The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Fig. 98. Thick fleshy leaves of Stone-crop.
Other plants, instead of having leaves of this type, have exceedingly small leaves which may soon drop off, while the stem is green and does some of the food building. Small leaves are assisted by the green stem in gorse (fig. 99), which often lives in very dry places, though it can grow equally well under usual conditions.
Fig. 99. Gorse, with green stem which does the work of leaves.
Many plants roll up their leaves when it is dry, so that the surface with the transpiring pores is on the inside, and protected by the outer side with its hard skin (see fig. 100). In damp weather these leaves unroll, and do all the work they can. Leaves like this are to be seen in many of the grasses, particularly those growing on sand dunes and moorland; while a number of the heaths and heather do the same thing to protect their transpiring surfaces.
Fig. 100. Leaf of the Sand-grass. A, rolled up; B, open. (a) and (b), sections across the same.
You will find that in nature, water is one of the most important things in the surroundings of plants, and in their struggles to get it and keep it they have changed their forms in many ways, and in some cases have become extraordinary-looking creatures as a result.
SPECIALISATION FOR CLIMBING
If you go into a wood, or even a thicket, in summer, you can see how the leaves of the big trees make, what is for us, a delightful shade. But look at the ground under these tall trees, at a place where they are growing thickly together, and you will find that there are very few plants below them, and that the earth is almost bare except for dead leaves, twigs, and a few mosses. In deep pine-woods there are great patches without even the mosses, where are only dead pine-needles and some toadstools. You can well understand, when you remember how very important light is for the plants, that it is too dark for them to grow under the heavy shadow of thick trees. Even in gardens you may see how the tall, quickly growing plants kill off the smaller ones beneath them.
When many plants are growing together, it is easy to see that the taller ones get most light, but if a plant grows very tall it requires a strong stem to hold it up right, and that means the building of a large amount of wood which takes a quantity of material, so that the growth must be slow and costly.
Some plants, however, have learned to grow up into the light without building a firm stem for themselves, because they use instead the support of other plants, and especially of trees. You must often have noticed in a wood great sprays of honeysuckle sprawling high up over the trees; sometimes one of the festoons of honeysuckle may lie over the branches of several trees, and so get into the best positions for the light. The Travellers’ Joy, or white clematis, grows all over the tall hedges, and may sometimes completely smother a young tree, so that one can see nothing but the leaves and light green and white flowers of the clematis. Then, too, there is the ivy, which you know may sometimes grow up trees to a very great height, covering over the leaves so that the whole looks like a giant ivy bush. These plants all get their support from trees, which have built themselves strong stems. Pull down a big branch of honeysuckle or Travellers’ Joy from the supporting tree-trunks, and you will see that it cannot remain upright but falls limply to the ground. It is true that these plants have some wood in their stems—sometimes clematis and ivy may have woody stems several inches thick, but they are never strong enough to support the weight of the crown of leaves and branches. By clinging to others in this way these plants can economise much building material and reach the light far quicker than they could do otherwise.
If you examine their wood, you will see that it is not quite like that of usual plants. Cut through the stem of a clematis which is about an inch thick, and even before you look at it with a magnifying-glass you will see how very loosely built the wood is, with wide rays of soft tissue and very large water vessels. It is not built for strength and support, but merely to carry supplies of water up to the leaves, for although these plants use trees as supports, they do not get anything more from them, and must supply themselves with all else they need. You may often see that the central part of the wood is not in the true centre of the stem, but is pushed to one side, and the rings of the year’s growth are very irregular, being much more to one side than to the other. This is because they lean against the supporting branches, and so must grow chiefly on the side away from them. Sometimes as the ivy grows right round the support, it will grow more, first on one and then on the opposite side of its stem, and so the centre does not remain in one place, but shifts round.
The other parts of these woody climbing plants are but little out of the common. They have merely learnt to economise their own stem-material, and at the same time to reach a good position in the light, so that it is in their stems that we find their chief differences from usual plants. The honeysuckle and clematis have no special climbing organs, but the Ivy has clusters of adventitious roots which come out from the back of its stem, and hold it on to the support (see p. 56 and fig. 101).
