of reproduction, where the two variants of reproduction – sexual and asexual – appears alternatingly, generation for generation. Alternation of generations appears in several protists, red algae, mosses, ferns, and seed plants, but also among cnidaria and tunicates. It combines the advantages of both ways of propagation: The asexually reproducing generation may produce a large number of offspring, without having to find a sexual partner. The sexually reproducing generation ensures a mixing in the gene pool.

As you may see for yourself, the article contains subsections dealing with terminology and with details in botanic alternation of generations. I find the terms heterophasischer Generationswechsel and homophasischer Generationswechsel particularly interesting; they are explained as alternation of generations with different or equal chromosome set-up, respectively.

• The history of our article includes
  • this edit, where our earlier formulation

...in which a sexual reproductive phase alternates with an asexual reproductive phase

was changed to

...in which a multicellular diploid phase alternates with a multicellular haploid phase

not by any reference to sources, but by a consistency argument; the edit comment is:

BOTH stages are part of a sexual life-cycle; neither stage is "asexual";

and

• the main edits from an older form to a newer, by PDH (talk · contribs), in 2005.

IMHO, these edits fixated the semantics still employed in the article. Now, these edits by PDH essentially appear to be good contributions, and they also include adding the two sources for the article; but note that these sources do not concern general biology as much as some more restricted botanic areas. It is interesting to see that the older version was rather unsure about whether the term alternation of generations properly was applicable to the subject (based on one interpretation of the term "generation"); PDH removed the question marks, and also added the claim that what the Germans would call "homophasischer Generationswechsel" actually is not an alternation of generations at all, but instead should be termed Heterogamy.

• On the other hand, our disambiguation page Heterogamy actually describes heterogamy in the relevant sense more or less as a special case of alternation of generations:

In reproductive biology, heterogamy is the alternation of differently organized generations, applied to the alternation between parthenogenetic and a sexual generation.

• This item also essentially was introduced by PDH, here.

Thus, it is not quite clear whether our present definition is a strightforward application of definitions, as found in the literature, or there also is an element of original research involved. In the former case, some explicit references should be added, together with comments on differences between older and younger terminology. In either case, something should be written somewhere about the concept examplified by the alternating polyp and medusa 'generations' in coelenterata. JoergenB (talk) 18:07, 15 February 2012 (UTC)

It would be perfectly reasonable to have two articles, one covering the use of the term in plants and on the use of the term one covering the alternation of diploid phases in animals. At one time there was a muddled attempt to explain both uses in the same article. The article only covers "alternation of generations" as applied to plants; it's complicated enough already and would not be helped by adding more information, in my view.

What I would suggest is a change to the very last paragraph. Currently it reads:

In some animals, there is an alternation between parthenogenic and sexually reproductive phases (heterogamy). Although in some ways similar to alternation of generations, the genetics of heterogamy is significantly different.

It could be changed to something like:

In some animals, there is an alternation between parthenogenic and sexually reproductive phases (heterogamy). This has also been called "alternation of generations".[REF]

Then there could be a "see" linking to the other article. There would also be a hatnote pointing to the other article. As a temporary measure, I have added a hatnote linking to "Heterogamy", but a fuller article describing "alternation of generations" in animals should be created and then linked here. (This article could then be moved, e.g. to "Alternation of generations (plants)".)

(What the Swedish or German terms mean is irrelevant; there's no a priori reason to expect a 1:1 translation between these terms and English. As far as I can tell, the term in now used in English overwhelmingly for plants.) Peter coxhead (talk) 00:30, 16 February 2012 (UTC)

After doing some more research, I'm clear that the term is rarely if ever used in English in zoological contexts. I've revised the hatnote I first put on the article and also the last paragraph. Peter coxhead (talk) 09:45, 16 February 2012 (UTC)

I agree that two separate articles may be warranted. I was a zoology undergraduate in the early 1970s, and at the undergraduate level I never encountered the term used for animals, although then, and to the present day, the term is widely used in English-language botany texts.--Curtis Clark (talk) 02:16, 16 February 2012 (UTC)

