286 DISEASES OF FIELD & GARDEN CROPS. [on. 



poverished thread, if grown in dry air, will quickly 

 become strong and robust in growth if transferred to 

 warm moist air, as in the thread illustrated at Q. In 

 many species of Peronospora the branches which carry the 

 conidia only produce one conidium, and do not continue 

 growing and producing new conidia. Owing to the mode 

 of spore production in the potato fungus, Professor De Bary 

 has recently suggested that the parasite should be placed 

 in a new genus by itself under the name of Phytophthora. 

 Other botanists, however, as the Kev. M. J. Berkeley 

 and ourselves, would prefer reducing rather than increas- 

 ing the genera of the Peronosporece ; and so include in 

 Peronospora not only Phytophthora but Ovularia and even 

 Ramularia. 



At Fig. 128 we have engraved a fragment of the 

 potato fungus to 400 diameters, so that the parts may 

 be compared with the other species of Peronospora en- 

 graved to the same scale in this work. A considerable 

 difference will be observed if Fig. 2 and Fig. 16 are 

 referred to. The spores or conidia are shown at A, 

 Fig. 128, and the peculiar constrictions in the jointed 

 branches are better seen. If ripe conidia are placed 

 in water it will be noted that a differentiation of the 

 contained protoplasm takes place, as shown at BB ; and 

 that the interior mass of each conidium becomes divided 

 into from five to nine or more portions, each contained 

 portion being furnished with one or two lustrous vacuoles. 

 These differentiated portions speedily emerge from the 

 top of the conidium when placed on any moist surface 

 as at C ; and each portion now free, becomes quickly fur- 

 nished with two extremely fine hairlike cilia, tails, or 

 vibrating hairs, as at D. These secondary spores or 

 zoospores are able to sail about in the slightest film of 

 moisture. After a brief time the little motile zoospores 

 or animal -like spores rest and take a globular form, as 

 at E, and the vibrating hairs dissolve away or drop into 

 the finest dust. After a short rest the now quiescent 



