CHAPTER XX.

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.

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 threads, twisted round and round the clover stems and hanging in festoons over them. Pull off these fine threads carefully, and you will find that at intervals along them there are little sucker-like pads which hold the dodder quite firmly on to the plant on which it is growing. If you cut through the middle of one of these pads and the clover-stem while they are still attached, and look at the cut with your magnifying-glass, you will see how the tissue of the dodder pad enters right into the tissue of the clover stem (~see~ fig. 107). These pads act as suckers for the dodder and draw from the clover all the ready-formed nourishment that the dodder requires, so that it has no work to do in food building. It has no roots because it needs none; the suckers act as roots in getting all the water and also the manufactured food the plant uses; for the same reason it requires neither leaves nor green chlorophyll, and its body is only a colourless or pinkish mass of thread-like stems and sucker pads.

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.

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.

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.