The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
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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.
Fig. 46. Underground stem of the Solomon’s Seal, called a Rhizome. It has many scaly leaves, s and a shoot A which will come out into the air bearing green leaves. B is the scar left by the similar shoot of last year. r are the adventitious roots which come out all over the stem.
Fig. 47. A Potato: s, the stem attaching it to the main stem; e, scale-leaf; b, bud in its axil; t, tip of the Potato with several buds, some of which are sprouting.
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.
Fig. 48. Cactus plant, showing its fleshy stem, which is green, and does the food building for the plant. The tufts of spines and hairs represent 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.
Fig. 49. Leafy branch of Bramble which has bent into the earth and given rise to a cluster of adventitious roots at the tip; l, level of soil; P, point where the branch was cut from the parent plant.
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.
LEAVES
The late spring and summer are the best times to study leaves, for, as you must have noticed, the woods begin to lose their green in the autumn, and the leaves have fallen in the winter. This tells us that the fresh greenness of the leaves (which you know is so important for the plant) does not last very long, and when they are no longer green the leaves are useless and drop away. As you know, the chief work of leaves is to build starchy food, for which they require their green colour.
Fig. 50. Simple leaf of the Cherry.
When you go into the woods or gardens to study the leaves, first look at single ones, collecting as many kinds as you can. Though their shape varies very much, you will find that in almost all cases they are green, expanded, and flat. Let us first examine a single simple leaf, like that of a cherry. You will see that the expanded part (called the leaf blade or lamina) narrows down to a small stalk, which connects the blade with the stem from which the leaf is growing; this stalk is called the leaf stalk or petiole. Then at the base of it, just where it joins on to the stem, there are two little leaf-like structures which are not true leaves, but which belong to the leaf and are called stipules; they are attached to the base of the petiole, which spreads out to clasp the stem, and is called the leaf base (see fig. 50). Such a leaf shows us all the parts of a simple leaf; but some leaves have no stalks, others no stipules, and so on.
Fig. 51. Compound leaf of the Rose.
Let us compare a rose leaf with the simple leaf of a cherry, oak, or beech. In the rose you will find five or seven small leaflets arranged on a single main stalk, and each of these leaflets separately is very much like a single leaf of the beech. Such a leaf as this we call compound, for it is divided up into several parts, each of which looks like a whole leaf (see fig. 51).
Leaves are of very many different kinds and shapes, and special names have been given to each kind, which you can look up in a book if you want to classify them.
Fig. 52. Peltate leaves of Nasturtium, showing the stalk attached in the middle of the lamina.
Let us just notice a few of the types. The cherry, beech, and others which are simple with slightly pointed ends, we may call by the proper term ovate. Then there are leaves like those of the nasturtium, where the leaf blade is circular and the leaf stalk does not come in at the base of the leaf, but is attached to the middle of it; such leaves as that are called peltate.
The broad or rounded leaves, which spread out like the palm of a hand, such as the ivy (see fig. 26), are called cordate or lobed, and when compound, as are those of the horse chestnut, palmate.
Fig. 53. Needle leaves of Pine growing in pairs.
All the grasses and the many plants belonging to their family have very long, narrow leaves, which we call linear, while those of the pine trees are sharp and pointed, and are called needle leaves.
Fig. 54. Seedling of Rose; (c) cotyledons; (a) next leaf, simple, but toothed; (b) next older leaf divided into three leaflets.
As we noticed in comparing the leaves of the rose and cherry, some plants have very much more complicated leaves than others. Now such complicated structures do not develop on a plant all at once, as you can see if you examine a very young rose seedling. The first pair of leaves or cotyledons do not remain inside the seed as they do in the bean, but grow outside into the air and become green; they are quite simple leaves with smooth edges. The next leaf which unfolds is also simple, but it has a deeply toothed edge (see fig. 54), while the leaf following that is a compound leaf, divided into three leaflets. The other leaves gradually get five and then seven leaflets as the seedling grows up.
This is just one example of what usually happens in the history of plants with compound leaves, or leaves with any special shape; the young seedling’s earlier leaves are much more simple than the later ones. You should collect as many seedlings as possible and make drawings of them if you can, to show the various stages leaves pass through before reaching the full-grown complex form.
Fig. 55. Skeleton of a leaf, showing the fine network of the small veins.
Now let us look again at the actual structure of leaves. Hold up those of the rose, or lilac, or lime tree to the light, and look at the “veins” running in them. There is a chief central vein or mid-rib, and from it a number of side branches come off and divide and branch again and again till they form a fine net-work throughout the whole of the leaf blade (see fig. 55). If you now look at a grass
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