To add further to Curtis Clark's point, I was a zoology undergraduate in the late 1960s and had the same experience. I have the electronic version of R.S.K. Barnes et al. (2001) The Invertebrates: a synthesis, 3rd ed., Blackwell (a major undergraduate text) and so could search it. The phrase "alternation of generations" occurs only once, on p. 321, where it says "An alternation of generations of this type never occurs in animals, although there may be a striking alternation between diploid phases which reproduce sexually and asexually (see Section 14.2.1)." So it's clear that whatever may be the case in other languages, this term is used in English for plants, not animals. However, an article on the alternation of sexual and asexual phases in animals would be good, as noted above. Peter coxhead (talk) 09:25, 16 February 2012 (UTC)

Two separate articles seems to be a sensible solution. In one of the dictionaries I found (a Norwegian), there were actually two articles under the same heading, written by two different people, for the zoological meaning and the botanical meaning of the term.

I can also confirm that the usage in botanics seems to dominate over the usage in zoology, in the more modern material.

I also agree that the usage in other languages technically does not have bearing on the English usage. However, if the usage is different, this should be remarked rather explicitly, and some care should be taken with the iw linking. Note that biological dictionaries translate the terms between various languages as if they were exactly correspinding. This also more or less may hold for e.g. translations from German to English.

I strongly disagree with claiming that "true" alternation of generations must be heterophasic, without explaining that in older and some modern English usage, the term alternation of generations also is considered as correct in the homophasic case.

@Peter coxhead, IMHO, to judge from the small citation that you gave, you have overinterpretated the claim. That authors write that "this is not a.o.g. in this sense" is not at all the same as if they wrote "this is not true a.o.g.". Scientists are quite used to terms which have different meanings in different contexts. Actually, I am rather interested in the context from where you quoted; seemingly, it contains some kind of definition of alternation of generations. Even if it probably nowadays is used much more for plants than for animals, at least in English, this is not te only "correct" usage. e also should avoid WP:Recentism; older usage should be mentioned. If that usage today explicitly is considered as improper by appropriate sources, this should also be mentioned.

Is the on-line version you mentioned openly accessible? Else, could you give details, so that I may check if our university has it?

I've just spent a couple of hours at the only library in Stockholm which possesses the Concise Biology Encyclopedia to which Staticd (talk · contribs) referred the relevant heterogamy item, thus: Scott, Thomas (1996). Concise encyclopedia biology. Walter de Gruyter. ISBN 9783110106619. ({{Cite book| publisher = Walter de Gruyter| isbn = 9783110106619| last = Scott| first = Thomas| title = Concise encyclopedia biology| year = 1996}}). This rather modern work introduces a.o.g. thus:

"Alternation of generations: the usually regular alternation between a sexually reproductive generation and one or more generations that reproduce asexually. A.o.g. occurs in both animal and plant."

The article continues with a rather long paragraph about a.o.g. in plants, and ends with a rather short one about animals. In the plant swection, the term heterophasic is introduced, and it is explained that the a.o.g always is heterophasic in higher plants. The short final paragraph runs thus:

"In animals, heterophasic A.o.g. occurs only in unicellular organisms. Homophasic A.o.g., in which the chromosome number does not change, occurs in Heterogony (see) and Metagenesis (see)."

The articles Heterogony and Metagenesis were rather interesting, and probably contains factual and 'definitory' content that we should make use of. Both are defined as "form(s) of homophasic a.o.g" among animals. Heterogony (also called heterogamy) is explained as alternating between a sexual and one or several parthenogenic generations; the given principal example is the aphid reproduction. Metagenesis is explained as involving morphologically different generations, and the asexual generation as reproducing "by cleavage or budding"; the given principal example is reproduction among "many coelenterates".

The article Heterogamy, on the other hand, gives three meanings; it is a bit like our disambigs; one of the meanings is explained only thus

"2) See Heterogamy."

(Citations copyrighted by Walter de Gruyter (I think). Quoted only for illustrating terminological usage; the formulations may not be used freely in our articles.)

The Concise Biology Encyclopedia is thus the only source in Engish I've found for the terms heterophasic and homophasic, which clearly correspond to the German terms in our German sister article. They are not contained in the OED. However, even Peter coxhead might agree that there is an a posteriori:-) reason to consider a connection between German terminology and the Concise Biology Encyclopedia: Indeed, it is translated from German. I like the terms; but I would prefer confirmation that they (and the terms haplophase and diplophase) have been used also in other English-language contexts, before introducing them to enwp. (If we don't find such confirmation, we might use them with an explicit reference to the source.)

