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rust fungus is carried from one leaf to another, from one plant to its neighbouring plant, and from weeds to foodplants, in damp, rainy, or misty weather, by the microscopic zoospores sailing about over moist surfaces. There can

be no doubt that they are also carried about in damp air, in currents of wind, and that birds, insects, and other animals help to carry the living conidia and zoospores from place to place.

As a rule conidia, zoospores, germ tubes, and fungus spawn are very liable to perish; too much dryness, a superabundance of moisture or frost, will quickly destroy

them.

Three questions now present themselves to us-How does the white-rust fungus tide over the winter ? Where is it hidden? How does it suddenly reappear in the spring? In the case of many plant diseases, as in the surface mildew of turnips, Oidium Balsamii, Mont., already described, no one is able to answer such questions; but with the white-rust fungus and several of its allies the knowledge has been obtained, and a satisfactory answer can be given. It has been already stated that the spawn or mycelium of Cystopus grows within the leaves and stems and burrows amongst the intercellular spaces of the host plant. It not only bears the chains of spores already described, which, when ripe, are blown away by the wind, but it carries other bodies within the substance of the leaf. These latter organs roughly answer to the pistils and anthers of flowering plants. The first bodies are female, and are termed oogonia; these are large globular cells in which the female reproductive bodies, or oospheres, or sometimes zoospores, are formed. They generally grow on terminal branches of the mycelium; sometimes they are sessile or nearly so, or they may be intercalated in the mycelium itself. An oogonium is illustrated, enlarged 400 diameters, at A, Fig. 33. The oosphere, filled with granular protoplasm or vital formative material, is seen within. Other organs borne on the mycelium are male,

and termed, in reference to their nature, antheridia, or organs answering to the anthers of flowering plants. In the course of growth the antheridium comes in contact with the oogonium as at B, and projects a fine beak through its wall, till it pierces the oosphere within as at C. This is the act of fertilisation answering to the discharge of pollen on to the stigma in flowering plants. In the same way as an ovule becomes a seed after

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Oogonium with Antheridium and Resting-spores, or Oospores of Cystopus candidus, Lev. Enlarged 400 diameters.

fertilisation in flowering plants, the oosphere becomes an oospore or egglike spore, after the contact of the antheridium with the oosphere. The now fertile oospore within the oogonium grows and matures itself whilst still within the supporting leaf or stem for many months, generally for the greater part of a year, and it does not become perfectly ripe till the host plant has decayed. Although the

cabbage leaf which carried the Cystopus may be dead and putrid, or reduced to tinder by drought and frost, the oospores or resting-spores remain alive and uninjured, in a dormant state. They change in colour and form, from almost colourless smooth spheres to amber-coloured, warted, globular bodies, as illustrated at D. They are best seen in the putrid remains of plants which have been destroyed by the white-rust fungus. In good material the ambercoloured oospores will be seen closely packed together in the decayed leaf or stem in enormous numbers. The best plan for obtaining resting-spores is to collect leaves infested with white rust and allow them to decay upon a garden bed; after the diseased leaves have perished the oospores will be found during the winter or the following spring in the decayed fragments of foliage.

The oospores germinate on the ground during wet weather in the spring; but the germination may be easily observed in water under the microscope. After a few ripe resting-spores have been placed in a drop of water they will speedily burst, either at once, or in a day or two, according to the state of their maturity. They germinate by bursting, as illustrated at Fig. 33, E; a transparent inner membrane is protruded, and the contained protoplasm, which at first is differentiated into numerous polyhedric portions, at length resolves itself into a large number of oval zoospores as at F. Soon the transparent investing membrane is ruptured, and the zoospores sail out as at G, thus repeating, after from six to ten months' rest, the phenomenon described as belonging to the chains of conidia illustrated in Fig. 32. The zoospores produced by the conidia are precisely the same in size and habit with those produced by the oospores; in both instances they germinate in the same manner after swimming about for three or four hours in water. The difference in size of the zoospores shown in Fig. 32 and Fig. 33 is owing to the fact of the former being enlarged 1000 diameters, whilst the latter illustration is only enlarged 400 diameters.

Cruciferous plants in the spring owe their infection with white rust to the zoospores germinating upon the seed-leaves. No one, of course, has seen such extremely minute objects as zoospores with the unaided eye, so no one has ever seen them naturally transferred from the germinating oospores on the wet ground, to the young seed-leaves of cabbages, cauliflowers, and other cruciferous plants. It is said that the zoospores cannot effectually germinate and form mycelium upon and in leaves and stems of cruciferous plants unless the latter are very young. But as cruciferous weeds infected with Cystopus are extremely common, the oospores must occur in profusion in all districts every spring. No doubt the little motile zoospores are carried through moist air by currents of wind, and distributed in every direction throughout the country.

Alternation of crops must tend to diminish white rust. Cabbages, cauliflowers, etc., should not be grown for two years in succession where white rust has prevailed. Cruciferous weeds should be gathered together and burnt, especially when they exhibit the well-known white sprinkling of the white-rust fungus. No cabbage, cauliflower, turnip, or mangel refuse should be allowed to remain in a decaying state throughout the winter in the fields, for in those positions not only the white-rust fungus, but the putrefactive mildew of the cabbage tribe and the fungus of club-root hibernate. Clean and intelligent farming will greatly reduce the attacks of these two, as well as of many other pests.

CHAPTER XVII.

CLUB-ROOT OF TURNIPS, CABBAGES, MANGELS, AND ALLIED PLANTS.

Plasmodiophora Brassica, Wor.

THE disease of turnips, cabbages, and allied plants, known in some districts by the popular name of club-root, is recognised in other places as anbury and finger and toe. On the continent the disease is popularly known as hernia or rupture.

Until the last six or seven years no one knew the cause of club-root, but in 1876, after three years' constant attention, M. Woronin, a Russian botanist, as completely explained the nature of club-root in turnip and cabbages, as the Rev. M. J. Berkeley expounded the nature of the murrain of potatoes in 1846.

The observations made by M. Woronin, which have several times been confirmed by others as well as ourselves, seem to place the fact beyond all doubt that clubbing is caused by a fungus named, by M. Woronin, Plasmodiophora Brassica. Plasmodiophora means a bearer or carrier of a plasmodium, and a plasmodium is an Amœba-like mass of protoplasm or vital formative material of changeable form; Brassica, of course, means that the fungus is peculiar to the turnip and cabbage class. The family to which the fungus belongs is a remarkable one, and is known as the Myxomycetes or family of slime-fungi. These fungi have appeared so animal-like to some observers that, by a misinterpretation of analogies, an attempt has been made to transfer them to the Protozoic division of the animal kingdom. With the same idea in view they have been

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