The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
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In the numbers of the parts of the flower, we also find differences between the two groups; for example, the Dicotyledons have generally two, four or five, or a multiple of these numbers such as ten, as we see in the poppy, primrose, rose, and many others; while the Monocotyledons have the parts of their flowers in three or multiples of three, as in the lily, tulip, and daffodil.
These differences between the Monocotyledons and Dicotyledons, however, are not nearly so important as their likenesses, for they agree in the main points (1) to (6), and therefore belong equally to the great family of the flowering plants, which is the most important family now living.
THE PINE-TREE FAMILY
Since trees such as the oak, beech, and lime all belong to the family of flowering plants, you may be surprised to find that the pine-trees are separated from them. Yet all the trees like pines, Christmas trees, larches, and many others, form a family of their own. You will see why this is, if you look at a pine-tree carefully, and compare its characters with those we saw in the flowering family. In the first points the two families are alike.
1. We find that the pine-tree body is clearly marked out into root, stem, leaves, and cones.
2. Also that the stem and root have definite strands of “water-pipe” cells, and that the stem has rings of wood, one of which is added every year.
3. The leaves vary a little in the different members of the family, but the commonest kind of leaf is the fine sharp “needle” leaf of the ordinary pine (see fig. 53). In almost all cases the leaves remain on the tree for more than a year; they are evergreens (it is only the larch among the English-growing members of the pine-tree family which has new leaves every year), and the leaves are simple and strong, and well protected.
4. There are no “flowers,” but the two kinds of cones which take their place are easily recognised. The two kinds of cone generally grow on different branches of the tree, the small ones only live a short time and scatter the pollen, and the larger ones often remain two or three years on the tree, and form the seeds. The wind scatters the pollen; you will remember in the spring-time before the leaves are out, how the “sulphur rain” showers down from the pine-trees; this is the yellow pollen, which is blown in clouds on to the seed-bearing cones. There are millions of pollen grains scattered in this way, and but few of them ever reach a cone. You will remember that many of the flowering plants could afford to make small quantities of pollen, as they had special carriers in the insects to take it from flower to flower.
Fig. 123. A branch of Pine with a small young seed-bearing cone, and a large ripening one.
Besides the pollen cones, you should find two sizes of seed-cones on the tree: some quite small, and green or pink, and some large ones which are brown and ripe. It will be easier to see their structure at first in the big ones; they consist of a number of brown scales packed neatly one over another. If you pull these apart you will see that each of them bears two seeds on its upper side.
Fig. 124. Larch. A and B, young scales, showing (i) inner seed-bearing scale, and (o) outer protective one. A, side view; B, front view; C and D, old scales, C from the side, D from the front, showing how the inner scale increases more rapidly than the outer.
5. The seeds are always seen to be lying quite openly on the upper side of the scales, and are not covered in by closed carpels as they are in the flowering plants. Each of the scales (which bears its two seeds) corresponds in a way to the carpel in a flower, but there is an important difference in the fact that it leaves the seeds open. In old pine cones there seems to be only one scale to each pair of seeds, but there is really a second smaller one outside it—which is sometimes quite difficult to see. It shows better in the larch, where the outside one is much the bigger of the two in the young cones, and gradually gets left behind, as the inner scale grows very fast (see fig. 124). Notice, too, how the ripe seeds have one-sided wings, which split off from the inner scale, as you can see if the cone is not too ripe. This wing is on the seed itself, not on a fruit, as is often the case among the flowering plants. The wing helps the seed to fly, and in the late autumn (in many cases two years after it began to grow, for some pines grow very slowly) it is scattered with its brothers. If you are ever near pine trees when there has been snow, you may see it sprinkled with these winged brown seeds.
Fig. 125. Winged seed of the Pine.
6. You may never have seen a baby pine tree. If not, you must get some seeds and grow them. They grow very slowly at first, and may take six weeks to show above ground even in summer; but they are well worth waiting for. Notice how they come up (see fig. 126), and that at the beginning of their growth, as they come out from the seed, they have seven or even as many as twelve first leaves, and these leaves are really the cotyledons, as you may see by cutting a seed across. So that instead of the one or two cotyledons of the flowering family, we find in the pine family that there are many cotyledons, and that their number may vary from five to ten or more.
