The stem:
The stem is the ascending organ of the plant which originates from the plumule of an embryo and grows upwards to the source of light. It can be distinguished by the following characteristic features:
- It has got nodes and internodes.
- Stem bears leaves, branches, flowers and fruits.
- Leaves and branches are developed from the nodes.
- Stems are green in colour when young.
- Stem often bears hairs which are multicellular in nature.
- Buds are developed in the axil of the leaves, at the nodal region and at the apex of the stem.
Branching of the stem:
Mainly two types of branching are found – Racemose or indefinite and Cymose or definite.
Racemose branching:
In this type of branching, the terminal bud grows indefinitely, and thus, the main stem continues to grow indefinitely. Lateral branches are developed in acropetal succession from the main stem. In Racemose branching, the main stem is distinguished as a single unit from the lateral branches. This type of branching is known as monpodial branching, example, Casuarina etc.
Cymose branching:
In this type of branching, the terminal bud does not grow indefinitely. After certain period, it ceases to grow so that the growth of the main stem is definite. As the terminal bud becomes inactive, the lateral bud grows just below it and becomes more active. After certain period, the growth of these branches ceases and again, lateral branches are produced from them. The process is repeated and the type of branching is called Cymose branching. It may be of the following types.
- Uniparous cyme: When only one lateral branch develops in a cymose branching, it is known as uniparous cymose branching. The successive lateral branches may develop on the same side so that the branching appears like a helix, and this type of one-sided branching is known as helicoid cyme – example Saraca. When the successive lateral branches develop on the alternative sides so that, they appear to be arranged in a zigzag manner, the branching is known as scorpioid cyme – example, Cissus.
- Biparous cyme: When two lateral branches develop below the apical bud, it is known as biparous cyme or true cyme – example, Mirabilis jalapa etc.
- Multiparous cyme: When more than two lateral branches are developed below the apical bud, it is known as multiparous cyme – example Nerium.
Fig: Branching of stem
Dichotomous branching:
This type of branching is generally found in the lower group of plants. In this case, the terminal bud bifurcates and gives rise to two branches which develop in a forked manner – example, Marchantia, Psilotum etc.
Forms of stem:
Stems may be aerial, subaerial or underground. They may be erect or prostrate. In general, all these diverse forms of stem can be broadly grouped into two classes – normal and modified.
Normal stems:
- Erect stems: Some stems are strong and grow upright. They can hold the leaves well exposed to sunlight, by means of their lateral branches. Such stems are known as erect stems as in sunflower, mango etc. Some erect stems have distinct nodes and internodes, but the nodes are solid whereas, in some other cases, internodes are hollow. Such hollow erect stems are called culm, as in bamboo. In some other cases, the erect stem is unbranched, stout, and is marked with scars of fallen leaves. Such stem is called caudex, as in palm, coconut etc. In some monocots, the stem is condensed and underground, but at the time of flowering an erect unbranched aerial shoot develops bearing a single or cluster of flowers. Such a flowering shoot is known as scape, example, onion, Aloe indica etc.
- Weak stems: Stems which are unable to stand erect are called weak stems. They are of the following types:
Creeping stems: These stems creep over the surface of the soil. Roots are developed from the nodal regions of the stem. A creeping stem may be described as prostrate, when it lies flat on the ground as in Oxalis. When a creeping stem is lifted slightly after creeping for some distance, it is called a decumbent, as in Tridax. When the branches of a creeping stem spread in all directions, it is known to be diffuse as in Boerhaavia.
Fig: Creeper
Twiners: The stem is long and slender and it twines spirally around a support without any organ of attachment. The support may be a tree or some non-living object. Such stems are known as twiners, example Clitoria etc.
Fig: Twiners
Climbers: When a weak stem climbs a support with the help of some special organ of attachment like tendrils, hooks, or climbing roots, it is known as a climber. According to the nature of the organ of attachment, the climbers may be classified as tendril climber – example, Passiflora, Cucurbita, hook climber – example Bougainvillea or root climber – example, Pothos, betel etc.
Fig: Climber
- Underground stem: Stems growing under the soil surface are called underground stems. These are often confused with the roots. An underground stem has got some characteristic features by which it can be distinguished from a root.It has nodes and internodes. It bears scale leaves. Buds are developed in the axil of scale leaves. It has got no root cap and it possesses adventitious roots. Some underground stems are like the normal weak stems but others are modified to serve certain special functions.