The main functions of the stem are to support the leaves; to conduct water and minerals to the leaves, where they can be converted into functional products by photosynthesis; and to transport these products from the leaves to other parts of the plant, as well as the roots. The stem conducts water and nutrient minerals from their site of absorption in the roots to the leaves by means of convinced vascular tissues in the xylem. The movement of synthesized foods from the leaves to other plant organs occurs chiefly through other vascular tissues in the stem called phloem. Food and water are also often stored in the stem.
There are mainly two types of function of stem; those are normal and special function of stem. The normal function of stem is carried out by normal stems, while the special functions of stem are carried out by special stems. Both of them may be further classified as mechanical and physiological functions.
Normal function of stem
Mechanical function of stem :
The mechanical fubction of stem are mainly of two types. Those are as follows:
[j] The stem support the entire plant by bearing the entire weight of the shoot system.
[ii] The stem bears the leaves, flowers, fruits etc of the plant.
Physiological function of stem
The physical function of stem are of mainly three types, those are:
[i] Conduction function of stem :
The upward conduction of water and minerals from the roots to the leaves takes place through the xylem tissue of the stem, while the downward or lateral or upward translocation of prepared food (synthesized in the leaves) takes place through the phloem elements of the stem.
[ii] Storage function of stem:
Some parts of the stem, especially the pith and the parenchyma tissue of the stele help in the storage of food.
[iii] Food manufacturing function of stem :
The shoots of the herbaceous plants contain chlorophyll and help in the preparation of food by photosynthesis.
Special function of stem
[i] Support function of stem:
The terminal part of aerial branches like passion flower, grape-vine gives support to these climbing plants and help them in climbing up.
[ii] Self defense function of stem:
The stems of Duranta and Hygrophyla contain thorn, while that of rose contain prickles, which protect them from grazing animals and that is the self defense of the said plant.
i) Food storing function of stem:
The underground modified stems like rhizome of ginger, tuber of potato, bulb of onion store carbohydrate food in the form of starch for future use.
ii) Water storing function of stem:
There are some fleshy stems in cactus or succulents storing water for future use. This is because, they survive in xerophytic conditions and do not get adequate water in the soil for survival.
iii) Assimilation function of stem:
The aerial metamorphosed, fleshy stems of plants like Opuntia, Asparagus exhibit carbon assimilation by fixing CO2 from nature and prepare simple carbohydrate type of food by photosynthesis.
iv) Perennation function of stem:
The underground modified stems like tuber, bulb and corm serve as outstanding perehnating organs and help the plant to survive during unfavorable conditions.
v) Propagations function of stem:
The sub-aerial modified stems like runner in Oxalis, offset of Pistia, sucker of Chrysanthemum, stolon of Mentha (pippermint) help in vegetative propagation and also perennation.