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In still other forms, in place of the zoöspores, more highly differentiated cells, known as eggs and sperms, are developed, and these unite to produce the new individuals. Having once originated, the sexual type of reproduction possessed a definite survival value which assured its continuation.
Agardh, and other of the botanists of Cuvier's generation, who occupied themselves with the lower plants, had observed that, under particular circumstances, the contents of the cells of certain water-weeds were set free, and moved about with considerable velocity, and with all the appearances of spontaneity, as locomotive bodies, which, from their similarity to animals of simple organisation, were called "zoospores."
But the protoplasmic contents of some of the cells of which the plant is made up occasionally divide, by a method similar to that which effects the division of the contents of the Peronospora spore; and the severed portions are then set free as active monad-like zoospores. Each is oval and is provided at one extremity with two long active cilia.
But, instead of the terminal cell being filled with the usual densely crowded cluster of dark green granules constituting the rapidly forming spore, it contained hundreds of actively moving, nearly transparent zoospores, and nothing else. Not a single chlorophyl granule was to be seen.
Since, again, the zoospores set free from each spore, in virtue of their powers of locomotion, swiftly disperse themselves over the surface, it is no wonder that the infection, once started, soon spreads from field to field, and extends its ravages over a whole country.
"I determined the nature of the conidia by comparing them with authentic conidia directly removed from diseased plants; by there being attached to some of them portions of the characteristic conidiaphores; and by cultivating them in a moist chamber, the result of which was, that five conidia, not having been immersed in the glycerine, retained their vitality, which they showed by bursting and producing zoospores in the manner characteristic of Peronospora infestans."
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