Conclsions and sugestions: All references I've found (including the modern English one) include the alternation between asexual and sexual reproduction as part of the criteria. No reference I've found or heard about claims that employing the term a.o.g. for generations with the same chromosome structure is improper. (That includes Peter's short citation supra.) Most modern English sourcses seem to employthe term a.o.g. freely for plants, but rarely for multicellular animals. (This includes my own amateurish findings, but also the testimonies from others supra). Possibly the modern semantics in the usages of a.o.g. in botanics and zoology might diverge a bit. I therefore would suggest that we

• move the present page Alternation of generations to Alternations of generations (botanics);
• leave the thus created new page Alternation of generations as a redirect to the a.o.g. (botanics) page (since most people looking it up probably are looking for application for plants);
• create a new, relatively short page Alternation of generations (biology), where we give the general definition, discuss terminology and varying usage, and have brief sections for the various uses, referring to main articles;
• mreplace Peter's link to the disambig page Heterogamy to a new article Heterogony, written in accordance with the usage in the Concise Biology Encyclopedia;
• create a new article titled either Metagenesis or possibly Metagenesis (zoology) (if indeed the term is employed in botanics, too);
• re-include the traditional reference to sexual and asexual reproduction early in the a.o.g. (botanics) article, but add that within botanics, emphasis now have shifted from reproduction to chromosome set ups (widening the application of a.o.g. to situations where actually the haplophase is purely transitory and does not involve any reproduction of more than one "offspring");
• and, finally, use the a.o.g. (botany) as a 'hat reference' in a.o.g. (botanics), and as a first sentence reference in the heterogony and metagenesis articles.

JoergenB (talk) 16:56, 16 February 2012 (UTC)

At first I was in favour of renaming this page as something like "Alternation of generations (plants)", but now I'm not. The overwhelming use in biology in English is for plants, so there's no reason to disambiguate the title. But there should certainly be an article on the alternation of sexually and nonsexually reproducing diploid animals, under whatever is the most plausible title (namely the term most used today in English in zoology). The point is not that Barnes et al. explicitly say that the term doesn't apply to animals, it's that they don't ever use it other than as quoted above. They discuss alternation between sexual and nonsexual reproduction but never using "alternation of generations" for this.

I would happily add to the lead a statement about the use of the term "alternation of generations" in zoology if I could source this in one or more zoology textbooks (I didn't only use the electronic one, I looked in the index of two other paper textbooks I own on invertebrate zoology; neither include it, but of course the term may be used and not be in the index, which is why I quoted R.S.K. Barnes et al. (2001), where I could search the whole book [Google Scholar finds this work if you search, but you may or may not be able to access it]. I'm very sceptical about using dictionaries or general encylopaedias of biology to determine usage; my experience is that they often just copy from older versions and are reluctant to drop terms, so that they do not reflect current usage. However, it's fine to add this at the end, I think, and I have now done so. Could you please add the page in Scott? Peter coxhead (talk) 19:13, 16 February 2012 (UTC) (UTC)

Stupid me - I thought that references to the items rather than the pages were the norm, and didn't write down the page numbers. For "a.o.g." I could reasonably well recognise the page number (as being one of two deleted pages in a Google preview version); but I can't do that for most of the other items. Is there no alternative "item-wise" template argiment?

The text is better now, I think. There should still be an article about the more "general" concept a.o.g., for the reasons I lined up. Do you agree? What do you think it should be called? If we don't have such an article, then, what should we do with iw links to articles like de:Generationswechsel?

We cannot use negative arguments for concluding that all general definitions are "wrong", since the concept nowadays is not frequently used in zoological contexts. We need to separate definitions from usage. The general article might note that using the term a.o.g. in the "homophasic" cases, and in particular for multicellular animals, seems to be "somewhat obsolete". Actually, in order to avoid WP:OR, we should use the sources (not our synthesis or rationalisations) both for the definitions and for the usage. If there is a discrepancy betwen a (retained) definition and (modern only) usage, then we should note that, not "correct" the definition.

