Stems are modified in several ways to perform different functions.
In some plants stem grows into soil, not only stores food materials but also acts as organs of pennations.
Such stems are called undergound stem modifications. Further, they also help in vegetative propagation.
Ex : The stem tuber of potato, Rhizome of zingiber, corm of colocasia and Bulb of onion.
In some plants, Aerial stems show many modifications.
1) Tendrils : Slender, spirally coiled structures which may develop either from Auxillary Bud [Cumcumber, watermelon] or from Terminal Bud [Grapes] called stem Tendrils, help in climbing.
2) Thorns : In some plants, buds are modified into woody, straight, pointed thorns, which protect plants from grazing animals. Ex : Citrus, Bougainvillaea.
3) Phylloclades : In some plants of arid regions, stems are modified into fleshy flattened (opuntia) or fleshy cylindrical (Euphorbia) or needle like structures (casuarina), called phylloclades. They carry out photosynthesis as their leaves are reduced to scales or spines to reducethe transpiration.
4) Bulbils : In some plants, the vegetative buds (Diascorea) or floral buds (Agave) store food materials. At maturity they detach from the parent plants, develop adventitous roots and help in vegetative propagation. These are called Bulbils.
In some plants, some part of the stem is aerial and some part is underground. Such stems are called sub aerial stem modifications.
Sub aerial stems of ..oxalis.. spread to new niches and form new plant when older parts die. Such plants are called runners. In plants like Nerium, Jasmine, a slender lateral branch called .stolon. arises from the base of the main axis, grow vertically, arches downwards and touch the ground, produce adventitious roots. In some aquatic plants like pistia and Eichhornia, a lateral branch of one internode length called offset which bears a rosette of leaves at each node and a tuft of adventitious roots arising from the base of the discoid stem.
In plants like banana, chrysanthemum, the lateral branches originate from the basal portion of the main stem, grows horizontally, beneath the soil and then come out obliquely upwards giving rise to leafy shoots. These branches are called suckers. All these sub-aerial stem modifications help in vegetative propagation.
