STEMS
If a structure has leaves and buds, it is called a stem. Underground stems have scale leaves.
Functions of stems:
- They conduct water with dissolved mineral salts from the roots to the leaves through xylem vessels.
- They conduct food manufactured by photosynthesis from the leaves to the storage organs like the roots through the phloem tubes.
- They expose leaves to the atmosphere for air and sunlight.
- They expose flowers to pollinating agents like insects and wind.
- They store food in some plants for example potatoes.
- They carry out vegetative reproduction.
- Green stems carry out photosynthesis.
Types of stems:
There are two major types of stems: aerial stems found above the ground and underground stems found below the ground.
Aerial stems:
Some aerial stems are erect i.e. stand straight while others are prostrate i.e. can not stand straight on their own.
Erect stems: These are divided into two:
- Woody stems which are supported by wood for example hibiscus.
- Herbaceous stems which are not supported by wood for example beans.
Prostrate stems: These are divided into three:
- Twining stems are stems which twine around other objects. Some twine using the whole stem round the support for example dutchman's pipe; while others twine using tendrils for example pumpkins, passion fruits.
Creeping stems are runners which creep over the ground for example sweet potatoes, wandering jew (Commelina bengalensis).
- Climbing stems are stems which have hooks that catch on to the support for example bougainvillea.
Underground stems:
Some underground stems may store food. These are the groups of underground stems:
- Rhizomes are underground stems which creep under the ground horizontally for example spear grass (Imperata cylindrica). Only the swollen rhizomes store food for example ginger.
- Tubers are enlarged underground stems with buds from which new plants will grow. Unlike rhizomes, they do not creep for example Irish potatoes, yams.
- Corms are swollen underground stems which do not creep and the buds are not found all over the stem as in tubers and the axes for the buds are vertical for example cocoyams. Corms have contractile roots.
- Bulbs are very short underground stems with fleshy overlapping leaves and short internodes for example onions.
Internal structure of a dicotyledonous stem:
The outer layer is called the epidermis. The inner cells form the cortex and the pith. Between the cortex and the pith are the vascular bundles.
The vascular bundles contain special cells for carrying food, water and mineral salts i.e. the xylem and the phloem.
The epidermis is impermeable to liquids and gases but gaseous exchange in the stem can take place through small gaps in the bark called lenticels.
The epidermis protects the inner cells from damage and bacterial invasion and also reduces water loss by evaporation.
The cortex and the pith help in packing and strengthening the stem.
In plants like the pawpaw, the pith is hollow.
The vascular bundles consist of xylem vessels which carry water with dissolved mineral salts from the roots through the stem to the veins in the leaves.
The vascular bundles also consists of the phloem tubes which are perforated living cells for carrying manufactured food from the leaves to other parts of the plant for example ripening fruits or storage organs like the roots.
Between the xylem and the phloem is the cambium which is also to be found between the cortex and the pith. The cambium consists of actively dividing cells. So the cambium carries out cell divisions to form the xylem on its inside and the phloem on its outside.
An internal structure of a monocotyledonous stem reveals that the vascular bundles are scattered all over the stem and that there is no pith.