On the other hand, several of you botanics guys have argued that the genetical differences are the essential parts of the (modern) a.o.g. concept. This could and should absolutely be included, if it can be supported by positive arguments. In other words, do you have actual references to e.g. botanics text books that make the 'heterophasism' part of the introductory definition of a.o.g.? (N. b., it is not enough to have it mentioned in the introduction; not if it could be interpretated as stating a fact about a.o.g in higher plants, rather than being a part of the definition.) It would also be sufficient - and rather excellent - to have a few explicit references to a.o.g. being applicable in cases of an extremely reduced haplophase, where "reproduction" should not be said to end the phase, by a text-book that explains that this still is an a.o.g. precisely since it is 'heterophasic'. (Hardly using the word "heterophasic", of course; but employing that concept.) JoergenB (talk) 20:15, 16 February 2012 (UTC)

I think what Peter is trying to say is that in modern usage, the English phrase "alternation of generations" is only used for plants, and that the zoological phenomenon needs to be at another name. The conventions for article titles in English Wikipedia strongly support using a title that is broadly recognized, as long as it does not cause confusion. Using "alternation of generations" for the zoological phenomenon will cause confusion among English speakers who were educated in the United States or the Commonwealth. This is not to say that the widespread use in zoology of the equivalent term in other languages should not be mentioned; it is obviously important and a potential source of confusion for the unwary. But the article title is best as it is.--Curtis Clark (talk) 05:43, 17 February 2012 (UTC)

Exactly. The way forward, I think, is for articles to be created on the other phenomena which have been called or are related to "alternation of generations". For example Heterogeny is too important a phenomenon in my view to be dealt with just inside Parthenogenesis, so there should certainly be an article on it. Then, and only then, there can be appropriate explanations/wikilinks in each of the articles to the other ones – there could be a page "Alternation of generations (disambiguation)" perhaps; then hatnotes at the more specific articles could use this for redirection. Peter coxhead (talk) 10:17, 17 February 2012 (UTC)

(1) There was a time when alternation of generations did not refer to chromosome structure (haplo/diplo). (2) Current use is not restricted to plants, but maybe to botany, but botany has rather obscure boundaries.

(1) Chamber's Encyclopedia (1880) says: "a phrase devised by Steenstrup, a Danish naturalist, about the year 1840, to signify " the remarkable and till now inexplicable natural phenomenon of an animal producing an offspring, which at no time resembles its parent, but which, on the other liana, itself brings forth a progeny which returns in its form and nature to the parent animal".

(2) Biology of plants, Peter H. Raven et al, 1999, defines AOG. as here (haplo/diplo). It mentions AOG in plants, of course, and also in chytrids (fungus) and red algae. --Ettrig (talk) 06:50, 18 February 2012 (UTC)

I have never encountered the use of the word heterophasic in botany. The word would be kind of tautological since if generations or phases are alternating there must by definition be two different ones. What we do say is that phases can be (and I emphasise can be) heteromorphic. All modern (extant) plants have heteromorphic alternating generations or phases, but there is growing evidence that this derived from isomorphic ancestors. The essential fact about all known embryophytes=land plants is that in every case there is an alternation between multicellular diploid and haploid phases. There are examples of asexual (clonal) reproduction in both phases - e.g. in the haploid gametophytes of bryophytes, and in the diploid sporophytes of higher plants such as Liliaceae - but this is not relevant to the concept of alternation of generations or phases which is essentially about alternation of multicellular haploid and diploid phases (whether isomorphic or heteromorphic is immaterial) in a complete sexual/asexual life cycle. Although the term Alternation of generations is widely used by botanists, the term Alternation of phases is often preferred since it can be argued (see e.g. Stewart & Rothwell 1993 Palaeobotany and the evolution of plants, Cambridge University Press) that a generation is a complete diplohaplontic (haploid/diploid) life cycle which involves alternation of two multicellular phases with different chromosome complements. Plantsurfer (talk) 11:21, 18 February 2012 (UTC)

The article is clear, I think, that "alternation of phases" is an alternative use, as explained in Alternation of generations#Definition. I myself slightly prefer it, but it still seems to be less widely used than "alternation of generations". The key point on which we agree is that it's the alternation of haploid and diploid forms that is the subject of the article, whatever the title. I don't think it would be helpful to readers to mix this up with alternations of any other kind; the subject is already very complicated! Peter coxhead (talk) 14:33, 18 February 2012 (UTC)

@Curtis Clark, we should not confuse usage with the definition. May I give an example from (undergraduate) mathematics? A rectangle is defined as a quadrangle where all four angles are right (90 degrees) angles. A square is defined as a quadrangle, where the four angles are right angles, and all four sides are of equal size. An immediate consequence is that by definition every square is a rectangle.

