Page:Popular Science Monthly Volume 26.djvu/632

614 is a familiar destructive fungus pest that will serve our purpose in an attempt to show the preparation these low forms of plant-life make for passing over hard times, including the winter season. In early-summer the lower surface of affected grape-leaves is covered with a minute forest of white filaments. The threads of the fungus run in all directions through the tissue of the leaf, and, coming to the surface, pass through the breathing-pores of the epidermis, and afterward branch and bear oval spores on the tips of the filaments. Fig. 2 represents a small portion of a branch of the lettuce-mildew, which is in the same genus, and closely related to the mildew of the grape. The branch is magnified two hundred times, and the spore A and its attachment are enlarged five hundred diameters. The multitudes of spores borne upon the tips of the branches are for



the rapid propagation of the mildew. They germinate by producing zoöspores—that is, the contents divide into six or more oval bodies, which soon rupture the spore-wall and escape, each provided with two hair-like appendages termed cilia. These motile zoöspores reproduce the mildew in a new place upon the same or another grape-leaf. Later in the season, and within the substance of the leaf or fruit, a second form of spore is found. This is termed a sexual spore, and requires the union of the contents of two peronospora filaments for its production. One of the thread-tips becomes much enlarged, as shown in Fig. 3, o (on the left), which represents the female cell charged with granular protoplasm. Another thread, n, arises near by, becomes club-shaped, and applies itself to one side of the female sphere. The contents of the male cell enter and mingle with those of the female, after which the latter matures into a spore. The process of fertilization is further shown in the portion of the engraving on the right, all parts being magnified three hundred and fifty times. The dark central portion is the spore, the contents of which become securely protected by the thick, hard coverings. Unlike the small, thin-walled spores borne upon the tips of the aërial branches, these large sexual spores remain through the winter before germinating. The vitality of the grape-mildew is