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Stem is a part of the plant which lies above from surface of soil i.e. it shows negative geotropic growth. It has nodes and internodes. Branches, leaf, flower bud and bracts are developed from nodes. Stem arises from plumule.
FORMS OF STEM
- Caudex: It is unbranched, erect, cylindrical stout stem and marked with scars of fallen leaves. Crown of leaves is present at the top of plant. eg.: Palm
- Culm: Stem is joined with solid nodes & hollow internodes. eg. Bamboo (Graminae)
- Excurrent: The branches arise from the main stem in acropetal succession and the tree assumes a cone-like appearance e.g. Pinus, Eucalyptus, Casuarina, etc.
- Decurrent (Deliquescent): The lateral branches grow more vigorously and outcompetes the main trunk, giving a dome-shaped appearance, e.g., mango (Mangifera indica), shishem (Dalbergia sissoo) and banyan (Ficus bengalensis).
TYPES & MODIFICATION OF STEM
Aerial stems (Epiterranean stem) :
It may be reduced, erect and weak.
Reduced – Stem reduced to a disc. eg., Radish, Carrot, Turnip.
Erect stem – It is strong and upright e.g., maize, wheat, mango.
Weak stems – These are thin, soft and weak and need support. They can be upright or prostrate.
Creepers – The stem creeps on earth and the roots arise at the nodes, e.g., Grasses, Strawberry, Oxalis.
Traliers – The stem creeps on the ground, but the roots do not arise at the nodes.
They may be of two types :
- Prostrate or procumbent – The stem creeps on ground totally, e.g., Evolvulus, Tribulus.
- Decumbent – When prostrate stem projects its tip, e.g., Portulaca, Linderbergia.
Lianas (Stem climber). Woody perennial climbers found in tropical rain forests are lianas. They twin themselves around tall trees to secure sunlight, e.g., Hiptage, Bauhinia vahlii (Phanera).
Climbers – Plants are with long weak stem and have organs of attachment to climb the object. They may be of following type .
- Rootlet climbers – Roots produced at nodes help in climbing e.g., Tecoma, Pothos, Piper betal (pan).
- Hook climbers – In Bougainvillea, Duranta and Carrisa, the thron is modification of axillary vegetative bud which helps in climbing. In Bignonia, terminal leaflet is converted into hook.
- Tendril climbers – Tendrils are thread like structure which help the plants in climbing. Tendrils are modifications of :
- Entire leaf e.g. Lathyrus sativus.
- Leaflet e.g. Pisum sativum
- Petiole e.g. Clematis, Nepenthes.
- Stipule e.g. Smilex.
- Leaf apex e.g. Gloriosa
- Inflorescence e.g. Antigonon.
- Stem e.g., Vitis (grapevines), gourds, Passiflora (modified axillary bud).
Twiners – The stem body twines around the support without any special organ of attachment. e.g., Cuscuta, Dolichos.
Sub-aerial modification :
- Runner – When stem grows and spread on the surface of the soil. Roots are developed at lower side and leaves from upper side from node eg. Cynodon dactylon (Doob grass), Oxalis.
- Stolon – In it, branches are small and stem condensed and grow in all direction. After some time, of growing, their apical region comes out from the soil. eg. Fragaria (Wild strawberry), Jasmine Peppermint.
- Sucker – In it the main stem grows in the soil horizontally and branches develop obliquely from nodes above the soil, eg. Mint, Pineapple, Chrysanthemum.
- Offset – A lateral branch with short internode and each node bearing a rossette of leaves and tuft of roots at base. eg. Pistia, Eichhornea.
- These modifications are also involved in vegetative reproduction.
Underground modification :
This type of modification occurs generally for food storage and vegetative propagation.
- Tuber – The tips of underground branches become swollen in the soil. Eyes are found on then which are axillary buds and covered with scaly leaves. eg. Potato, Helianthus tuberosus
- Rhizome – It is fleshy and horizontally stems found below in soil. Small nodes and internodes are found which are covered by scaly leaves. eg. Ginger, Turmeric, Canna, Waterlily, Banana.
- Corm – It is condensed structure which grows vertically under the soil surface. They are having spherical node and internode eg. Colocasia, Alocasia, Zaminkand, Saffron, Gladiolus, Colchicum
- Bulb – This stem is reduced and has disc-like structure and surrounds with numerous fleshly scaly leaves. Many roots arise from its base. Food is stored in flashy leaves. They show apical growth eg. Onion, Garlic.
Special stem modification :
- Phylloclade – It is green photosynthetic flattened or rounded succulent stem with leaves either feebly developed or modified into spines e.g., Opuntia, Casuarina, Euphorbia, Cactus.
- Thorn – It is modification of axillary bud, e.g., Bougainvillea, Duranta, Carissa. Thorns of Alhagi possess flowers, while thorns of Duranta bears leaves.
- Cladode – Phylloclade usually having one or two internodes long & succulent is called cladode, e.g., Asparagus, Ruscus.
- Stem tendrill – it is a leafless, spirally coiled structure found in climbers. It may be a modification of Axillary bud, e.g. Passiflora or terminal bud e.g., Vitis.
- Bulbils – A condensed, axillary fleshy bud is called bulbils. It helps in vegetative reproduction. eg., Dioscorea, Globba, Agave, Oxalis.
FUNCTIONS OF STEM
The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It conducts water, minerals and photosynthates. Some stems perform the function of storage of food, support, Protection and of vegetative propagation.