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Microphylls and megaphylls

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The term microphyll has two independently derived and unrelated uses:

  • In plant anatomy and evolution a microphyll is a type of plant leaf with one single, unbranched leaf vein.[1] Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of a microphyll, the leaf vein emerges from the protostele without leaving a leaf gap. Leaf gaps are small areas above the node of some leaves where there is no vascular tissue, as it has all been diverted to the leaf. Megaphylls, in contrast, have multiple veins within the leaf and leaf gaps above them in the stem.
  • In ecology, microphyll and similar terms based on blade size of the leaf are used to describe a flora, for example, a "microphyll rainforest" is often defined as a forest where the dominant trees have leaves less than 7.5 cm in length.[2]

Leaf vasculature

Microphylls contain a single vascular trace.

The clubmosses and horsetails have microphylls, as in all extant species there is only a single vascular trace in each leaf.[3] Despite their name, microphylls are not always small: those of Isoëtes can reach 20–100 centimetres in length, and the extinct Lepidodendron bore microphylls up to 78 cm long.[3]

Evolution

The enation theory of microphyll evolution posits that small outgrowths, or enations, developed from the side of early stems (such as those found in the Zosterophylls).[4] Outgrowths of the protostele (the central vasculature) later emerged towards the enations (as in Asteroxylon),[4] and eventually continued to grow fully into the leaf to form the mid-vein (such as in Baragwanathia[4]).[1] The fossil record appears to display these traits in this order,[4] but this may be a coincidence, as the record is incomplete. The telome theory proposes instead that both microphylls and megaphylls originated by the reduction; microphylls by reduction of a single telome branch, and megaphylls by evolution from branched portions of a telome.[4]

The simplistic evolutionary models, however, do not correspond well to evolutionary relationships. Some genera of ferns display complex leaves that are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap.[1] Horsetails (Equisetum) bear only a single vein, and appear to be microphyllous; however, the fossil record suggests that their forebears had leaves with complex venation, and their current state is a result of secondary simplification.[5] Some gymnosperms bear needles with only one vein, but these evolved later from plants with complex leaves.[1]

An interesting case is that of Psilotum, which has a (simple) protostele, and enations devoid of vascular tissue. Some species of Psilotum have a single vascular trace that terminates at the base of the enations.[3] Consequently, Psilotum was long thought to be a "living fossil" closely related to early land plants (Rhyniophytes). However, genetic analysis has shown Psilotum to be a reduced fern.[6]

It is not clear whether leaf gaps are a homologous trait of megaphyllous organisms or have evolved more than once.[1]

While the simple definitions (microphylls: one vein, macrophylls: more than one) can still be used in modern botany, the evolutionary history is harder to decipher.

  • Megaphylls have a complex network of veins.
    Megaphylls have a complex network of veins.
  • Psilotum has secondarily lost leaves, and bears enations resembling the microphylls of early land plants.
    Psilotum has secondarily lost leaves, and bears enations resembling the microphylls of early land plants.

In ecology

See also: Leaf size

Christen C. Raunkiaer proposed using leaf size as a relatively easy measurement that could be used to compare the adaptation of a plant community to dryness.

We have for a long time been aware of a series of different adaptations in the structure of plants enabling them to endure excessive evaporation, and thus allowing them to live in place where the environment determines intense evaporation, or where the conditions of water absorption of the ground are unfavourable either physically or physiologically. Examples of such structures are: (1) covering of wax, (2) thick cuticle, (3) sub-epidermal protective tissue, (4) water tissue, (5) covering of hairs (6) covering of the stomata, (7) sinking of the stomata, (8) inclusion of the stomata in a space protected from air currents, (9) diminution of the evaporating surface, &c. The matter however is so complicated that it is very difficult to reach an exact appraisal of these adaptations in characterizing the individual plant communities biologically. ... In general we must content ourselves with showing the most frequently occurring adaptations, without going farther into the statistical investigation. ... A preliminary direct consideration of a series of evergreen phanerophytic communities, ... show that amongst the adaptations named, diminution of the transpiring surface, diminution in leaf size, is one of the adaptations generally in evidence; and since this adaptation is easy to observe and comparatively easy to measure, it is convenient to begin with it if we wish to use the statistical method on this domain.[7]

Raunkiaer used the following size classes:

  • Leptophyll: less than 25 square millimetres
  • Nanophyll: 25–225 square millimetres
  • Microphyll: 225-2,025 square millimetres
  • Mesophyll: 2,025-18,225 square millimetres
  • Macrophyll: 18,225-164,025 square millimetres
  • Megaphyll: greater than 164,025 square millimetres

Later authors have modified the classes and have sometimes used leaf length as a simpler measure than leaf area if the leaf shape is approximately an ellipse. For example, L.J.Webb[8] used size classes:

  • Microphyll: less than 2,025 square millimetres
  • Notophyll: 2,025–4,500 square millimetres
  • Mesophyll: greater than 4,500 square millimetres

See also

References

  1. ^ a b c d e Kaplan, D.R. (2001). "The Science of Plant Morphology: Definition, History, and Role in Modern Biology". American Journal of Botany. 88 (10): 1711–1741. doi:10.2307/3558347. JSTOR 3558347. PMID 21669604.
  2. ^ Microphyll rainforests and thickets of the wet tropics bioregion (PDF), Wet Tropics Management Authority (Australia), retrieved 18 January 2016
  3. ^ a b c Gifford E.M. & Foster, A.S. (1989). Morphology and evolution of vascular plants. WH Freeman, New York, USA.
  4. ^ a b c d e WN Stewart & GW Rothwell (1993) Palaeobotany and the evolution of plants. 2nd edition. Cambridge University Press.
  5. ^ Taylor, T.N.; Taylor, E.L. (1993). "The biology and evolution of fossil plants". {{cite journal}}: Cite journal requires |journal= (help)
  6. ^ Qiu, Y.L.; Palmer, J.D. (1999). "Phylogeny of early land plants: insights from genes and genomes". Trends in Plant Science. 4 (1): 26–30. doi:10.1016/S1360-1385(98)01361-2. PMID 10234267.
  7. ^ Raunkiaer, C. (1934), "The use of leaf size in bioloical plant geography", in A.G.T. H. Gilbert-Carter, and A. Fausbøll (ed.), The life forms of plants and statistical biogeography, Oxford: Clarendon, pp. 368–378
  8. ^ Webb, L.J. (1959), "A Physiognomic Classification of Australian Rain Forests", Journal of Ecology, 47 (3): 551–570, doi:10.2307/2257290 Figure 2
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