288 



DISEASES OF FIELD & GARDEN CROPS. [on. 



zoospores burst and produce a thread of spawn, as shown 

 at F ; this germinal thread is capable of carrying on the 

 existence of the potato fungus. Zoospores were first dis- 

 covered in the genus Cystopus, allied by some authors, with 

 Peronospora (see chapter xvi.), by Prevost in 1807; and 

 Mr. Berkeley described and illustrated them in the potato 

 fungus, though he did not see the vibrating hairs, in 

 1846, Journal of the Royal Horticultural Society, vol. i. pi. 

 4, fig. 18. Sometimes the conidium, which, when it 

 bears zoospores, is really a sort of spore-case, sporangium, 

 or zoosporangium, does not differentiate within, but bursts 



X-1000 



FIG. 129. 



Conidium or zoosporangium and zoospore of the potato fungus, Peronospora 

 infestans, Mont. Enlarged 1000 diameters. 



and protrudes a small mass of protoplasm or vital material, 

 as at G, Fig. 128. This mass speedily elongates into a 

 mycelial thread capable (like the thread from the zoospore) 

 of carrying on the life of the potato fungus. A ripe 

 conidium or zoosporangium and zoospore of the potato 

 fungus are farther enlarged to 1000 diameters at Fig. 129, 

 for comparison with other reproductive bodies illustrated 

 to the same scale in this work. A considerable difference 

 in the size of zoospores will be noted if Figs. 33 and 39 

 are referred to. The soft papilla or bursting point of 

 the zoosporangium is shown at A, Fig. 129; and 

 the minute footstalk by which the zoosporangium was 



