MORPHOLOGY OF STEM

THE STEM

The ascending part of axis bearing branches, leaves, flowers and fruits is called stem. It develops from Plumule of the embryo.

Stem bears nodes and internodes. The regions of stem where leaves are born are called nodes and portion between two nodes are called internodes.

The main function of stem is spreading lateral organs like branches, leaves, (exogenous in origin) flowers and fruits. It also conducts water and minerals and products of photosynthesis from roots to leaves.

Some stem perform special functions like storage of food, support, protection and vegetative propagation.

BRANCHING OF THE STEM;- in some plants stem remains unbranched called caudex or columnar stems (coconut, maize, sugar cane) in angiosperms the branching of stem is always lateral. The lateral branching can be;-

Racemose brancing (monopodial); the terminal bud grows indefinitely and branching occurs in acropetal succession (youngest at apex and oldest at base) e.g eucalyps.

Cymose branching: when the apical bud stops its growth or gets modified into flower, tendril etc and further growth occurs by branching. If growth occurs by only a single branch (sympodial branch) it is called monochasial. The sympodial branches may arise on both the sides of main axis (scorpoid) like in grape vine or on one side (helicoid) like in saraca indica.

If the further growth of plant takes place by two branches it is callked dichaesial branching like in four o clock plant(mirabilis jalapa), viscum, silene etc  and if it takes place by more than two branches it is called polychasial branching like in euphorbia, croton, nerium odoratum etc and the axis is said to be multipodial.

Types or forms of stems;-

1) Reduced stems:- when the stem is reduced and not differentiated into nodes and internodes like in wollfia, onion, garlic etc it is called reduced stem.

2) Erect stems:- where the stems are upright and vertically erect. If the nodes are swollen and give a jointed appearance to stem than it is called culm stem like in bamboo. If stem is thick at base and tapering towards apex with branches in acropetal succession it is called excurrent like in cedrus deodara, pinus spp. If the main stem stops growth after some height and further growth occurs due to the activity of branches making a dome like appearance it is called decurrent stem like in banyan tree, walnut etc.

3) weak stems;- these are thin soft and delicate stems. These can be upright or prostrate.

The upright stems may coil around a support (due to a special type of growth called nutation) are called twiners like in clitoria or lab lab.

Some weak upright stems climb over a support called climbers with the help of tendrils (vitis,smilex, clematis) adventitious roots (piper betel), curved pricles or hooks (rose).

The prostrate stems spread over the ground. Such stems may trail along the surface without rooting at intervals called trailers or stragglers like in tridex bessela. Some prostrate stems root at nodes and creep along the surface called creepers.

Many creepers develop adventitious roots downward into soil  after some elongation and an aerial branch upright, such creepers are called runners. The elongation of internodes in runners occur in all directions like in dramun (cynodon dactylon) oxalis etc.

In strawberry fragaria vesica, mentha the stem initially grows upward then arches down to develop new plant, such creeping stems are called stolons. In some plants like water hyacinth the stems creep only one inter-node and develop crown of adventitious roots bellow and a tuft of leaves above, such creeping stems are called offsets.

MODIFICATIONS OF STEM;-

underground stem modifications;-

In some plants the main stem or its branches lie below the soil and are called underground stems. these stems are non green and resemble roots. The underground stems are of the following types

Sucker;- in some plants like menthe, chrysanthemum  a branch of stem grows obliquely below the surface of soil and emerges out in the form of new shoot.

Rhizome;- (figA-B) in some perennial plants like ginger, nelumbo nucifera, alocasia indica the stem remains underground, stores food, and swells. It develops roots below and reduced shoots above it. These bear nodes and internodes and are non-photosynthetic.

Corms;- fig ( A-E) in some plants like saffron (crocus sativus), amorphophallus the stem is underground, spherical, swollen with a large apical bud over and small adventitious roots bellow. The parent corms develop daughter corms which develop into new shoots.

Tubers;- in some plants like potato the stems branch underground and each branch swells at its tip which forms a tuber. In tubers the nodes are represented by small depressions called eyes. The eyes are more crowded at apical end (rose end) and less at distal or stolen end (heel end). Tubers store food in the form of starch.

Bulb;- in some plants like onion garlic the stem is much reduced in the form of a small disc with several fleshy scales enclosing a terminal bud.

AERIAL MODIFIED STEMS;-

Stem tendrils;- in some plants like cucurbita, antigonon, passiflora the stems are modified into long thin and spirally coiled climbing organs called tendrils.

Stem thorns;- in some plants like citrus, pomegranate the stem is modified into stif, pointed and sharp structures called thorns. Thorns are protective in function.

Phylloclades;- in some plants like opuntia, euphorbia royleana the stem gets modified into flattened or cylindrical, green leaf like structures called phylloclades. The leaves in such plants are modified into spines.

Cladode;- in some plants like asparagus, ruscus the stems and branches of unlimited growth get modified into small one internode long sylendrical (asparagus) or flat (ruscus) leaf like structures called cladode. These are born in clusters.