6i6 



THE POPULAR SCIENCE MONTHLY. 



The various rusts and brown mildews furnish very complicated 

 methods of propagation, there being no less than four kinds of spores 



Fio. 5— Sexual Spores op White 

 Molds. 



Fig. 6.— Barberex Leaves with " Clusteu-Cups." 



Fig. 7.— Section of Brown Mildew Pustule. 



produced before the whole life-history of some species is complete. 

 Beginning with the cluster-cup form, found abundantly on the bar- 

 berry-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 an- 

 other 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 mil- 

 dew, two hundred times mag- 

 nified, with the spores congre- 

 gated 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 sjyoridia. 

 These wnll 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 



Fig. 8.— Winter Spores of Brown Mildew ger- 

 minating. 