However, in actual every-day life usage, the term "rectangle" is almost never applied for squares. Instead, you even may contrast rectangles and squares:

"Where shall I put the cup, on the square table or on the rectangular one?"

in general may get the answer "On the rectangular table, please!", without any risk for confusion. (A math teacher like me, on the other hand on a day when I feel extra didactic, or just peevish, might answer "Well, both tables are rectangular, as you should know; put the cup on the oblong table!". However, in most situations, that answer is not recommendable.)

Thus, we have a general usage, which is more restricted than the definition. The main reason of course is that square says all that rectangle says, and more; so that a shorter word actually conveys more information. This makes it unneccesary to call a single square a rectangle. However, the old definition of rectangle is retained and rather practical e.g. for discussing properties of all rectangles (including the squares). (This is also a question of well-organised concept forming. If we defined rectangle as "quadrangle with four right angles but not all sides of equal size", this would be a "paraphyletic concept", from the mathematician's point of view.)

Thus, again: Let the consensus definition in both modern and older literature dictate the article names, so that this articles be moved to Alternation of generations (botanics) (botany), and a shorter article Alternations of generations (biology) be written briefly writing about the main occurrences of a.o.g. (in all senses) in various biological contexts. (If real references for a definition restricted to the heterophasic situation turns up (e.g., in Raven et al., as I think Ettrig claims), then this should be noted in both these articles.) On the other hand, let the modern common usage dictate that Alternation of generations redirects to the botanics article, since there is a greater chance that people look up the term after having found it in a botanical context.

@Plantsurfer, there is no contradiction of terms, since the sources that use the terms heterophasic and homophasic as referring to generations or phases which are genetically different, with (at least) one haplophase and one diplophase, but also recognise some alternations of diplophases as a.o.g. The concept is defined in our German sister article, see de:Generationswechsel#Heterophasischer Generationswechsel, and also in the CBE (which incidently was translated from German). In the zoology examples, the successive diplophases may be rather morphologically different (as the polyps and medusae), or more or less equal, except for the means of propagation (as the parthenogenic and the sexual generations of aphids). Self-evidently, if you redefine a.o.g. as presuming at least one haplophase and one diplophase, then indeed the distinction between heterophasic and homophasic a.o.g. is void. Since as yest no reliable source has been quoted as adhering to this redefinition, my only reservation against using the terms is that there is too little evidence of them being actually used in English. They are logical, describe the differences in an excellent manner, and are fairly easy to understand - but that does not matter, as long as they are not used (to a reasonable amount) outside wp.

@Ettrig: Could you please quote the definition of a.o.g. in Raven et al., with sufficient context? I ask, since in a few other cases here there was a confusion between definitions and enlisted properties. I also wonder whether they mentioned a shift between sexual and asexual phases as a part of the definition and/or the description. If Raven et al. indeed define (and not just describe) a.o.g. as heterophasic, then this clearly should be added to the relevant articles, as a reference for an alternative definition being around. JoergenB (talk) 22:43, 18 February 2012 (UTC)

Once again, English Wikipedia does not support your view on the article title. There is a long history of editors wanting to title articles to conform to specific definitions, but in contradiction to general usage, and with the exception of politicized issues such as Ireland and Macedonia, they have uniformly lost. As my relatives, speakers of Southern American English, would say, "That dog don't hunt." I am fully prepared to oppose a move to either Alternation of generations (botanics) (which is not correct English) or even Alternation of generations (botany), since both titles are unnecessarily disambiguated.

