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THE PARASITIC SLIME-MOULDS.
by
W. R. IVIMET COOK.
THE UNIVERSITY, BRISTOL, ENGLAND.
The parasitic slime-moulds are a small group of organisms belong- ing to the group Plasmodiophorales. They are all microscopic and develop in the stems and roots of Flowering plants, where they live as parasites within the cells. In nearly every case their presence is manifested by swelling or hypertrophy of the infected tissue. As a group they are of considerable interest since they represent some of the simplest, if not the most primitive, parasitic fungi known. Owing to their structure and mode of living they have been considered by many as parasitic animals rather than plants, but in their method of reproduction they more closely resemble plants, producing spores within peculiar sporangial structures. In their vegetative state there is little to distinguish the species from one another, but in their reproduction the arrangement of the spores differs markedly in the different genera.
Only twelve species are recognised and all, with two exceptions, are very local and rare in distribution. The two common species arc Plasmodiophora Brassicae and Spongospora subterranea, which attack Crucifers and potatoes respectively. These two species have followed the cultivation of their hosts into most countries where cabbages and potatoes have been extensively grown. The other species are known only from a few isolated localities, though there seems evidence that when further collecting has been done, both new species, and a wider distribution of the existing ones, will be found. Of the rare species one occurs in the East Indies, one in South Africa, two in Great Britain, and seven in various parts of Europe.
My purpose in contributing this paper, giving the characteristics of all the known species, is to encourage the search for species in Eastern Asia from where, at present, no species at all have been recorded.
I propose first to give a general account of the life-history of a typical member, and I shall select Plasmodiophora Brassica, which causes a disease of Crucifers known as Finger-and-Toc disease, as my example of the group, For purposes of description it is convenient to divide the life-history into three phases; (4) the infection of the host by the parasite; (b) the growth of the organism at the expense of the host, and (c) the multiplication of the fungus by the development of spores within the host cell. It is proposed to begin with the second phase, since we know comparatively little about the infection stages in most species.
น. The growth of the fungus within the host tissue.
The first clearly recognisable stage in most of the genera is the occurrence of uninucleated masses of protoplasm lying within the host cells. In Plasmodiophora Brassica these are generally found in the cells of the cortex of the root. The bodies are more or less regular in shape, but occa- sionally those with an amaboid outline are found. These bodies are capable of very slow movement, of the same sluggish character which occurs in
Supplement No. 1, 1934.
110