A stem is the main axis or stalk of a plant, which develops from the plumule of a germinating seed. The stem bears branches, leaves, buds, flowers and fruits. A stem usually has many nodes and internodes. The buds growing on the stem can be either axillary or terminal. As the stem grows, many lateral buds emerge, which grow into lateral branches. These branches further develop other lateral shoots called twigs or branchlets.
The stem along with its branches hold the leaves, buds, flowers and fruits and also transport water, minerals and food between the roots and the shoots. Apart from these functions, the stem of many plants are modified for storing food, providing protection and support and aiding in vegetative propagation.
Underground stems of potato, turmeric, ginger and colocasia store food. In watermelons and grapes, the stem tendrils help the plant to climb. In bougainvillea, lemon and orange, the axillary buds on the stem modify into thorns, which protect the plant from pasturing animals. In jasmine and mint, slender lateral branches arise from the base of the main stem, which bend to touch the ground and form a new plant. In pineapple and banana, branches grow out horizontally from the underground base portion of the main stem, which later emerge out of the soil in the form of leafy shoots. In opuntia and cactus, stems are modified into flattened or fleshy cylindrical stems that perform photosynthesis.
These modified stems have thorns to deflect sunrays and maintain the moisture content. In grass and strawberry, some stems get modified and grow underground, and form new plants as the old ones die. Aquatic plants like Pistia and Eichhornia have modified stems with short internodes. The nodes of these plants have clusters of leaves and small bunches of hair-like roots, which help the plant to stay afloat.