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.
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.
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.
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.
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.
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 or lily leaf, you will find that there are very many veins about equally important, running from end to end of the leaf and remaining nearly parallel to each other. This difference between parallel veins and net-work (or reticulate) veins is quite important, and is one of the characters which help to separate two very big families of flowering plants (see Chapter XXIII).
Now let us see in what way the leaves are arranged on the stem. If you pick a branch of dead nettle you will see that the leaves are attached by their stalks to the stem in pairs, two leaves coming off from the same level at opposite sides of the stem (fig. 56); while fig. 57 shows that the leaves of honey-suckle really do the same thing, only they grow out directly from the stem as they have no leaf stalk. Now look once more at the leaves of the dead nettle, choose one particular pair to start with, and then look how the pair above it are placed. You will see that they do not lie directly above the pair you chose, but are arranged on the opposite sides of the stem, so that the two pairs alternate. If then you look at the pair next above them, you will see that they are arranged in just the same way as the first pair, and so alternate with the second. In this way every pair of leaves on the stem alternates with the pair above and below it. Now examine a pear or cherry twig, and you will see that the leaves are arranged singly on the stem. Fasten a piece of thread to the stalk of one leaf and twist it round the base of the next, then on to the next above and so on. You will find that the thread makes a spiral round the stem, and finally comes to a leaf higher up it, which lies exactly above the one you started from. Very many plants have their leaves arranged like this in a spiral on the stem with the youngest at the top. There are different kinds of spirals for the arrangement of leaves in the different plants. You can see this by making the spiral of thread and counting how many leaves you pass on your way up the stem till you reach the leaf which lies just immediately above the one you start from.
Fig. 58. Branch of Cherry (leaves cut off to make it clearer), with a string twisted from leaf stalk to leaf stalk, showing the spiral arrangement. Note that leaf 5 is the first to come immediately above the one you started from.
Sometimes the leaves are arranged in a circle all round the stem at the same level; this is the case in the horsetail (see fig. 59), and such an arrangement is called a whorl, but it is not very common in plants.
In the goose grass the leaves look very much as though they were really in a whorl (see fig. 60), but there are only two true leaves; the others are the stipules, which are so much like the leaves that it is very difficult to tell them apart.
As we found out already, leaves require light and air, and usually arrange themselves so as to get them; hence, in a general way, we may observe that the leaves all grow to face the light. If you go under a beech tree, for example, and look up, you will find that you can see nearly all the big branches on the inside, while the leaves form a covering or dome on the outside. Special cases of leaves so arranged as to get a good light we noticed before (see pp. 36 and 37).
As well as their own particular work, leaves may take on extra and different work, so becoming modified to suit their different occupations, and unlike true leaves. We already noticed in the cactus (see fig. 48) that the leaves become like sharp spines which protect the fleshy stem, and can do none of the usual work of leaves, because they have lost their green colour.
In some plants leaves, or parts of leaves, may change into fine tendrils which become very sensitive to touch, and can twine round supports and cling to them, and so help the plant to climb. Such tendrils we saw (fig. 31) move very quickly; they are quite different in their structure from ordinary leaves. This happens in many plants, and you may see it very well in the sweet pea (see fig. 61), where only two leaflets of the compound leaf remain leaf-like, the others having been changed into tendrils.
When we come to look at Flowers, with all their special shapes and brightly coloured parts, we are really looking at modified leaves. But they are so very much modified that we have come to consider flowers as things by themselves, and so we will study them later on.
Some plants which do not have true flowers, yet have leaves of two kinds. For example, the “flowering fern” has the usual green leaves and others which form rather brownish golden spikes, and which are covered with spore[6] cases. Then again, some leaves are very specially modified and are changed from the usual structure in order to act as traps for insects (see Chap. XXI.).
Other leaves, instead of being very much developed, or specially developed along some line, are simply reduced, that is, are very little developed indeed. For example, as you saw in the under-ground part of the potato and many rhizomes growing horizontally, the leaves never become large and green, but remain as simple brown scales. Some scale leaves have quite a special work to do in the way of protecting the very young green leaves while they are in the buds, and we will look at these carefully in the next chapter.
We have now seen that leaves, like all the other parts of the plant, can modify themselves in a very great number of ways, and may do many extra pieces of work above and beyond their chief work of food manufacture.