The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Fig. 66. Branch of Sycamore, showing leaf stalk (l) with bud (b) in its axil. Scars of leaf stalks (s) and large terminal bud (t) with scars (a), (a1), and (a2) left by the terminal buds of past years.
The buds which come in the axils of the leaves along the stem may form new leaves, or may develop into side shoots with new stems and leaves. There is another bud, generally bigger than these, which grows at the end of the shoot (t, fig. 66). This has just the same structure as the others, but it will certainly grow out to form a stem and carry on the line of growth of the main shoot, unless it is injured.
The amount that the shoot grows in one year depends on very many things, on the light and warmth it gets, on its food and the growth of its neighbours. Hence, in the growth of different shoots in the same year, or the same shoot in different years, we find very great differences. Sometimes a number of bud scars lie very close together, showing that for several years it had only grown a small amount, while in the years following it may have added very much to its length. In some plants there are little side shoots which never grow much, and always remain quite short; for example, in the larch each tuft of leaves grows on a little stunted stem which represents several years’ growth, and which never reaches any length (see fig. 67).
Fig. 67. Larch, with tufts of leaves growing from short side shoots.
Not only do we get leaves and stems packed away in buds, but the flowers for next year are there also. For example, examine several of the big horse chestnut buds from the outer branches of the tree, and you will be sure to find tiny sprays of young flowers packed away in the hearts of some of them.
There are some quite special buds which we must notice, and which at first sight appear very different from real buds. They have been given a different name, and are called Bulbs. Cut right through a tulip or hyacinth bulb lengthways, and compare it with a horse chestnut bud to which you have done the same. The arrangement of the parts of the two things seems to be very similar. If you examine the bulb in detail, you will find that it is protected on the outside by brown, hard scales, and that the softer leaves within are folded over each other very much like those in the true buds. Now the bud of the horse chestnut is attached to the parent stem—is there nothing corresponding to the stem in the tulip bulb? Look carefully at the base, and you will see a little mass of tissue which holds the scales together (see S, fig. 68); this is the stem, which is short and very much reduced, being unlike a usual stem. There is also one great difference between the scales in the bud and the bulb. In the bud they are rather thin and dry, but in the bulb they are thick and white and very fleshy, and if you test them with iodine, you will find that they contain much starchy food. They form the storehouse of the tulip, and this food will be used by the plant when it begins to grow. In the axils of these thick fleshy leaves you may often find small buds, which will get large and fleshy by next year and form the new bulbs (see fig. 68 b).
Fig. 68. Bulb cut through, showing the overlapping scales (s) packed with food attached to the shortened stem S. B is the bud, which will grow out into the air, and (b) the bud which will form a new bulb next year; (r) adventitious roots growing from the base of the stem.
Sometimes little bulb-like structures grow in the axils of ordinary leaves, for example, in the tiger lily; these drop off when they are ripe, and can grow into whole new plants. They are really half-way between bulbs and true buds.
FLOWERS
If you have ever noticed a pea-flower fading, you will have seen that from its heart there grows a little green pea pod which ripens till there are full-grown peas in it (see fig. 80); and a yellow dandelion flower turns in the end to a white puff ball which scatters a hundred floating fruits. In fact, almost all flowers which have not been spoiled by the gardeners and “over-cultivated” leave in their place when they die fruits and seeds in some form or other. This gives us the key to the secret of the structure of the flowers themselves. They are the forerunners of the fruits, containing living seed, and their structure and all their parts are adapted in some way to help in the formation of fruit. Now let us examine the flowers, never forgetting that fact.
Fig. 69. Flower of Harebell; (s) sepals; (p) petals.
Let us choose, for example, a harebell. On the outside we find five separate green parts, and if we examine a bud which has not yet opened we shall find that these fold quite tightly over the inner portions of the flower and protect them, as they do in the rose and in almost all flowers (see fig. 69). In this they correspond to some extent to the bud scales, and their special work is that of protection. In all harebells and roses there are five of these parts, but in the wallflower you will find only four, and in poppies two, and so on. There are different numbers of them in the different kinds of flowers. They are also of different shapes and sizes; sometimes each of the five parts is free, and sometimes they are all joined up together to form a true cup, as in the primrose (see fig. 72). These outer green protective parts have a special name, and are called the calyx or cup, while each of the separate parts which makes the cup is called a sepal.
Fig. 70. Buds of the Rose; A with the calyx covering the inner parts, B with the petals opening out.
Fig. 71. Flower of the Rose, with separate petals.
Directly within the calyx we come to the parts which are generally bright and prettily coloured, and which give the chief beauty to the flower. In the harebell which we are examining these parts are joined up to form a bell, but in the rose they are each separate (see fig. 71). In both harebell and rose we find five of these parts, and the same number in the primrose, where you will find that they are joined up at the base to form a long, narrow tube, and then spread out separately like those of a rose. Both in the harebell and primrose, where they are joined up, we can tell the number of parts which go to make the whole bell or tube (and this is nearly always the case in bell flowers), while, of course, where the parts are free it is quite easy to count them, and we find that for each kind of flower their number is always fixed. For example, we find five in the harebell, rose, primrose, and many others, four in the poppy, wallflower, cress, and so on. These parts are called the petals, and in almost all flowers you will find that they are bright and pretty, and stand out from the surrounding green leaves, so that they are easily seen. When the cups or bells hang down they may protect the parts within from the rain, but that is not generally their chief work. The first duty of the petals is to be attractive. You will understand better why this is so after we have gone further into the flower.
Fig. 72. Primrose flower cut open, showing the stamens (s) attached to the tube formed by the petals.
Fig. 73. Flower of Speedwell, with only two stamens (s).
Within the petals, and, in most cases, lying at the base of the bell, you find several yellowish dusty sacs, on fine thread-like stalks. In most flowers they are all free from each other and from the petals, but in the primrose they are fastened to the tube of the petals (see fig. 72). In some flowers you will find a great many of these, as you do in the wild rose (see fig. 71) and the poppy, where there are so many that you can hardly count them. In other flowers there are very few; for example, there are only two in the blue speedwell (see fig. 73). In most flowers the single stamens, as they are called, are very much alike in their structure, and they all have the same work to do. Look at these structures in a tulip or lily, where they are very big, and carefully pull one off and examine it (fig. 74). You will find that it consists of a stalk which we call the filament, with two long sacs at the tip which hold the yellow dust, and which we call the anthers. If you examine a fully blown flower of the tulip or lily, you will find that the sacs split open right down their length and let out a fine yellow powder. This powder is the important thing about the stamens, and is called the pollen. In all stamens you will find the anthers or pollen sacs, while the stalk, which is less important, is not always developed. Sometimes the sacs split right open like those in the lilies, but there are other ways of opening; as for instance, in the rhododendron you will see a little round hole at the tip of each anther, which lets the pollen shake out like pepper out of a pepper-pot.
Fig. 74. Single stamen from Tulip flower; A, anthers, or pollen sacs; F, filament, or stalk of stamen.
Fig. 75. Flower of Tulip laid open, showing the three-cornered central green box containing the young seeds.
Now we have come to the heart of the flower, and find there the most important thing in it. Examine a sweet pea, for example, and you will find in
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