The Stem

The stem is the ascending, usually above-ground part of the plant axis, developing from the plumule of the embryo, and is distinguished from the root by bearing nodes -- the points where leaves, branches and buds arise -- separated by internodes. A young stem is typically green, photosynthetic, and grows towards light, but across the plant kingdom the stem has been modified for an unusually wide range of alternative purposes, from underground food storage to climbing to defence to taking over photosynthesis entirely.

Underground stem modifications. Several kinds of underground stem serve chiefly to store food and help the plant survive an unfavourable season, then resume growth once conditions improve; all remain recognisable as stems, not roots, because they bear nodes, internodes and axillary buds. A rhizome is a horizontal, underground stem that grows partly beneath the soil surface, bearing nodes and scale leaves, storing food, and producing new aerial shoots -- ginger and turmeric are common examples. A tuber, as in potato, is the swollen tip of an underground stem branch, packed with stored starch; its surface bears small depressed 'eyes', each of which is a node carrying a dormant bud capable of sprouting into a new plant, which is exactly how potato is propagated. A bulb, as in onion, is a highly condensed underground stem reduced to a small flattened disc, surrounded by numerous fleshy, overlapping scale leaves that do the actual food storing. A corm, as in Colocasia, is a vertically oriented, condensed underground stem, broader than it is tall, that similarly stores food and bears nodes with buds.

Aerial stem modifications. Above ground, stems are reshaped for other purposes. In climbing plants with weak stems that cannot stand upright on their own, part of the stem -- often an axillary bud -- may develop into a slender, coiling stem tendril that wraps around any nearby support, as seen in the grape vine and in Passiflora. In other plants, an axillary bud develops instead into a hard, pointed thorn, a defensive structure that discourages browsing animals, as seen in citrus and Bougainvillea (a stem thorn is distinguished from a superficial prickle, such as that of a rose, by the fact that a thorn arises from a node and contains vascular tissue continuous with the stem, whereas a prickle is only an outgrowth of the epidermis). Some plants that live in arid conditions, such as Opuntia (prickly pear), have flattened, fleshy, green stems called phylloclades that have taken over the job of photosynthesis entirely, while the true leaves are reduced to small spines that minimise water loss. A related but different structure, the cladode, is a phylloclade of only one or two internodes, as in Asparagus. Certain plants, such as Oxalis, develop bulbils -- small stem outgrowths that store food and can detach to grow into new plants, functioning as a means of vegetative propagation. …