Shoot System : Shoot system or stem is the part of the plant which grows above the soil.
Stem : Aerial axis of the plant, positively phototropic, bearing leaves, buds, flowers, fruits and branches, is called as stem.
Features of Stem
(i) Stem is usually an aerial part which grows vertically upwards, negatively geotropic and towards light (positively phototropic).
(ii) Apical bud maintains the vertical growth of the stem.
(iii) On the stem and nodes development of leaves takes place.
(iv) In the beginning the stem is green, but with the passage of time it becomes woody. Woody stems are brown in colour.
(v) A stem has nodes and internodes.
(vi) The shape of the stem is almost cylindrical.
Functions of Stem
1. Green stem is exposed to the sun for efficient photosynthesis.
2. Leaves, branches, flowers, fruits etc. are all attached with the stem.
3. Water and minerals are transported to the whole plant by stem.
4. Starch and water is stored by stems.
Types of Stem
Herbs : These are small plants without woody nature. They live only for a few years. Most of the vegetables are herbs.
Shrubs : They have big woody stems.
Trees : They have tall, large and woody stems with a distinct trunk.
Duration of Plants
Annuals : Plants which live only for one year, e.g. crop plants such as wheat, paddy etc. are called annuals.
Bienials : Plants which live for two years are known as bienials, e.g. carrot, beetroot etc.
Perennials : Trees which live for many years, e.g. Mango, Neem etc. are called parennials.
Ephemerals : Ephemerals are the herbs which survive only for few a weeks, e.g. Tribulus.
Acaulescent : In some herbs the stem remains below the soil and the leaves also develop beneath the soil. At the time of reproduction, flowers are developed aerially without aerial stems. These plants are called ‘acaulescent’.
Deliquescent trees : After producing some branches, the main stem stops its growth. Then the axillary buds produce many branches and the main stem is formed into an umbrella-like structure, e.g. Ficus.
Excurrent trees : The growth of the main stem is indefinite. All the branches formed from axillary buds are arranged acropetally to make the stem conical, e.g. Casuarina, Polyalthia longifolia.
Caudex : In caudex stem the main stem remains unbranched, woody, long and stout. e.g., Cocos, Borassus.
Culm : In some plants, the stems are differentiated into nodes and internodes. The stem looks jointed and this type of stem is called ‘culm’, e.g. Zea mays.
Pseudostem : In pseudostem the sheathing leaf bases overlap to form an aerial stem-like structure.
Prostrate Plants : In prostrate plants the long, slender stems and branches trail along the surface of the soil in various directions.
Decumbent plants : These plants are fixed at one place by the roots. The vegetative parts grow horizontally and reproductive organs grow vertically above the soil, e.g. Tridax procumbens.
Procumbent plants : In these plants, the stem parts having vegetative and reproductive parts, spread out on the ground, e.g. Tribulus terrestris.
Climbers : Weak stemmed plants grow vertically with the help of a support called climbers.
Twiners : In some plants the weak stem twines round the support and grows up. These stems are called twiners.
Tendrils : Some weak stemmed plants produce sensitive climbing organs, called ‘tendrils’. Tendrils coil round the support and help the plant in climbing, e.g. Passiflora, Posum.
Root Climbers : In root climbers, the weak stem produces climbing roots at nodes. With the help of these roots the plant climbs up, e.g. Piper betle.
Stragglers : The woody plants which climb up the support with the help of modified organs, like hooks and thorns, are called stragglers, e.g. Bougainvillea.
Lianes : The woody twiners are called lianes, e.g. Bauhinia.
Buds
A bud is a young shoot with a short axis covered by underdeveloped leaves.
Vegetative buds : The buds that arise from usual places of a plant are called vegetative buds.
Cotyledonry buds : Axillary buds are formed in the axils of cotyledons. They are called ‘cotyledonry buds’.
Collateral buds : The accessory buds are arranged side by side in the axil of a leaf, e.g., Melongena.
Serial buds : The serial buds are those which are arranged one above the other, e.g. Capparis and Aristolochia.
Adventitious buds : Buds developing from any part of the plants except in their normal positions are called adventitious buds.
Radical buds : Radical buds grow on the roots, e.g., Mullingtonia.
Cauline buds : The axillary buds present on woody stems are called cauline buds, e.g. Polyalthia, Artocarpus.
Epiphyllous buds : These are the buds growing on leaves, and are called phyll, e.g. Bryophyllum.
Floral buds : These buds always produce flowers and help in sexual reproduction.
