Stems Of Tropical African Plants

WHEN a seed starts to grow into the young plant two parts appear-the root and the shoot. The shoot comes up above the ground, grows towards the light and consists of stem and tiny leaves. In this chapter we shall see what the stem is like and what it does for the plant. The stem is the part of the plant between the root and the leaves; it bears the leaves on it and holds them up to where they can get plenty of air and sunlight.

The food material taken in by the root is manufactured into plant food in the leaves; this material soaks up through very small tubes or pipes in the stem; these tubes can be seen if the stem is cut across. This, then, is the work that the stem does, but stems are by no means all alike. Some are always soft and green, while others become woody as in trees. "Herbaceous stems" is the name given to soft green stems. Stems may be of many shapes: round, as those of most grasses and trees; square, as those of the granadilla or of dead-nettle, a common herb on abandoned farms; or ribbed, as those of the balsams Some of the stems are strong and stand erect' but many are not strong enough to stand alone, and these either climb up some other plant or creep along the ground.

Climbing plants climb in several ways; they may twist themselves round a stick or another plant, as the yam does, or the morning-glory; or the plant may have some special climbing growths called tendrils, which stick to or twist round any support they can find, as in Antigonon, passion-lower, cucumber, gourd etc. Creeping plants neither climb nor can they stand erect, but have to creep along the ground, as sweet potato, bindweed, etc. Some plants do this from choice, such as those growing on exposed places or on very loose soil. Woody stems are found in all trees and most shrubs; the wood is formed in the stem to strengthen it.

Woody stems are not all alike, and the stems of the mango, the iroko, or the cotton, are different from those of the palms or bamboos. If you the end of a cut stem of, say, a teak tree, you will see a number of circular rings; if you will count the number of rings this number will be the age of the tree; for the tree grows by adding one of these rings each year. Trees which shed their leaves for part of the year show these annual rings, as they are called, best. The trees with stems like a palm (i.e., the same thickness all the way up) never have these annual rings.

So far all the stems we have considered grow above the ground; but there are quite a number of stems of very important plants which grow underground. These, although at first sight they may look like roots, must never be mistaken for roots; for they bear buds and often leaves, which a root never does. Underground stems are useful to the plant for the storing of food or for the producing of young.

Yams and coco-yams are underground stems that are swollen with stored food; such stems are called tubers. On the tubers are to be seen numbers of small buds, which will grow into new plants the following season, making use at first of the stored food. In some cases the bases of the leaves get swollen and closely surround a short swollen stem. What is called a bulb is formed. The onion is a typical example of a bulb. if an onion is cut through lengthways in hail; the stem, the swollen leaves, and probably one or two buds will be seen, and also the flower of the plant. It is for the use of the food found in these underground swollen stems that we cultivate a great many of our best-known vegetables. Another type of underground stem is found in the caladium, the cobra-lily, and the gloriosa; this type looks like a bulb in shape, but if you cut it down the centre you will find that it is not composed of swollen leaves but of one solid piece of stem. In this case the new plant grows from the top of the corm, as the swollen stem is called.

In some cases the stem grows horizontally just below the surface of the ground, and gives off branches which grow up to the surface. These stems grow zigzag for many feet in all directions; they are swollen and contain plant food so that if they get broken the loose portion can form into a new plant entirely. At the nodes where the branches are given off there are generally a number of roots. Such stems are called rhizomes, and common examples of these are cannas, ginger, plantains, ekong grass, ordinary river sedges, etc.