Morphology Angiosperm- Botany
Morphology of Angiosperm
The stem develop from the plumule of the germinating seed. Normally it is the aerial part of the plant body. The stem with it's branches, leaves, buds, flower and appandages is known as shoot system.
The stem shows the differentiation of nodes and internodes. The place where the leaf develops on the stem is called the node. The portion of the stem between two successive nodes is called the internode.
- Stem is an ascending axis of the plant and develops from the plumule and epicotyl of the embryo.
- It is generally erect and grows away from the soil towards light. Therefore, it is negatively geotropic and positively phototropic.
- The growing apex of stem bears a terminal bud for growth in length.
- 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.
- The lateral organs of stem (i.e., leaves and branches) are exogenous in origin (from cortical region).
- The young stem is green and photosynthetic.
- Hair, if present, are generally multicellular.
- In mature plants, stem and its branches bear flowers and fruits.
- 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.
- Erect stems : Majority of angiosperms possess upright, growing-ascending, vertically-erect stems. They are fixed in the soil with the help of roots. Erect stems belong to four categories :
- Clum : Erect stems with solid nodes and hollow internodes. The nodes are swollen giving the stem a jointed appearance e.g., Bamboo (Bambusa arundinacea)and wheat (Triticum vulgare).
- Caudex : The main stem remains unbranched and bears a crown of leaves at its top. e.g., Coconut (Cocos nucifera), Palm, etc.
- Excurrent : The main stem is trunk like. It is thickest at the base and gradually tapers towards the apex. The branches arise in acropetal succession, i.e., oldest at the base and youngest at the apex. The tree appears cone-shaped. e.g., Casuarina, Eucalyptus, etc.
- Decurrent or Deliquescent : The apical bud of main stem is weak as compared to the buds of lateral branches. Thus, the lateral branches are prominent and spreading. The main stem grows upto a certain height after which it gives several branches. These branches dominate by giving the branches of several orders. The whole tree looks like dome-shaped. e.g., Banyan (Ficus bengalensis).
- 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.
- 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 :
- Tendril climbers : Tendrils are thread like 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).
- 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.
- 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).
- 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.
- Prostrate weak stem
- Trailers : The stem creep on the ground but roots do not arise at the nodes. They are of three types :
- Creepers : These weak-stemmed plants grow prostrate and develop adventitious roots from their nodes. Creepers are of three kinds – runners, stolons and offsets.
Procumbent : The stem creeps on the ground totally e.g., Tribulus, Bassela, Evolvulus.
Decumbent : Branches, after growing horizontally for some length, grow vertically upwards, e.g., Portulaca, Tridax, Lindenbergia, etc.
Diffuse : Branches grow profusely in all directions, e.g., Boerhaavia.
Runners : This prostrate aerial stem has a long internode and creeps horizontally. Axillary buds arise from nodes to form aerial shoots and roots. Several small (daughter) plants are thus linked by runner which may break off later. e.g., Cynodon (doob grass) and Oxalis.
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).
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.
In angiosperms, always the branches are produced by the growth axillary buds or lateral buds. This type of branching is known as lateral branching. The lateral branching is classified into two kinds racemose and cymose.
- Racemose branching : In this type of branching, the terminal (or apical) bud of the main stem grows indefinitely and the axillary buds grow out into lateral branches in acropetal succession.
This type branching is also called monopodial branching. Due to monopodial branching the shoot system of plant appears conical e.g., Eucalyptus, Polyalthia (Ashoka tree).
- Cymose branching : In cymose branching the terminal bud is active for a short period and becomes modified into some permanent structure like tendrils, thorns of flowers. Due to the terminal bud modification the growth of the main stem is definite. Further growth in the plant is carried by one or more axillary buds. Cymose branching may be of three types :
- Uniparous or Monochasial type : In uniparous type of branching only one lateral branch is produced at each time below the modified terminal bud. Here the successive lateral branches that are formed unite to form a stem. Such a stem is called false axis or sympodium. The uniparous branching is of two kinds, helicoid and scorpoid.
- Biparous or Dichasial type : When the activity of terminal bud stops, further growth of plant takes place by two lateral branches, e.g., Viscum (Mistletoe), Silene, Stellaria, Mirabilis jalapa (Four O’ clock), Dianthus (Pink), Carissa carandas (Karonda), etc.
- Multiparous or Polychasial type : When the activity of terminal bud stops, further growth of plant takes place by a whorl of three or more axillary branches. The axis is said to be multipodial, e.g. Euphorbia tirucalli, Croton, Nerium odoratum (Oleander).
Helicoid branching : If the successive lateral branches develop on one side it is called helicoid branching. e.g., Saraca, Canna and Terminalia.
Scorpioid branching : If the successive lateral branches develop on either side alternately, it is called scorpioid branching, e.g., Cissus, Gossypium and Carissa.
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