The stem is the most prominent, ascending and generally aerial part of the plant which develops from the plumule and epicotyl.
The stem along with its branches and leaves etc. is called shoot system. Normally the stem bears and provides support to vegetative and reproductive shoots.
The stem and its branches arise from the buds. Cabbage is an example of the largest vegetative bud in the plant kingdom.
Characteristics of Stem:
1. Stem develops from the plumule and epicotyl of the embryo.
2. Generally, it is an aerial and ascending part of the plant axis.
3. A terminal bud is present at the apex of the main axis and lateral branches and is responsible for growth in length.
4. A stem is differentiated into nodes and alternating internodes.
5. Leaves emerge from the nodes of the axis and its branches.
6. The young stem is green and performs photosynthesis.
7. Multicellular hair may be present on the stem.
8. Branches of the stem and its leaves have exogenous origin.
9. Flowers and fruits develop on the stem of mature plants.
10. Generally, a stem is negatively geotropic and positively phototropic.
Functions of Stem:
1. Stem support and bears leaves, buds, flowers and fruits.
2. Conduct water and minerals from roots to rest of the plant.
3. Translocate organic food from leaves to the rest of the play.
4. The green stems take part in photosynthesis.
5. In many plants stem modified to perform functions like food storage, vegetative propagation and perennation i.e. to tide over the unfavorable periods.
Forms of Stems:
1. Aerial Stem:
Such a stem remains outside the surface of the soil.
2. Erect Stem:
It grows straight upright without any kind of support above the surface of the soil.
3. Weak Stem:
It is incapable of growing straight upright, and under natural conditions trail on the surface of the soil or climb with the help of some support.
These are weak stems which climb with the assistance of tendrils, hooks, spines, prickles, roots etc. e.g., pea, passion flower and vine etc.
Such weak stems ascend by coiling around some support e.g., Ipomoeo palmata.
They lie and grow prostrately on surface of the soil e.g., Portulaca, Evolvulus etc. The trailing stems maybe decumbent, i.e., when a trailing stem tends to rise at the apex, or diffuse, i.e., when it branches profusely and spreads out on the surface of the soil.
These stems creep on the ground by various means and accordingly may be runner, stolon, offset or sucker.
4. Reduced Stem:
Sometimes the stem is reduced to an exceedingly small remnant persisting at the upper end of the root e.g., in carrots, turnips, radishes etc.
5. Underground modified Stems:
Except when the roots or leaves are fleshy and especially modified for food storage the stem is usually the chief organ in which food is stored. Some stems are especially modified for the storage of food. These are usually underground stems such as the rhizomes of ginger, the tubers of potatoes, or the corns of aroids.