Page:Popular Science Monthly Volume 26.djvu/635

Rh rapid propagation of its kind during the growing season, and finally stores up its vitality in dark, thick-coated spores that remain on the stubble through the winter, and are not injured by sudden and severe changes in the weather.

It is due the reader, in passing, to state that one or more of the four forms of spores here briefly mentioned may be omitted. In warm climates it is possible for the winter spores to be dispensed with, the ordinary rust-spores being able to remain alive and continue the life of the pest. It is also believed that in regions unknown to the barberry the cluster-cup form may also be omitted. Like all other living things, the rusts accommodate themselves to circumstances, though watchful that their members do not decrease from any lack of vigilance on the part of these parasites.

The order Perisporiaceæ illustrates our subject still further. The members of this group of fungi are mainly parasites upon higher plants, forming a whitish, web-like film over the surface of the affected parts. In the early life of these white mildews, the horizontal threads send up vertical filaments in which partitions rapidly form at regular intervals. The cells thus produced are spores which fall away in succession from the top. A single vertical filament is shown at I, Fig. 9.



The spores thus produced are minute, the winds easily disperse them, and they quickly germinate, giving rise to new filaments of mildew. The formation of the sexual or winter spores begins late in the season and is shown in the remaining portions of the engraving. When these spores are to be formed, two filaments, crossing each other as shown at III, send out short projections. One of these, c, becomes the female part, and the other, b, the male portion. As a result of fertilization, eight or more branches, IV, h, grow up from the base and envelop the female cell. These branches continue to grow until a thick, hard