Page:Popular Science Monthly Volume 26.djvu/634

 The various rusts and brown mildews furnish very complicated methods of propagation, there being no less than four kinds of spores



produced before the whole life-history of some species is complete. Beginning with the cluster-cup form, found abundantly on the barberry-leaves, as shown in Fig. 6, it is known that the spores from these "cups" produce the common rust upon the wheat leaves and stems. Later in the season another form of spore is formed in the same ruptured patches before occupied by the orange-rust spores. These last spores are double, and form slowly on the tips of slender filaments. Fig. 7 represents a cross-section through a pustule of brown mildew, two hundred times magnified, with the spores congregated beneath the ruptured epidermis. These dark patches and streaks remain until spring. When the spores germinate, as shown in Fig. 8, magnified five hundred times, each twin-spore sends out a filament that bears from three to five small oval bodies, known as sporidia. These will germinate on the barberry-leaf and develop the cluster-cups with which we started. It is seen that the rust has ample means for a