Fig. 101. Adventitious roots growing out from the stem of Ivy between the leaf stalks.
In climbing plants in which the above-ground parts live only for one year and then die down, we do not get a woody stem. Such soft green plants as the hop and convolvulus, for example, are entirely dependent on others for their support. They have specially sensitive tips to their stems, which feel the support and definitely twine round it in a close spiral, which clings ever closer to the support as they grow (see fig. 102).
Fig. 102. Soft twining stem of Convolvulus.
Climbers of this kind have only modified their stems; the rest of their parts are not in any way specially altered by this habit.
Some plants which sprawl about on others hold themselves up by the power of clinging and twining in their leaf-stalks, for example, in the nasturtium we find that the plant is held up entirely by the leaf stalks, which catch on to anything in their way (see fig. 103).
Fig. 103. Nasturtium stem held up by the support given by the leaf-stalks, which cling around any suitable prop.
Very many plants which depend on others for support modify their leaves, or parts of leaves, to form sensitive tendrils which twine quickly round any prop they can find, and thus hold up the stem (see fig. 104). The young tip of the stem continues to grow upwards, the next young leaves unfold their soft green tendrils which twist round a support directly they feel it, and so the plant goes on growing higher and higher. You can see the fate of a pea-plant which does not find supports, by growing one in a big pot all by itself. It will grow upright at first, but it will soon have to creep along the earth and fall over the edge of the pot, for its stem is not strong enough to support its own weight.
Fig. 104. Sensitive tendrils of the Pea. (t) tendril at the end of foliage leaf, (o) ordinary leaflets.
In vines and marrows we also get tendrils, but they are not modified leaves, but special branches which have become sensitive.
In some plants the sensitive tendrils do not twine, but instead form little sticky suction pads at their tips whenever they come in contact with the support, and these hold the tendril very firmly on, as you can see in the ampelopsis, which grows right up the walls of houses. If you look under the thick covering of leaves, you will find these tiny padded tendrils clinging tightly to the wall (see fig. 105). This is the reason that the ampelopsis grows so well up the walls without being held up artificially.
There are many other things you may find out about climbing plants, but you will have seen enough to be able to look for more for yourself, and to understand how it is that the climbing plants can reach such a great height so quickly. They have learnt to avoid the trouble and expense of building strong supporting stems for themselves, and by getting their support from others, they are able to grow quickly out into the good positions for the light which they could not otherwise have reached.
Fig. 105. Ampelopsis, which supports itself by the little suction pads developed at the ends of the tendrils.
PARASITES
We call a plant or animal a Parasite when it does no food-building for itself, but adapts its whole structure to obtain and use the food made by the work of other plants or animals. Plant parasites generally attach themselves to a “Host” plant so closely that they suck their food from it, and sometimes remain with it till they have finally killed it, and so have destroyed their only source of food and means of life.
Among plants, most of these degenerate creatures belong to the group of Fungi. The rust and smut on wheat, the mildew on fruit, and nearly all the thousand spots, blemishes, and diseases of cultivated and other plants, are the result of the parasitism of some members of the family of fungi. Plants which prey like this on others are without very many of the characteristics of true plants; they become colourless, losing their green substance, and with it all power of building food for themselves, so that they are quite dependent on the host plant, without which they must ultimately die.
Fungus parasites, of which there are many thousands, have become so specialized that they are quite a study in themselves, and we will leave them for the present and follow the history of a few of the higher plants which have taken to this mode of life.
Fig. 106. Dodder plants growing over Clover. (a) clusters of Dodder flowers.
One of the most completely parasitic of the flowering plants is the dodder, which you may often find growing on clover. In fields of clover sometimes there are colonies of dodder, which live together and kill the clover in great patches so that it almost looks as though it had been burnt. Dodder grows on other plants, such as gorse, as well as clover, and even on nettles. If you find a plant of dodder you will see that it seems to consist of nothing but fine, white or pinkish
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