The Study of Plant Life - M. C. Stopes (korean ebook reader TXT) 📗
- Author: M. C. Stopes
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Fig. 107. Section, A, across the Clover stem, with the Dodder D attached. S, suckers of the Dodder, entering the Clover.
There is one thing, however, that the clover plant cannot do for the dodder, and that is, make its seeds. When the clover builds seeds, then they are clover seeds and will grow up as new clover plants. The dodder must build its own seeds if dodder plants are to grow from them. That is why we find growing out from the simple reduced thread of a stem, relatively large tufts of flowers (see fig. 106), which are very little different from usual flowers and which form seeds. The dodder belongs to the same family as the convolvulus, and though its flowers are small, if you examine them with a magnifying-glass you will see that they are very much the same in structure as those of the convolvulus.
When the young dodder plant grows out from the seed, it is a simple little thread with no leaves, and it keeps on growing at the tip, which it moves round till it feels some suitable host, then it quickly fastens on to it and lives on its food.
This is the general history of all kinds of parasites, for when any living thing ceases to use its structures and becomes a complete parasite it loses nearly all its parts, as there is no longer any need for them. So that parasites tend to sink to a lower level of development simply as a result of their way of living.
A plant which is largely a parasite, but yet does a little work for itself, is the mistletoe (see fig. 108). Its leaves are greenish, but not the true healthy green of a hard-working plant. If you can find a bough of mistletoe growing on an oak or apple tree, you will see that it has no root in the earth, but grows out of the bough of the host tree. It has sucker-like roots at the base of its stem, which go right into the stem-tissues of the host and get much nourishment from them.
Fig. 108. Young Mistletoe attached by its sucker-like roots S to a twig of apple A, split open.
In the winter, when the flow of food is very slow in the host, it is likely that the mistletoe does some of its own food building in its yellow-green leaves, which would be exposed to the full light, as the host’s leaves would have fallen away. The mistletoe has soft, white fruits which are scattered by birds, and as they are very sticky, they hold for some time on to the branch where they are dropped, and there the seedling sprouts and fastens itself on to the tissues of the host, growing every year with its growth.
Fig. 109. P, parasite attached to the root R of a host plant H (which is the Ivy). A is the host root on the other side of the parasite.
Quite a number of plants which grow in the ground attach themselves with suckers to the roots of other plants, from which they get all their ready-made food. Plants which do this are generally colourless or brownish yellow, like the broomrape, which has only whitish leaves which cannot do the proper work of leaves (see fig. 109).
Then there are several plants which are partly parasitic, but which you would never guess were anything but ordinary plants. For example, the little eyebright with its green leaves, which do most of the food-building, is yet partly parasitic. If you very carefully get out a whole plant with its complete roots (this is rather difficult to do, and you must not pull it hastily, or you will break the connections), you will find that there are tiny suckers on them which connect them with the roots of the plants which are growing near. So that the eyebright gets some of its food ready-made from the neighbouring plants. The meadow cow-wheat does the same thing, and so do the lousewort and several others; but they are not complete parasites, for they are green and do a lot of work for themselves, even though they are not quite self-supporting, and tap the supplies of other plants to some extent.
Among flowering plants, parasites are not common. We see in plants like the eyebright and cow-wheat, which do a little thieving, that the results are not very serious, and they are little altered by their habit. In those which are entirely parasitic, however, like the dodder, the result is the loss of nearly all the organs of the plant except the flowers, which have to be kept in order to build seeds.
PLANTS WHICH EAT INSECTS
As a rule, plants are the sufferers and are eaten by animals, but there are cases known in which this state of things is reversed; the plants catch and devour the tinier animals and small insects such as flies. But, you may ask, how can they do that, for the insects move so quickly, and the plants are fastened by their roots to one spot. Just as a spider builds a web and then waits quietly beside it till the flies are caught, so the plants build traps which catch the unwary insects. There are not very many plants growing wild in England which do this, but there are one or two that you might be able to find.
