Page:Treatise on Cultivation of the Potato.djvu/49

 that Mr. Smith discovered the rest-spores. With the view of separating the tissues for more exact examination, he placed in water some of the diseased leaves obtained from plants grown at Chiswick. He observed that the mycelium grew with greater rapidity in the water, and after ten days he found it producing a large number of minute spherical bodies of two kinds, the one considerably smaller than the other. He further observed specimens in which the already known fruits of Peronospora Infestans were growing from the same mycelium as the newly-discovered bodies.

These bodies exactly correspond with the sexual organs that De Bary had already described in several species of Peronospora under the name of oogonia for the larger, and antheridia for the smaller bodies. Mr. Smith perceived that he had discovered the sexual organs in this species of Peronospora, and continuing his observations he traced the relation between the two bodies. He observed the small antheridia attaching themselves to the oogonia, and fertilising them, by discharging part of their contents into the larger cells through a small tube which was protruded into the substance of the oogonia. The growth of the fertilised oogonium, now called an oospore, was traced by him until it arrived at maturity, when it is a spherical body covered with warts or coarse reticulations and of a black-brown colour. It is but slightly larger than the cells of the leaf, being about one-thousandth of an inch in diameter.

When the rest-spores are mature, they separate themselves from the mycelium on which they grow, and lie as free bodies in the substance of the potato. And when in course of time the whole of the plant perishes, these small hardy bodies remain, able to endure through the winter, and ready to renew the life of the destructive fungus with the restored vegetable life of another year.

Mr. Smith has found the rest-spores in the haulm and tuber as well as in the leaf.

Having thus discovered the means by which the fungus maintains its life through the winter, we are able to look at the question of the possibility of doing something efficiently to mitigate if not destroy the evil. The malady which so extensively destroyed the silk-worms in the south of Europe some years ago, was unconsciously augmented by the producers