Fig. 126. Stages in the growth of Pine seedlings; (c) cotyledons.
If you go back over these points, you will see that we have found a large number of differences between the flowering plants and the pines. Of these, the most important are the points (5) and (6), which alone would be enough to make us place the pines and flowering plants in separate families, though point (4) is also very important. We find, however, that the pines are more like the flowering plants than are any of the other families, so that they are the nearest relatives the flowering plants have, even though they are rather far-away ones.
PLATE III.
TREE FERNS, SHOWING THEIR TALL THICK TRUNKS AND LARGE LEAVES, WITH SMALLER FERNS GROWING BENEATH THEM
FERNS AND THEIR RELATIVES
Perhaps there is no family of plants so easy to recognise as the ferns. It is nearly always a simple matter to know whether or not a plant is a fern, for although there are hundreds of different kinds, they all have the family characters plainly marked.
We have not very many ferns growing commonly in England, for they generally require a moister air than is usual in this country. By far the commonest is the bracken, which grows in all parts of the country, and sometimes in very large masses (see Plate I.). Some people separate the bracken fern from the others, and speak of “bracken” and “true ferns,” but this is not at all correct, for the bracken is just as much a true fern as the others, only as it is so much commoner, people are apt to value it less.
In some countries, particularly in the tropics, there are (as well as ferns like ours) very large ferns with tall, strong, upright stems, and crowns of large spreading leaves. Such ferns you can see in Plate III., and they are called tree ferns. Notice how thick the stem is, and how large the leaves are compared with it, while the trunk seems to be all rough and hairy, which is due to the jagged bases of the old leaves which have fallen away. Yet even the tree ferns are easily recognised as belonging to the fern family.
Let us examine ferns in order to find out what are the points about them which are specially characteristic for their family, and which help us to separate them from the other plants.
1. We find that the fern body is clearly marked out into roots, stem, and leaves, but there appear to be neither flowers nor cones.
2. The stem and roots have definite “water-pipe” cells, as you can see if you examine a thin slice with your magnifying-glass, but there are never rings of wood formed year by year, as in the higher families. The stems are frequently short and stumpy, and often run underground. They are usually covered by the rough leaf-bases of old leaves and by dry scales.
3. The leaves are generally few in number, often only three or four, but they are highly compound, and are split up into very many side leaflets. They are generally thin and delicate. When they are young they are rolled up in the bud in a close coil (see fig. 127), and as they unfold they bend back. This way of coiling up is quite a special character of ferns. The buds are generally covered with flaky, shining scales, which stick all over the young leaf-stalk.
Fig. 127. Young leaf of a Fern rolled in a close coil.
4. You have never seen a fern with flowers or seeds, yet there are always plenty of new ferns every year. How are the young ones formed? For a long time botanists did not know, so that people thought there was some magic about it, but now we know the whole story, and it is a very interesting one.
5. There are no seeds, and
6. Therefore there are no seedlings to have cotyledons.
You must have noticed little dark brown spots on the backs of some fern-leaves. It is in them that you must look for the beginning of the new fern plants. The little patches are at first hidden by green coverings, but when they are ripe these bend back, and expose the little brown clusters within. If you look at one of these ripe patches with a magnifying-glass, you may be able to see a number of little roundish boxes on stalks. Each of these contains a number of tiny “spores” (which are single cells with the power to grow), and when the spore-cases are ripe they open and shoot out the spores, as you may perhaps see if you look closely at a ripe patch when it is taken into warm, dry air.
Fig. 128. A small piece of Fern leaf showing the patches of spore-cases on the under side.
These brown patches are not at all like flowers, but in some way they do the work of flowers, for they give rise to cells which can carry on the life of the fern to a second generation. The way in which they do it, however, is totally different from that of the seed, and is quite the most special character of the ferns and their relatives.
The spores grow slowly when they come on to moist earth, but as their development takes a long time, you had better get some from a gardener which have already grown. As the spore grows, the one cell composing it divides and divides again, until there is formed a little filmy heart-shaped green structure called a prothallium (see fig. 129), which is not in the least like a fern
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