Modification of Stem - Underground, Sub aerial and Aerial Modification

The stem is the aerial portion of the plant axis which develops from the plant plumule of the embryo. It is differentiated into node and internode. Stems grow erect above the soil and towards light performing their normal functions like support, protection, storage of food and photosynthesis. Broadly, there are three types of modifications in the stem: underground modifications, sub-aerial modifications and aerial modifications.

a) Underground Modification of Stem: Some plants develop underground stem for the purpose of perennation, i.e. surviving from year to year through unfavorable season. These stems are thick and fleshy due to storage of food. The stem bears nodes, scale, leaves and buds. The underground modifications are in the form of rhizome, tuber, bulb and corm.

The Rhizome: This is a thickened, horizontally creeping stem. Its axillary buds grow into short, stout branches. It remains dormant underground and undergoes vegetative growth during the vegetative season, e.g. alocasia, canna and ginger.

The Tuber: This grows from special underground branches which swell at the apex. It has a number of buds or eyes on its surface, e.g. potato.

The  Bulb: This is a modified shoot found underground. It has a condensed stem which bears terminal and axillary buds apart from numerous scale leaves. The inner scales are fleshy while the outer ones are dry, e.g. onion and lilies.

The Corm: This is a condensed form of a rhizome, consisting of a stout, solid, underground stem, spherical in shape. It grows vertically with the help of its terminal bud. It has prominent nodes with scale leaves, e.g. gladiolus, calocasia and crocus.

b) Sub-aerial Modification of Stem: The stems of certain plants grow horizontally on the soil surface or just below it. These are modified for vegetative propagation. They never store food but conduct it away to the daughter plant until they are fully developed. According to their nature and mode of propagation, there are four types of sub-aerial modifications: runner, stolon, offset and sucker.

Runner: A runner is a thin, slender, prostate branch, generally with long internodes, creeping on the soil surface. It arises from the axillary bud, and after growing for a distance away from the mother plant, it strikes roots at the nodes. Thus, a new daughter plant is formed, e.g. wood sorrel, doobo grass, strawberry, penny-wort.

Stolon: Stolon is similar to runner but is subterranean. It is a long and thick branch which develops from the base of the stem. It grows horizontally just below the ground. After covering some distance, the terminal bud of the stolon emerges from the soil and gives rise to a new plant which develops root below. When detached from the parent plant, the new plant can become independent, thus surviving as a means of vegetative propagation, e.g. colocasia, dracaena.

Offset: Like the runner, the offset arises from the axil oh the leaf, but is distinguished by short, thick internodes. It is found in the rosette type of water plants. At the nodes adventitious roots are given off below, and a tuft of rosette leaves at the apex, e.g. water lettuce, water hyacinth.

Sucker: Like the stolon, the sucker is also a lateral branch that develops below the soil from the underground part of the stem. However, it grows obliquely for some distance below the soil and directly gives rise to the daughter plant on touching soil surface. A sucker is much shorter and stouter than a stolon, e.g. chrysanthemum, pineapple, bamboo.

c) Aerial Modification of Stem: In some plants stem or their parts are structurally modified to perform specialized functions such as climbing, protection, food manufacture, vegetative propagation or provide a support system for the plant. These modifications occur in the form of tendrils, twinner, thorns, phylloclade, cladode and bulbils.

Stem tendril: In certain plants, the terminal or axillary bud is modified into a tendril and appears as a thin wiry spirally curled branch. These plants attach themselves to neighbouring objects and plants by means of tendrils and take support for climbing, e.g. passiflora, antigonon and gouania.

Twinner: Certain weak stemmed plants take support by twinning around other plants and climb towards sunlight. These plants do not have any special organs for climbing, e.g. morning glory.

Thorn: A thorn is a sharp, pointed defensive organ developed for the protection of the plant. it may sometimes be hooked and hence used as a climbing organ, as in bougainvillea. It is regarded as a branch modification since it arises either in the axil of a leaf or at the end of the stem, both of which are normal positions of the bud. A thorn may be branched and may sometimes bear leaves, flowers and fruits, e.g. lemon, duranta and carissa.

Phylloclade: In desert plants, the leaf surface is reduced to minimize water loss by transpiration. The leaves may be detached completely, reduced in size or modified into spines. Therefore, the main function of the leaf is taken over by the stem.
As such, the stem becomes green, flattened or cylindrical and often fleshy due to retention or storage of water and mucilage. It also develops a thick cuticle which further reduces transpiration. A phylloclade is a branch of unlimited growth, consisting of a succession of nodes and internodes modified to prepare food and store water for plants growing in water scarce regions, e.g. opuntia, phyllocactus and euphorbia.

Cladode: A cladode is also a flattened green branch similar to a phylloclade developing from the node or branch in the axil of a scale leaf. However, it is of limited growth consisting of one internode only, e.g. asparagus, phyliocladus, lemna and ruscus.

Bulbils: A bulbil is a vegetative or floral bud swollen with the accumulation of food and responsible for reproduction of the plant. The bulbils detach from the mother plant and grow into new individuals during favourable conditions, e.g. dioscorea, globba and agave americana.