And in response to your statement about definition, it is widely held (although not universal) among current Anglophone scholars of the English language that definitions are descriptive, not prescriptive, and that they can change over time. Even the Oxford English Dictionary, which I, an American, consider to be the most authoritative dictionary of the language, will provide definitions in current use as well as definitions used in the past. For you to call the current use of the term a "redefinition" is perhaps technically correct (although I am not convinced it was ever widely used by working zoologist), but that is not a negative appraisal.--Curtis Clark (talk) 03:17, 19 February 2012 (UTC)

Biology of plants, Peter H. Raven et al, 1999, Glossary: "A reproductive cycle in which a haploid (n) phase, the gametophyte, produces gametes, which, after fusion pairs to form a zygote, germinate, producing a diploid (2n) phase, the sporophyte. ..." --Ettrig (talk) 19:40, 19 February 2012 (UTC)

@Ettrig: Many thanks! That seems to be highly relevant! (Am I right in assuming that the quotation refers to a.o.g as an item in itself, without further qualifications?)

@Curtis, thanks for correcting my English! Apart from that, I assume that you've already checked the on-line version of the OED, and thus have seen that it has the same broader definition as all the other sources - except Ettrig's - including an excerpt about a.o.g. among cnidarians from the year 2000. (The item is found at the end of the article Alternation. If you don't have access to it, I'll happily reproduce it here.)

Thus, I'm perfectly happy with using OED as the "norm", in this case, and thus to move this article to a.o.g. (botany):-); and, besides, I agree with you that the OED mostly is a very good norm.

As for the difference between prescriptive and descriptive definitions: I'm not proposing a new usage. I do oppose any idea that the dictionary definitions should be replaced by an original research based estimate of what current usage implicitly indicates as the most common meaning. For most sciences, I (and seemingly the wp consensus) think this is a bad practice. Thus, you'll find numerous exceptions from the "most common usage" titles within scientific fields; especially when some recognised international scientific community has decided on a fixed terminology. E.g., not only the definitions in articles such as rectangle deviate from the most common usage; but we have such controversial and well debated cases as Pluto (which was reclassified as a dwarf planet by a decision of the International Astronomical Union).

I doubt that botany is fundamentally different from other sciences, except perhaps by concentrating the terminological effort to taxonomy. I found this interesting article by Nadia Talent (alias Nadiatalent (talk · contribs)), whith an appended Table 1 (p. 42 ff) which IMHO is very much devoted to trying to bring order in the terminology. Of course, concepts are much more important than mere words; but a clear terminology is a great help in trying to organise our thoughts, I think.

@Curtis and Peter: I proposed the name Alternation of generations (biology) for the more general concept (as given and exemplified in the Chamber's encyclopedia, the New universal encyclopedia, Encyclopædia Britannica, the Concise Biology Encyclopedia, and the Oxford English dictionary, apart from numerous other sources, older and newer, including the quotation about cnidarians from Colin Tudge: The Variety of Life, 2000, as quoted by OED). Do you agree, or do you have a counterexample; or do you consider such an article as unnecessary? JoergenB (talk) 20:37, 20 February 2012 (UTC)

As the overwhelmingly dominant use, this needs to stay here. The other uses are dealt with by the hatnote and a small section in this article. Anything else appears to be a solution in search of a problem. Guettarda (talk) 20:51, 20 February 2012 (UTC)

Complex life cycle errors

It is unfortunate that the Complex life cycle section begins by citing Salix as an example, because several errors follow as a consequence, together with other errors which need the attention of a proper botanist. Extant seed plants including Salix do not have either antheridia or motile sperm. The sentence "The whole pollen grain is moved (typically by an insect) to an ovule (megagametophyte), where a sperm is produced which moves down a pollen tube to reach the egg." is riddled with errors. Salix is wind pollinated, the pollen grain is delivered not to the ovule but to the stigma where it germinates to produce a pollen tube which grows down the style to the ovary and then to the ovule, where a non-motile sperm nucleus is delivered to the egg. Salix is not exclusively dioecious - some species, and particularly hybrids have bisexual catkins. Carpels are not megasporangia but enclose an integumented megasporangium. The ovule article makes it clear that the megagametophyte is enclosed successively in the nucellus=megasporangium and integuments, the whole constituting the ovule. The ovule may in turn be enclosed singly in a carpel (e.g. Ranunculus), or with others in a follicle (e.g. Eranthis) or in an ovary comprising several fused carpels (Papaver). Plantsurfer (talk) 12:57, 19 February 2012 (UTC)

This bit "The whole pollen grain is moved (typically by an insect)" is easy to fix, as I have done. (Actually some Salix species are often insect pollinated as well as wind pollinated.)

