Budding
Budding is the formation of a new organism by the protrusion of part of another organism. This is very common in plants and fungi, but may be found in animal organisms, such as the hydra, as well. Usually, the protrusion stays attached to the primary organism for a while, before becoming free. The new organism is naturally genetically identical to the primary one (a clone). When yeast buds, one cell becomes two cells. When a sponge buds, a part of the parent sponge falls off and starts to grow into a new sponge. These are examples of asexual reproduction.
In virology
Budding is the process by which enveloped viruses acquire their external envelope, often as fragment of the host cell membrane, which bulges outwards and takes the virion inside. The Gag protein is essential for this process. Some viruses hijack the host cell proteins normally involved in endocytosis to facilitate this process.
In embryology
The term budding is also applied to the process of embryonic differentiation in which new structures are formed by outgrowth from preexisting parts.
In horticulture
Budding is also a process that consist of ingrafting the bud of a plant into another plant. This is a frequent technique for fruit trees (see fruit tree propagation), but can also be used for many other kinds of nursery stock. An extremely sharp knife is necessary; specialty budding knives are on the market. The rootstock or stock plant may be cut off above the bud at budding, or one may wait until it is certain that the bud is growing.
Fruit tree budding is done when the bark "slips," ie. the cambium is moist and actively growing, which is usually August in the US. Rootstocks are young trees, either seedlings as Mazzard cherries for many cherry varieties, or clonal rootstocks (usually propagated by layering) when one wants highly consistent plants with well defined characteristics. The popular Malling-Merton series of rootstocks for apples was developed in England, and are used today for the majority of the commercial apple orchard trees.
T-budding is the most common style, whereby a T-shaped slit is made in the stock plant, and the knife flexed from side to side in the lower slit to loosen up the bark. Scion wood is selected from the chosen variety, as young, actively growing shoots. Usually buds at the tip, or at the older parts of the shoot are discarded, and only 2-4 buds are taken for use. The buds are in the leaf axils. They may be so tiny as to be almost unnoticeable.
Holding the petiole of the leaf as a handle, an oval of the main stem is sliced off, including the petiole and the bud. This is immediately slid into the T on the rootstock, before it can dry out. The joined bud and rootstock are held by a winding of rubber band, which will hold it until sealed, yet the band will deteriorate in the sunlight so that soon breaks and does not pinch new growth, girdling the shoot.
The percentage of "take" of the buds depends on the natural compatibility of the stock and scion, the sharpness of the knife, and the skill of the budder. Even the experts will have some buds die.