The ascending part of the plant axis is called stem. The stem bears branches, leaves, flowers and fruits. The stem develops from the plumule of the embryo. Nodes and internodes are present on the stem. Leaves and branches come out from the nodes. The portion between two consecutive nodes is called internode. The buds on stem can be terminal or axillary. Terminal bud is present at the tip, while axillary bud is present at nodes. Young stem is usually green in colour and subsequently becomes woody and brown.
Functions of Stem:
- Bearing branches, leaves, flowers and fruits.
- Conduction of water and minerals.
- Green stems also carry out photosynthesis.
- Food storage, support, protection and vegetative propagation.
Modifications of Stem
Modification for food storage: The underground stems of potato, ginger, turmeric, zamikand, Colocasia, etc. are modified for food storage. There are different names for them, depending on particular shape of the modified stem. Ginger and turmeric are called rhizome, potato is called tuber and colocasia is called corm. Such modifications are the tools of perenation to tide over unfavourable conditions.
Modifications for climbing: In some plants, stem tendrils develop form axillary buds. These are slender and spirally coiled structures. A tendril helps the plant to climb to a support, e.g. cucumber, pumpkin, grapevine, etc.
Modification for defence: In some plants, the axillary buds are modified into woody, straight and pointed thorns. Such thorns protect the plant from browsing animals, e.g. Citrus, Bougainvillea.
Modification for photosynthesis: In some plants of arid regions, the stems are modified into flattened or fleshy cylindrical structures. The flattened structure can be seen in Opuntia and the cylindrical structure can be seen in Euphorbia. Such structures contain chlorophyll and photosynthesis happens in them.
Modification for vegetative propagation: Stems are modified for vegetative propagation in many plants.
In some plants, the underground stems spread to new areas and give rise to new plants when the older plants die, e.g. grass and strawberry. Such a modification of stem is called runner.
In some plants, lateral branches come out from the main axis. These branches arch downward to touch the ground, after some growth. These branches then give rise to new plants, e.g. mint and jasmine. Such a modification of stem is called stolon. While the main prupose of runner is to facilitate horizontal growth and proliferation of plant, the main purpose of a stolon is vegetative propagation.
In some aquatic plants, a lateral branch bears a bunch of leaves and a tuft of roots at nodes. New plants emerge from such nodes, e.g. Pistia and Eichhornia.
In some plants, the lateral branches emerge from the basal and underground portion of the main stem. Such branches grow horizontally beneath the soil and come out upward to give rise to leafy shoots, e.g. banana, pineapple and Chrysanthemum.