The stem is the main axis of the plant body arises from the plumule of an embryo. It bears branches, leaves, and flowers. The stem is characterized by its positively phototropic nature and the presence of nodes, internodes, and buds. On the basis of position, buds can be categorized into two types:
In some herbaceous plants, the aerial part die off during unfavourable conditions and the reserve food materials are stored in their underground portion due to which they become thick and fleshy. These structures are although underground are considered as the modified stems because they have distinct nodes, internodes, scaly leaves and buds on them.
These underground modified stems can grow into the new plant with the coming of favourable conditions. Thus, they are also known as organs of perennation. Its types are discussed below:
Rhizome is underground, branched, dorsiventral horizontally growing modified stem having distinct nodes and internodes. Thin and membranous scaly leaves arise on the nodes. Apical as well as auxiliary bud arise on the rhizome which grows into new aerial shoots. Adventitious root arises from the lower surface of a rhizome. Eg; Zingiber officinate (Ginger).
It is spherical, an underground structure having reduced conical stem known as disc in which the internodes are highly compressed and nodes closely arranged. Thick and fleshy scaly leaves arise on the nodes which store reserved food materials. Apical bud is enclosed by the scaly leaves and auxiliary buds are sometimes developed. Adventitious root arises from the lower surface of the stem. Eg; Allium cepa(Onion), Allium sativum (Garlic).
It is underground swollen vertically growing modified stem having circular nodes and internodes. Thin membranous and brownish scaly leaves arise on the nodes. Single apical bud is prominent and auxiliary buds are sometimes developed. Adventitious roots arise normally all over the body of the corm. Eg; Colocasia, Croccus sativus(Safron).
Tubers are swollen terminal portions of an underground stem which are covered with a thin corky sheath having lenticels(pores). Many depression known as eyes are present which represent nodes. Thin scaly leaves and buds arise on the eyes or nodes. Adventitious roots are absent. Eg; Solanum tuberosum (Potato).
The sub-aerial modifications are found in many herbaceous plants with a thin, delicate and weak stem. In such plants, a part of the stem lives underground whereas remaining part of the stem is aerial. These plants bear adventitious roots and aerial branches at nodes. Based on type of growth and part of plant that provides them, it is divided into four types:
The runner is a specialized weak stem that has long and thin internodes and the branches creep over the surface of the soil. The branches develop adventitious roots from the lower sides of each node. The aerial branches develop from the axil of the scale leaves at the nodes. The runner gives rise to new plants either from axillary or terminal buds. Eg; Cynodon dactylon (Doobo), mint.
It is a thin layered branch which grows horizontally outward and bears nodes and internodes. It produces adventitious root at the point of contact with the soil. Leaves are distributed all over the stolon due to normal apical growth. Eg; Strawberry
It is the lateral branch which develops from the axillary bud of an underground part of the stem. It grows obliquely and gives rise to leafy shoot. The adventitious root develops at the base of nodes of the underground part of a sucker. Eg; Mentha arvensis (Mint), Chrysanthemum.
The offset is also known as condensed aquatic runners. It is weak, elongated, horizontal branches at one internode that arise in the axil of a leaf. An offset produces a tuft of leaves above and a cluster of the roots below. The offset may break off from the parent plant and acts as an independent plant. Eg; Pistia(Water lettuce).
It is the characteristic feature of some xerophytic plants. Phylloclade can be defined as green, swollen, cylindrical or flattened aerial modified stem of unlimited growth. It has many depressions known as Aereoles representing on the nodes on which reduced leaves or spines and buds are developed. Phylloclade store water in the form of mucilage and is covered by thick cuticle so that the plant can survive in a dry habitat. Eg; Opuntia(Nagphani), Euphorbia royleana(Siundi)
Cladode is short, green aerial modified stem or branch of limited growth having single internode only. It can perform photosynthesis on itself because the leaves are modified into thin membranous scales. Eg; Asparagus racemoscus (Kurilo).
Tendrils are thin, elongated, thread-like spirally coiled modified stem which may be branched or unbranched and provide support to the climbers. Tendrils arising from the axils of leaves are known as axillary tendrils. Eg: Passiflora (Passion flower). Tendrils arising on the internodes are known as extra-axillary tendrils. Eg; Cucurbita (Gourd). Tendrils arising on the opposite of leaf are known as leaf-opposed tendrils. Eg; Vitis vinifera(Grapevine).
Thorns are short pointed hard often straight aerial, modified stem arising from the axil of the leaf which may be branched or unbranched and may bear leaves or flowers. Eg; Durant repens, Punica granatum(Pomegranate), Bougainvillea.
Bulbils are modified axillary buds which are meant for vegetative propagation of the plant. They develop from axillary buds, which become swollen and fleshy with food and drop off to form new plants. Eg; Agave.