The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕
- Author: M. C. Stopes
- Performer: -
Book online «The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕». Author M. C. Stopes
Fig. 56. Alternating pairs of leaves of the Dead Nettle.
Fig. 57. Pair of Honeysuckle leaves with no leaf stalks.
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.
Fig. 59. Leaves arranged in a whorl in the Horsetail.
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.
Fig. 60. Leaves of Goose grass looking like a whorl.
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.
Fig. 61. Leaf of Pea, showing leaflets modified as tendrils (t); expanded leaflets (o).
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.
BUDS
The proper time to study buds in nature is the spring, but then you will have to wait long to see all the different stages of their slow unfolding. But they can be made to open artificially, and it is really wise to study buds in winter, when there are not so many other things to do. You can arrange this very well if in the late autumn you cut off fairly big branches with buds on them (horse chestnut is particularly good for this) and keep them in a warm room. You must, of course, keep the cut ends of their stalks in water, which you should change every three or four days, sometimes cutting off a piece from the ends of the branches so that they have a fresh surface exposed to the water. In this way they should live for months, and may just begin to unfold and show fresh young green leaves about Christmas time, when the buds on the trees out in the cold are still tightly packed up.
Fig. 62. Buds of the Horse Chestnut beginning to unfold.
Watch the buds as they unfold, and you will find that round each bud are several dry brown scales; these drop off, and within them are more green, leaf-like scales enclosing the true young leaves, which are still curled up and very small when they first come out.
If you examine a big bud which has not yet begun to unfold, and carefully pull off all the parts separately with the help of a needle and knife, you will see how the outer scales fold over one another like a coat of mail, and where they are exposed to the outside air they are hard and shiny, and in many plants are covered over by a sticky waterproof substance like tarpaulin. These outer scales keep off the rain and snow, and keep the inner parts dry and unharmed. Within them the scales are softer and often quite green, and they, too, wrap round each other, so that there is no crack left which could allow the cold rains to enter to the little leaves within. In many cases also the young leaves are wrapped up in soft, long hairs which look almost like cotton wool. These hairs grow on the leaves themselves, and you can see them after they have opened out, but as the leaves are then much bigger, the hairs are scattered further apart and do not show so much.
Fig. 63. Bud of the Horse Chestnut, showing the overlapping of the scales.
If you cut right through the length of a bud with a sharp knife, you will see how all these scales and young leaves are packed together, as in fig. 64.
Fig. 64. Bud cut through lengthways, showing the bud-scales and young leaves packed within them.
Take another bud and carefully pull off all the scales one by one and lay them in a row, beginning with those right outside; you will see that they get less scale-like and more like real leaves as you go in towards the centre of the bud (see fig. 65). The outside simple brown scales scarcely look like leaves at all, but the inner ones are green and soft, and in some plants, those right inside have quite a leaf-like appearance.
Fig. 65. A series of bud scales from a Horse Chestnut; (a) and (b) are entirely hard and brown; (c) and (d) are brown at the tips and green at the base, where the others cover them; (e) is quite green, soft, and leaf-like.
This helps us to see that bud scales are really only modified leaves, which are altered for their special work of protection of the young leaves through the winter.
Of course, you know that the buds are already on the trees in the late autumn after the leaves have fallen; but have you seen the buds already there in the summer while the leaves are still fresh and green? If you look for buds you will be sure to find them, and at the same time you will learn where they grow on the stem. You must look right at the base of the leaf stalk, in the angle made by the leaf stalk where it joins the main stem; this is called the axil of the leaf, and it is in the axil of the leaf that you will find the small green buds in summer-time. These buds grow out in the following year, so that a new leaf comes in very nearly the same place as the old one, or, what is more usual, there grows out a new branch which may bear several new leaves. Now examine a twig of horse chestnut or sycamore from which the leaves have dropped; notice that, where the buds are to be seen on the stem, they lie immediately above scars of a definite shape, which are the scars left by the fallen leaf stalks, as you can see by comparing them in the autumn with leaf stalks which are just falling away (see fig. 66, l, b, and s).
On the stem there are
Free e-book «The Study of Plant Life by M. C. Stopes (best ebook reader for laptop TXT) 📕» - read online now
Comments (0)