11th Class Biology Morphology of Flowering Plants (Root, Stem And Leaf) The Stem

The stem develops from the plumule of the germinating seed. Normally it is the aerial part of the plant body.

Characteristics of stem

(1) Stem is an ascending axis of the plant and develops from the plumule and epicotyl of the embryo.

(2) It is generally erect and grows away from the soil towards light. Therefore, it is negatively geotropic and positively phototropic.

(3) The growing apex of stem bears a terminal bud for growth in length.

(4) In flowering plants, stem is differentiated into nodes and internodes. A node occurs where leaves are attached to the stem. Internode is the portion of stem between the two nodes.

(5) The lateral organs of stem (i.e., leaves and branches) are exogenous in origin (from cortical region).

(6) The young stem is green and photosynthetic.

(7) Hair, if present, are generally multicellular.

(8) In mature plants, stem and its branches bear flowers and fruits.

Diverse forms of stem

(1) Reduced stems : In some plants, the stem is in the form of a reduced small disc which is not differentiated into nodes and internodes. e.g., (a) A reduced green-coloured disc-like stem lies just above the base of fleshy roots of Radish, Carrot and Turnip ; (b) Green-coloured small discoid stem occurs in free-floating Lemna, Spirodela and Wolffia; (c) Highly reduced non-green discoid stem occurs at the base of Onion and Garlic bulbs, etc.

(2) Erect stems : Majority of angiosperms possess upright, growing-ascending, vertically-erect stems. They are fixed in the soil with the help of roots.

(3) Weak stems : They are thin, soft and delicate which are unable to remains upright without any external support. They are of two types : upright weak stems and prostrate weak stems.

(i) Upright weak stem

Twiners : The stems are long, slender, flexible and very sensitive. They twin or coil around an upright support on coming in its contact due to a special type of growth movement called nutation. They may coil the support to the right (anticlockwise from the top or sinistrorse) e.g., Convolvulus sp., Ipomoea quamoclit Clitoria ternatea, etc. or to the left (clockwise or dextrorse), e.g., Lablab.

Climbers : The stem is weak and unable to coil around a support. They usually climb up the support with the help of some clasping or clinging structure. They are of four types :

(a) Tendril climbers : Tendrils are thread like green structure which help in climbing the plants. They may be modified stem (e.g., Vitis), stem branches (e.g., Passiflora) and inflorescence (e.g., Antigonon).

(b) Root climbers : Adventitious roots arise from the nodes and penetrate into the upright support so that the climber climbs up, e.g., Betel vine (Piper betel), Tecoma, Ivy, etc.

(c) Scramblers or Hook climbers : These weak stemmed plants slowly grow over other bushes and rest there. They attain this position with the help of curved prickles (e.g., Rose), curved hooks on flowering peduncle (e.g., Artabotrys), prickles on stem (e.g., Lantana), spines (e.g., Climbing Asparagus) or spinous stipules (e.g., Zizyphus).

(d) Lianas : These are woody perennial climbers found in deep forests. At first, they are just like ordinary twiners but once they reach to the top and get sunlight, become woody. e.g., Tinospora, Ficus, Bauhinia, Bignonia, etc.

(ii) Prostrate weak stem

Trailers : The stem creep on the ground but roots do not arise at the nodes. They are of three types :

(a) Procumbent : The stem creeps on the ground totally e.g., Tribulus, Basella, Evolvulus.

(b) Decumbent : Branches, after growing horizontally for some length, grow vertically upwards, e.g., Portulaca, Tridax, Lindenbergia, etc.

(c) Diffuse : Branches grow profusely in all directions, e.g., Boerhaavia.

Creepers : These weak-stemmed plants grow prostrate and develop adventitious roots from their nodes. Creepers are of three kinds :

(a) Runners : This prostrate aerial stem has a long internode and creeps horizontally. Axillary buds arise from nodes to form aerial shoots and roots. e.g., Cynodon (doob grass) and Oxalis.

(b) Stolons : They are special kinds of runners which initially grow upwards like ordinary branches and then arch down to develop new daughter plants on coming in contact with the soil. e.g., Strawberry (Fragaria vesica), Peppermint (Mentha piperita), Jasminum (Jasmine).

(c) Offsets : They are weak, elongated, horizontal branch of one internode that arises in the axil of a leaf. At the tip, it produces cluster of leaves above and tuft of roots below. The offset may break off from the parent plant and act as individual plants. They are found usually in aquatic plants and rarely is terrestrial plants. They are helpful for vegetative propagation. e.g., Eichhornia (water hyacinth), Agave, Pistia.