We cannot afford much more space to plant diseases, the farmer has other troubles and we must mention some of them. We cannot leave the subject, however, without a word concerning the mildews. As we have mentioned, they are curious because they dwell outside the plants they attack. Rose mildew is unfortunately all too common in every garden, it may be recognised as a white powder covering leaves and buds. Under the microscope, in the summer we shall find that it consists of a number of thread-like structures, not unlike those of the common white mould and that there are a number of erect chains of spores. Towards autumn, a further examination will show us many round dark-brown structures from which project a number of minute threads. These brown spheres are the winter stage of the fungus, designed to withstand inclement weather. In the spring, the spheres burst and set free a number of minute sacs, each one containing eight spores. The spores germinate on rose leaves and start the disease anew.
There will be no difficulty in finding mildews; they are all very similar to the rose mildew in general but they all differ in detail. The gooseberry mildew for example, has a large number of threads running from its winter spheres and each thread is terminated by a little group of branches. The sacs which fall from the opened sphere in the spring, only contain four spores in this case.
The animal enemies of the farmer, so far as they concern the microscopist are more difficult to study. Many of them are internal parasites and to gain a real knowledge of their habits and life histories necessitates a good deal of rather unpleasant work for which the ordinary microscopist has neither the time nor the inclination.
In order to give our readers some idea of this work, let us take one of the commonest of all agricultural parasites and trace its life history whilst giving hints for its examination under the microscope. The common liver fluke is a worm which, in the adult stage, frequents the liver of some domestic animal, usually the sheep. A friendly butcher will probably be able to supply us with a specimen and, when we receive it, we shall probably dub it a very unwormlike creature. The worms form a large class in the animal kingdom and they do not all resemble the earthworm by any manner of means.
The liver fluke is a flat, almost leaf-like creature, it is not ringed like the earthworm and, under the microscope, we can plainly see all its internal organs. The fluke lays its eggs, each one enclosed in a little capsule, in the liver of the sheep. They are carried to the intestines and finally set free along with the animal’s excrement. If then the eggs are blown, or carried by some means to water they will continue their development, on dry soil they cannot long survive. Each egg gives rise to a little organism which swims freely in the water; it is shaded like a blunt-ended cone when extended and is roughly oval when contracted. Its body is covered with little whip-like structures similar to those of the slipper animalculæ, and it is due to the lashing of these little whips that the creature moves through the water. If we found one of these young flukes in some pond water we might be forgiven for thinking it to be some near relative of the slipper animalcule. When our subject finds a living water snail it enters its breathing organs, becomes affixed to their walls and loses its covering of little whips. It becomes transformed into a shapeless mass which later develops into an elongated structure, quite unlike the free swimming creature which took shelter within the snail. Next, a migration is made to the liver of the snail where birth is given to, from fifteen to twenty, curious little heart-shaped organisms each with a tail about twice as long as its body. These little creatures escape from the snail and swim freely in the water for a time. Eventually they make their way to herbage growing by the waterside, affix themselves thereto and become surrounded with a hard coat capable of resisting the effects of hot sun or drying winds. Should this herbage be eaten by cattle, the apparently lifeless young fluke bestirs itself, loses its tail and wends its way to the liver of its host, then the story begins again.
Having examined the adult liver fluke under the microscope, we shall probably wish to find both the free swimming young forms, and if we search carefully in ponds to which sheep have access we are likely to be rewarded. It is obvious that the life of a parasite such as the liver fluke is, of necessity, precarious. It is only chance or luck, or whatever one’s favourite term may be, that brings the egg to water, the young fluke to a snail, and the last free swimming form to herbage that will be eaten by a suitable animal. As usual in such cases, nature makes provision for emergencies by providing a large number of young, in order to insure that some at least may be able to complete their development. Owing to a series of changes, which we have omitted to describe for the sake of simplicity, each liver fluke egg may give rise to no less than three hundred and twenty of the final free swimming forms.
As we have remarked, the study of parasites is difficult but it is interesting. Very few of these creatures can complete their lives without living at the expense of two different animals. The liver fluke needs the water snail and some herb-feeding animal; there is another parasite which spends part of its life in the pig and another part in the grub of the cockchafer; a third parasite dwells for a time within the thrush, and for the rest of its time within the garden snail, and so on. Apart from the interest of the subject in itself, it brings us face to face with the fact that many quite unrelated forms of animal life are essential to the well being of a number of parasites. To the farmer the subject is all important.
Insects of various kinds are all important in agriculture; most of them are harmful, some few are useful. They have, however, been dealt with in [another chapter], so we will dismiss them here. The ticks are closely related, and anyone with access to a farm should be able to obtain some specimens. Whatever species we are able to obtain should be examined under the microscope. Their feet are always interesting, being furnished with powerful claws beautifully adapted to grasping the hairy coats of their hosts. Their mouth parts are quite unlike those of insects, and are always furnished with a number of backwardly directed teeth, which are useful for tearing flesh sufficiently to draw blood on which they feed.