Active buds : Active buds grow throughout the year.
Resting buds : They remain dormant during unfavourable weather conditions, e.g. Mangifera.
Branching
The branching is the mode of arrangement of branches on the stem.
Dichotomous branching : In this type the apical meristem of the stem is divided into two halves and each half produces a branch. In this way two identical branches are formed from the stem apex.
Lateral branching : In this, both the axillary and terminal buds of shoot system take part.
Racemose branching : In this the stem grows continuously and produces lateral branches from axillary buds in acropetal manner, e.g. Polyalthia.
Cymose branching : In this, the terminal bud of the main stem grows definitely as it is modified into a permanent structure, like a thorn, hook, tendril or inflorescence. The further growth of the plant is carried out by one of the many axillary buds which produce branches.
(i) Uniparous branching : This type of branching is found in plants showing alternate phyllotaxy and each leaf bears one axillary bud in its axil.
(ii) Biparous branching : It is found in plants showing opposite phyllotaxy.
(iii) Multiparous branching : This type of branching occurs in plants showing whorled phyllotaxy. In it, three or more branches are developed at each time from the axillary buds after the modification of apieal bud, e.g. croton.
(a) Helicoid branching : The successive lateral branches, grown from the axillary buds after the modification of terminal bud, lie on the same side.
(b) Scorpioid branching : The successive lateral branches, grown from the axillary buds after the modification of apical bud, lie on alternate sides in a zig-zag manner, e.g. cissus.
Stem Modifications
Underground Modifications
(a) Rhizome : It has nodes and internodes and grows horizontally below the soil. At the nodes, scale leaves are present. This remains alive even in the drought conditions, e.g. Ziviziber.
(b) Sobole : In sobole stem rhizome does not store food, and that is why the internode and branches remain tubular. This is found in grass plants.
(c) Root Stock : In root stock rhizome grows vertically below the soil.
(d) Corm : This stem type of grows vertically in the soil. It stores food material which is synthesized in aerial branches. It develops nodes and internodes, e.g. colocasia.
Stem Tuber : In tubers, branches grow from the lower part of the stem and goes into the soil. The underground branches store food and become thick, e.g. Solanum tuberosum.
Bulb : The bulb has disc shaped structure and does not store food. The disc bears many adventitious roots on its lower side and leaves develop on the upper side of the disc and grow above the soil. The leaf store food and water, and become thick. These bulbs are of two types;—tunicated bulb and scale bulb, e.g. Allium cepa and Lilium condidum.
Sub-aerial Stem Modifications
(1) Runners : Runners creep on the soil and are rooted at every node. When the internodes break off, the nodes lead an independent life.These weak stemmed plants are called runners. They help in vegetative propagation, e.g. Hydrocotyle vulgaris and Oxalis.
(2) Stolons : In stolons the stem grows downwards. When these branches touch the soil they produce adventitious roots. These branches are called ‘stolons’, e.g. Nerium and Jasminium.
Suckers : In suckers underground branches grow upward from the axillary buds of nodes present below the soil. These branches produce many adventitious roots from their ventral surfaces. The underground branches are called ‘suckers’. When separated, they develop into new plants, e.g. Chrysanthemum and Mentha.
Offsets : In offsets the stem is produced a disc like structure. Many leaves are developed from this stem in rosette manner. The axillary buds of the leaves develop into short, slender branches of one internodal length and grow horizontally above the water. These branches are called ‘offsets’, e.g. Pistia and Eichhornia.
Aerial Stem Modifications
(1) Tendrils : Tenderils are wiry, coiled, sensitive and delicate organs, and useful for climbing, e.g. In Passiflora the axillary bud develops into a tendril.
(2) Thorns : Thorns are hard, pointed structures meant for the protection of plant. They are provided with vascular tissues. They are formed either in the axil of a leaf or at the apex of a branch and control transpiration by reducing the vegetative growth, e.g: Bougainvillea, Duranta.
(3) Hooks : These are curved and woody structures developed either from axillary buds or terminal buds, e.g. Artabotrys, which helps in climbing.
(4) Phylloclades : Phylloclades are branches modified into green, leaf like structures to perform the function of photosynthesis, e.g. Opuntia.
(5) Bulbils : In some stems vegetative and floral buds are modified into condensed branches and store food materials. These modified buds are called ‘bulbils’, e.g. Diascorea.
(6) Flowers : The flowers are aerial stems modified for sexual reproduction.
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