92 DISEASES OF FIELD & GARDEN CROPS. [CH. 



cabbage leaf which carried the Gystopus may be dead and 

 putrid, or reduced to tinder by drought and frost, the 

 oospores or resting-spores remain alive and uninjured, in 

 a dormant state. They change in colour and form, from 

 almost colourless smooth spheres to amber-coloured, warted, 

 globular bodies, as illustrated at D. They are best seen 

 in the putrid remains of plants which have been destroyed 

 by the white-rust fungus. In good material the amber- 

 coloured oospores will be seen closely packed together in 

 the decayed leaf or stem in enormous numbers. The best 

 plan for obtaining resting-spores is to collect leaves in- 

 fested with white rust and allow them to decay upon a 

 garden bed ; after the diseased leaves have perished the 

 oospores will be found during the winter or the following 

 spring in the decayed fragments of foliage. 



The oospores germinate on the ground during wet 

 weather in the spring ; but the germination may be 

 easily observed in water under the microscope. After a 

 few ripe resting-spores have been placed in a drop of 

 water they will speedily burst, either at once, or in a day 

 or two, according to the state of their maturity. They 

 germinate by bursting, as illustrated at Fig. 33, E ; a 

 transparent inner membrane is protruded, and the con- 

 tained protoplasm, which at first is differentiated into 

 numerous polyhedric portions, at length resolves itself into 

 a large number of oval zoospores as at F. Soon the trans- 

 parent investing membrane is ruptured, and the zoospores 

 sail out as at G, thus repeating, after from six to ten months' 

 rest, the phenomenon described as belonging to the chains 

 of conidia illustrated in Fig. 32. The zoospores produced 

 Ly the conidia are precisely the same in size and habit with 

 those produced by the oospores ; in both instances they 

 germinate in the same manner after swimming about for 

 three or four hours in water. The difference in size of the 

 zoospores shown in Fig. 32 and Fig. 33 is owing to the 

 fact of the former being enlarged 1000 diameters, whilst 

 the latter illustration is only enlarged 400 diameters. 