Some of the other bits are over-simplifications, which can, and should, be corrected if they are seriously misleading. The problem is that the article is already very difficult for a non-specialist to understand; I worry that being over-fastidious will make it worse. However, this is not an argument for allowing errors to persist, rather for great care in correcting them.

It's not wrong to say "a sperm is produced which moves down a pollen tube to reach the egg"; the sperm nucleus (which is all there is of the sperm) does move down the pollen tube, although perhaps the passive "is moved" would be better. Peter coxhead (talk) 13:57, 19 February 2012 (UTC)

I agree it is hard to untangle but a confused interpretation does not aid understanding. In addition to the above it should perhaps be made clear that a pollen grain contains a packaged, endosporic microgametophyte consisting intially of four haploid cells derived by meiotic division of a diploid spore mother cell: pollen germination produces a pollen tube, not a microgametophyte. I will have a go at editing when I have the strength! However, I really wonder whether launching straight in at this stage with examples based on seed plant life cycles is jumping the gun. Are these complex lifecycles not also relevant to dioicous exosporic plants such as mosses and liverworts? Perhaps someone like EncycloPetey with solid expertise in bryophytes might be able to advise.Plantsurfer (talk) 14:31, 19 February 2012 (UTC)

It's not quite a question of "untangling" but explaining at the right level. EncycloPetey was heavily involved in the early editing of this article, by the way.

On another point, I've now checked several sources and they all agree with Flora Europaea (Vol. 1 p 53) that Salix is "primarily entomophilous"; many have nectariferous flowers (the number of nectaries per flower is used in keys to the species), and willow nectar and pollen is a major early source of food for many species of bee.

Good ref. They use both wind and insects, apparently in various proportions depending on species. see e.g. this abstract http://www.springerlink.com/content/v56614412l4j7038/ Plantsurfer (talk) 18:29, 19 February 2012 (UTC)

Several sources say flatly that not just Salix but Salicaceae is dioecious (e.g. Stace's New Flora of the British Isles, although this may be referring to the species found in this geographical area). However, I agree that this does not seem to be quite correct. Peter coxhead (talk) 17:34, 19 February 2012 (UTC)

1. Iirc, the sperm of most flowering plants is a cell, not just a nucleus.
2. Meiosis produces four microgametophytes; each one develops into a pollen grain.

I've taught this stuff successfully to lower-division undergraduates for years; I'll take a stab at it next week.--Curtis Clark (talk) 16:31, 19 February 2012 (UTC)

That would be very helpful indeed! Peter coxhead (talk) 17:34, 19 February 2012 (UTC)

Problem with dual meaning of Metagenesis

If we look up "metagenesis" in Wikipedia, it redirects to "alternation of generations". But it has a totally different meaning in petroleum geology:

metagenesis

1. n. [Shale Gas]

The last stage of maturation and conversion of organic matter to hydrocarbons. Metagenesis occurs at temperatures of 150° to 200°C [302° to 392°F]. At the end of metagenesis, methane, or dry gas, is evolved along with nonhydrocarbon gases such as CO2, N2, and H2S, as oil molecules are cracked into smaller gas molecules.[2]

We should have a separate page for metagenesis, with the main entry to this page "alternation of generations", and another entry for the geological meaning.173.10.202.66 (talk) 21:05, 20 February 2017 (UTC)

Most botanists call it Alternation of generations rather than metagenesis, so probably it would be better to keep that article as it is. One option is to convert Metagenesis to a disambiguation page and create a new article with a title like Metagenesis (geology). An alternative would be to make the Metagenesis article exclusively geological but to have a hatnote directing the biologist who arrives there to Alternation of generations. Plantsurfer 21:24, 20 February 2017 (UTC)

I'd vote for the last option, making the Metagenesis article entirely about geology, with a hatnote. Sminthopsis84 (talk) 10:27, 25 February 2017 (UTC)

I agree with Sminthopsis84. Peter coxhead (talk) 20:25, 25 February 2017 (UTC)

The initial statement, "This article describes alternation of generations in plants; for a similar phenomenon in animals see Heterogamy" is wrong.

