Artificial Methods of Vegetative Reproduction

Beyond the naturally occurring methods, agriculture and horticulture use a range of deliberate, human-assisted propagation techniques. Conventional methods, used for a long time by cultivators, include cutting, grafting and layering. In cutting, a plant part - root (as in Malus), stem (Hibiscus, Bougainvillea, Moringa) or leaf (Begonia, Bryophyllum) - is severed from the parent and placed in a suitable growth medium where it produces roots and develops into a new plant; stem cutting is the most widely used form. In grafting, parts of two different plants are joined so that they continue to grow together as a single plant: the lower part rooted in the soil is called the stock and the upper part joined onto it is called the scion, as seen in Citrus, Mango and Apple. Five distinct grafting techniques differ in how the scion and stock are united - bud grafting (a T-shaped incision is made in the stock, the bark is lifted, and a scion bud with a little wood is inserted beneath it and bandaged), approach grafting (both stock and scion remain rooted in separate pots, brought together, sliced and tied at matching cut surfaces for 1-4 weeks before the stock's tip and the scion's base are severed), tongue grafting (stock and scion of equal thickness are cut obliquely and fitted together), crown grafting (used on a large stock, where wedge-shaped scions are inserted into slits or clefts and sealed with graft wax) and wedge grafting (a slit or cut is made in the stock's bark, and a scion twig is inserted and tightly bound so its cambium fuses with the stock's). In layering, the stem of the parent plant is induced to develop roots while it is still attached; once rooted, the rooted portion is cut away and grown on independently, as in Ixora and Jasminum. Mound layering bends a flexible lower branch to the ground, buries part of the stem while leaving the tip exposed, and severs the newly rooted portion from the parent once roots emerge; air layering instead girdles a nodal region on an upright branch, applies rooting hormone, and wraps the wound in moist soil under a polythene sheet, after which roots typically emerge in 2-4 months and the rooted branch is detached and potted separately. Conventional methods produce genetically uniform plants quickly, are especially valuable for plants that set little or no viable seed (such as Solanum tuberosum) or for combining two desirable traits (such as disease resistance and high yield) onto one grafted plant, but they carry the risk of transmitting viral infection from an infected parent and the resulting propagules are often bulky and awkward to handle and store. Modern methods rest on the discovery, by Professor F.C. Steward of Cornell University in 1932, that mature phloem parenchyma cells removed from a carrot and placed in a suitable culture medium under controlled conditions could resume division and regenerate an entire new carrot plant - demonstrating that such cells are totipotent, meaning they retain the full genetic capacity to regenerate an entire plant. This principle underlies tissue culture, the controlled growth of plant tissue in a special medium, and specifically micropropagation - the regeneration of a whole plant from a single cell, tissue or small piece of vegetative structure through tissue culture. Modern methods allow rapid multiplication of desirable, genetically identical plants in any season, includi …