Modifications of Stem

The stem develops several modifications to carry out additional or accessory functions.

a. Underground stem. In some herbaceous plants the stem grows below the soil surface as an underground stem. It remains dormant in unfavourable conditions and, once conditions become favourable, produces aerial shoots. Underground stems store food and help in perennation (surviving from one growing season to the next) and vegetative propagation.

1. Stem tuber — a special underground branch of the stem whose tip becomes irregularly swollen because of stored food, mostly starch. The presence of distinct nodes (but no internodes) is what classifies a tuber as a stem rather than a root. At the nodal region, scale leaves are present with axillary buds, popularly called 'eyes', which can produce aerial shoots under favourable conditions; tubers are propagated vegetatively, e.g. Potato (Solanum tuberosum), Matalu (Helianthus tuberosus). A tuber has two distinct ends — the apical (rose) end and the basal (heel) end — and the number of nodes and eyes is greater towards the rose end.
2. Rhizome — a prostrate, dorsiventrally thickened, brownish underground stem that grows either horizontally or obliquely beneath the soil. It shows nodes and internodes, scale leaves, and terminal and axillary buds at the nodes. Where the terminal bud produces the aerial shoot under favourable conditions (the shoot dying back at the end of the favourable period) and growth continues through lateral buds, this is called sympodial growth, e.g. Ginger (Zingiber officinale), Turmeric (Curcuma domestica), Canna. Where the rhizome grows obliquely and its own terminal bud continues the growth of the rhizome, this is called monopodial growth, e.g. Nymphaea, Nelumbo (Lotus), Pteris (fern).
3. Bulb — an underground, spherical or pear-shaped (pyriform) stem that is highly reduced and discoid, bearing a whorl of fleshy leaves arranged concentrically around it; these fleshy (scale) leaves store food, and some of the outer scale leaves become thin and dry. The reduced stem produces adventitious roots at its base. Bulbs are of two types: a tunicated (layered) bulb, made up of fleshy leaves arranged concentrically with an outer dry scale leaf, e.g. Onion; and a scaly or compound tunicated bulb, as in Garlic, where fleshy scales (cloves) are arranged in an overlapping pattern.
4. Corm — a swollen, underground, spherical or sub-spherical stem representing a single vertically-growing internode. It is a condensed structure with circular, ring-like nodes, showing axillary buds and scales. Adventitious buds are produced that help in vegetative propagation, and adventitious roots arise from the lower part of the stem, e.g. Colocasia (Arbi), Amorphophallus (Zamikand/elephant foot). Presence of nodes with scale leaves and axillary buds is also seen in Cynodon (lawn grass), Centella (Hydrocotyl) and Oxalis.

b. Sub-aerial stem. These stems are generally weak or straggling and grow along the ground, needing support to persist; sometimes they also grow partly beneath the soil, so they are in contact with both air and soil. Sub-aerial stems help in perennation and vegetative propagation; scale leaves and axillary buds are present on the stem surface, and the buds produce aerial shoots. The different sub-aerial forms are:

1. Trailer — the shoot spreads over the ground without regular rooting at intervals; the branches may be flat/procumbent or partly vertical/decumbent, e.g. Euphorbia, Tridax.
2. Runner — special narrow, prostrate, green branches that develop at the base of erect shoots (called the crown) and spread in all directions, producing new crowns with a bunch of adventitious roots, e.g. Cynodon.
3. Stolon — a slender lateral branch arising from the base of the main axis; in some plants (e.g. wild strawberry) it grows above ground. It first shows upward growth like an ordinary branch, but when it bends and touches the ground its terminal bud grows into a new shoot along with adventitious roots, e.g. Mentha.
4. Sucker — a non-green, runner-like branch of the stem that develops from the underground base and grows horizontally below the soil before emerging above the surface to produce a new plant; it can be thought of as an underground runner, e.g. Chrysanthemum, Banana.
5. Offset — one-internode-long runners found in rosette plants growing at ground or water level, helping in vegetative propagation, e.g. water hyacinth/Jal kumbhi (Eichhornia) and Pistia.

c. Aerial modifications (metamorphosed stems). The stem or its vegetative part may modify to carry out specialised functions:

1. Thorn — a modification of the apical or axillary bud; it is hard, pointed and mostly straight (except in Bougainvillea, where it is curved and helps in climbing). It reduces transpiration and, being hard and pointed, protects the plant against browsing or marauding animals. The apical bud becomes a thorn in Carissa, while the axillary bud becomes a thorn in Duranta, Citrus and Bougainvillea.
2. Phylloclade — modification of the stem into a leaf-like photosynthetic organ. Because it is a stem, it still shows nodes and internodes; it is thick, fleshy and succulent, and contains mucilage that helps retain water, e.g. flattened in Opuntia, cylindrical in Casuarina, ribbon-like in Muehlenbeckia.
3. Cladodes — branches of limited (one-internode) growth that carry out photosynthesis, with the true leaves reduced to spines or scales, e.g. Asparagus. …