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.

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.).

[6] ~Spores~ are simple little structures which do much of the work of seeds. ~See~ the Chapter on Ferns.

~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.