CHAPTER XIII.

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.

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.

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~