This article (Permalink) Misleads potentially.

• Firstly, this article contain a little bit about animal's metagenesis. And animal's metagenesis is conceptually the same with plant's alternation of generation. There is no need to separate them.

• Secondly, Heterogamy is never similar with alternation of generation. The only similarity is both are related to reproduction.

• As well as heterogamy (heterozygosity) takes place in plants too. We can see heterogamous pea seedlings in Mendel's experiments. The linked article, Heterogamy, also mentioned several othe different usage of heterogamy, i.e. 1. "In cell biology, heterogamy is a synonym of anisogamy, the condition of having differently sized male and female gametes produced by different sexes or mating types in a species." and 2. "In botany, a plant is heterogamous when it carries at least two different types of flowers in regard to their reproductive structures, for example male and female flowers or bisexual and female flowers. Stamens and carpels are not regularly present in each flower or floret.". But they are found in plants.

If you are human-like but your daughter/son is box-like and your grand-daughter/grand-son is again human like, that could be called an alternation of "generations". There is nothing to do with heterogamy.

18:31, 18 September 2017 (UTC)

The term "heterogamy" has more than one meaning. The reason for separating animals is that they do not undergo a regular alteration of generations with differing ploidy levels consistent with the alternation of those generations. The two can be conceptually linked, yes, and that is done at biological life cycle. This article treats only of the alternation as it exists when accompanied by regular changes in ploidy. --EncycloPetey (talk) 00:11, 19 September 2017 (UTC)

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Article is too technical

I have added the technical template. I think it would help to have an simple example for a plant most people would recognize, even if that doesn't cover all cases. Also, it would be helpful to mention how frequent the alternation of generations is in the major plant divisions. Unfortunately I don't know enough about the topic to be of much help. Vpab15 (talk) 12:12, 1 April 2022 (UTC)

Alternation of generations is the norm for plants and algae. The phenomenon is often illustrated using the life cycle of a fern, in which the asexual sporing stage or sporophyte is the familiar fern, and the sexual gametophyte stage is the prothallus. Plantsurfer 20:47, 1 April 2022 (UTC)

Yes, the article is rather technical, but it seems to me that this is a rather technical subject. (Note the quote in the article: "the alternation of generations has become a terminological morass; often, one term represents several concepts or one concept is represented by several terms.") It could be introduced differently, e.g. for land plants by describing the fern lifecycle earlier, since (as Plantsurfer notes above) this is clearer as both the sporophyte and gametophyte are macroscopic. Peter coxhead (talk) 10:20, 2 April 2022 (UTC)

I think it would be helpful to compare the process to that of animals. As I understand for seed plants the main difference is the way gametes are produced by mitosis in plants rather than meiosis. But that seems more like a small detail rather than a fundamentally different reproduction process. The gametophytes do not have an independent existence from the parent plant. Since seed plants are the type of plant most people are familiar with I think we should stress more the similarity of the process and not focus so much on the differences. Vpab15 (talk) 21:59, 2 April 2022 (UTC)

No, the underlying difference is still quite major even if not apparent. People sometimes think of pollen as equivalent to sperm, for example, but it is not. Pollen may be a very reduced gametophyte, but it is still capable of growth, germinating and producing a pollen tube and gametes which move down the pollen tube. The pollen does have an independent existence from the parent plant – only the female gametophyte doesn't. By contrast, animals produce sperm directly, and the sperm is not capable of growth or division. Peter coxhead (talk) 08:33, 3 April 2022 (UTC)

To help readers understand a concept, it usually helps to link it to something they already know. In this case animal reproduction. I think we don't need to stress those differences in the intro, they can be explained later in the body. Vpab15 (talk) 09:19, 3 April 2022 (UTC)

But the whole point is that a lifecycle with alternation of a diploid sporophyte generation and a haploid gametophyte generation is radically different from a life cycle with only a diploid generation. So the "link" to animal reproduction is that it's different, not that it's the same. Peter coxhead (talk) 20:39, 3 April 2022 (UTC)