Fig. 110. Plant of Sundew, showing the round leaves covered with tentacles.
There is the sundew, which grows among bog-moss in wet, swampy places at the edges of lakes, or on the wet patches on hillsides. It is fairly common in such places, a little distance from big towns, but it does not like smoke, so that it will not live within a few miles of London, Manchester, or any big smoky town. It is a small plant with round, reddish-coloured leaves, covered over with little fingers or tentacles each with a sparkling drop of sticky moisture at the end, so that even in the heat of the day when all the dew is dried up, the whole plant looks as though it were spangled with tiny dew-drops. Perhaps it is this cool, sparkling appearance which attracts the insects to it, but when once a fly alights on one of the leaves, its fate is sealed. The tentacles with their sticky tips bend over one by one till the fly is quite covered in by them and cannot get away. It dies, and is digested by the juices given out by the leaf, which are very much like the digestive juices of animals.
Fig. 111. Single leaf of Sundew, with the tentacles closing over a fly.
You can watch the movement of the tentacles very well if you drop a minute piece of meat or white of egg on to the leaf. They will close over it one by one till it is quite shut in, and when the egg is all digested, they slowly open out again. The time that this takes depends a little on the health of the plant and the time of the year, but generally all the tentacles are bent over in a few minutes. The digestion takes longer, of course, at least several hours and often more, partly depending on the size and nature of the piece of food. The sundew leaves contain chlorophyll and do some of the usual work of leaves, but the plant gets much of its nourishment from the insects it catches.
Fig. 112. Butterwort, showing the rolled leaves which catch flies and other small insects.
In the butterwort there is a different arrangement for catching its prey. You will find its little clusters of broad, spoon-shaped, yellowish-green leaves growing in marshy places and beside streams in hilly districts. In the spring one or two lilac flowers on long stalks come up from the centre of the group of leaves. The leaves of this plant also act as insect traps; they are covered with little sticky glands, and when an insect settles on them, the edge rolls over and shuts it in, keeping it there till the juices given out by the glands have digested all that is worth digesting, when the leaf unrolls again, and the remains of the feast are washed away by the rain.
Fig. 113. A piece of leaf of Bladderwort showing the bladders on the branches.
There is one more animal eater which you must try to see, which grows in the water of slow-running streams and in ponds. It is the bladderwort, on which we find very many tiny bladder-like structures on the finely divided leaves under the water. The bladders are built on something of the same plan as a lobster pot, with bristly hairs pointing into the entrance, across which there is a little flap, which makes it quite easy for the very minute animals living in such abundance in the water, to swim into the bladder opening, but extremely difficult or almost impossible for them to swim out again (see fig. 114). So there they must finally die, and their nourishing juices are absorbed by little compound hairs, many of which are developed on the inside of the bladder.
Fig. 114. A single bladder of the Bladderwort, much enlarged, showing the pointed hairs and the flap at the opening.
In the tropical countries there are many kinds of “pitcher plants” with wonderful soup-kettle-like pitchers which catch insects. You may be able to see these plants in a big greenhouse, and should certainly find them in every botanical garden. Notice how large the pitchers are, and that they are really modified leaves which have become different from the other leaves of the plant because of their special work. They generally contain a considerable quantity of water as well as the flies they have caught, and are really “stock-pots” which keep the plant supplied with nourishing, ready-made food in addition to the food which it builds for itself in the green leaves.
Fig. 115. Pitcher leaf of Nepenthes, which acts as a “soup-kettle.”
Though these plants have specialised themselves to catch and use animal food, still there are not very many plants that do so, and the old fairy tales about trees with branches which caught men and devoured them, as a sea-anemone catches and devours its food, are only fairy tales, because no such plants exist.
FLOWER STRUCTURES IN RELATION TO INSECTS
The relation between flowers and insects is one of mutual help and advantage, and therefore is quite different from that in the cases
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