CHAPTER XII.

STEMS

Examine the stem of a sunflower; it is tall and straight and grows upright in the air, bearing leaves which stand out from it.

In a young holly, and many other plants, we find growing out from the central stem smaller side branches which bear the leaves. As we have found already (Chapter VI.), the leaves are the active parts of the plant and do the food-building, so that the stem is chiefly useful as a support, which keeps them in a good position as regards the light and air. In general, we do not see much of the stem because it is largely hidden by the covering of leaves, so that if you want to study stems you should go to the woods in the winter when there are no leaves on the trees, and you can see the form of the branches themselves.

In big trees, such as the oak and beech, the stems are very important, and the chief stem or ~trunk~ becomes very thick as it gets old. It is made of hard wood which is tough and strong, for such high trees have to bear great strain from the winds, as well as the weight of all the leaves. If you go into the woods when it is very windy, and watch the thick wooden boughs swaying, boughs which you could not move, you will see how much force the wind may sometimes have. The branches need all their strength in the summer to support the curtain of leaves which catches the wind. In a big tree we find the few chief branches thick and strong, but there are many hundred smaller ones, some of them dividing to quite delicate branchlets which bear the leaves, so that the whole tree body is very much complicated (~see~ fig. 43).

Each kind of tree has a way of branching which is characteristic of its species, so that even without leaves or flowers a woodman can tell what a tree is. This one can learn by practice in the woods, but to begin with it is rather difficult. Without going into detail, however, we may notice great family differences, such as exist between a larch or a Christmas-tree and an oak. In the first two there is one straight main trunk, with side branches at very regular intervals (~see~ fig. 44), and in the oak the main thick trunk soon bears several large branches nearly equalling the main stem; these divide again and again in a rather irregular fashion (~see~ fig. 43).

In many of the smaller plants the stems are not strong enough to stand up against the wind, and they simply lie along the ground or support themselves by growing among other plants, such, for example, as the common Stellaria, where the stem is very delicate indeed (~see~ fig. 45). Then again, if you pull up a large water-lily, you will notice how soft and limp the long leaf-stalks are. They cannot support themselves at all in the air, though they were upright in the water. This is because the stalks get their support from the water which allows them to float up, so that the plant does not build a strong stem. You will find that plants are very economical in their use of strengthening material, and never waste it where it is not wanted. If you remember this, and then study all the stems you can, and note when and where they are strengthened, you will find what good and economical architects plants are.

As well as supporting the leaves, the stems have another very important duty, something like that of the roots. Just as the roots absorb the water from the soil and carry it up, passing it on to the stems, so the stems carry it on to the place where it is finally used, that is, to the leaves. In both stems and roots there are channels or “water-pipes” which carry water about, as well as other special “pipes” which carry the manufactured food.

So that =the two chief duties of stems= are to act as supports for the leaves and flowers, and to carry the food materials and water between the roots and leaves.

Just as we found in the case of the roots, =there are many extra duties which the stems may take over=, and as a result, we find great variety in the appearance of stems. For example, in some plants the stem does not grow up into the air at all, but ~creeps along just below the surface of the ground~. This you may see if you dig up a Solomon’s Seal or an iris, when you will find that the stem looks very like a thick root running horizontally in the ground. That it is really a stem you can tell from the fact that the leaves grow out from it, and you can see the scars of old ones as well as the present leaves, and also some little brown scaly leaves, and a large number of adventitious roots. The stem is rather swollen with food materials which are stored up in it, and it is not coloured green like many of those growing in the air. Such a stem, creeping under the earth, and only sending its green leaves into the air, has a special name, and is called a ~Rhizome~. Many plants have such stems, particularly ferns, as you can see very well if you dig up a bracken.

~Some of the underground stems which store food are still more modified~, so that it is very hard indeed to tell what they really are. This is the case in the potato, which you would naturally think at first is a swollen root, like those we saw in the dahlia (fig. 39). That it is really a stem you can see by examining the “eyes” carefully. The eyes (~see~ fig. 47) are buds with scale leaves round them, and at the tip of the potato we can see several such buds together (fig. 47 ~t~). The whole potato is a very much swollen stem which is packed with food and has all its other parts so reduced that it is difficult to recognise them. Such special stems are called ~Tubers~.

Certain stems take on the work of leaves, and sometimes they are so much modified for this that the plant has no true leaves at all. This is what has happened in the case of a cactus. If you can get a cactus, examine it carefully and you will see that the whole plant consists of a thick mass of green tissue, which apparently is not divided into stem and leaves. But the truth is that the whole of the thick mass of tissue is the stem, and the little tufts of spines and hairs are really reduced leaves. So that in the cactus the ~green stem does all the food building work instead of the leaves~.

In some plants this is not so much marked, even though the stem does some of the work for the leaves. In such cases the stem is generally green and broad or winged and the leaves small, as in our common broom and the whortleberry, where the leaves very soon drop off. Quite a number of plants have stems which do this, and it is sometimes a great advantage to the plant, for the big leaves are often very wasteful of water, as you will see in Chapter XVIII.

In other cases we find that the side branches of stems may be modified to protect the plant, and so take on the form of strong spines or thorns, as in our blackthorn, where the sharp pointed ~spines~ are ~modified side shoots~.

There are many other pieces of work which stems may do; we must just mention the climbing and twining stems, where the stem is delicate and requires to be supported, which we are going to examine more carefully in Chapter XIX.

Sometimes, instead of continuing to grow into the air, the stem may bend over into the earth again, as often happens in big bushes of bramble (~see~ fig. 49), and then from the tip of the stem a number of adventitious roots (~see~ p. 56) grow out and hold it firmly in the ground. If, then, this branch gets separated from the rest of the plant, it can build a complete new individual.

In the case of the bramble notice how the leaves get smaller and smaller towards the tip of this branch as it bends down to the earth, and of course, they do not develop at all as true leaves under the soil (~see~ fig. 49).

From these examples, and the many others you should be able to find for yourselves, you see that stems may take on other duties beyond their two chief ones, but that, however much they change their form and appearance, we can always find out that they are really stems by studying them with a little care.