xvi. J WHITE-RUST DISEASE OF CABBAGES, ETC. 89 



zoosporangium, and enlarge it 1000 diameters, we shall see 

 it as at E, Fig. 32 ; the former point of attachment is seen at 

 F, and the sporangium is shown in the act of discharging 

 its differentiated contents, in the form of zoospores, from 

 its apex. These secondary spores were at first the poly- 

 hedric contents of the sporangium ; but as they emerge in 

 water or on any damp surface the angles become rounded, 

 and they are at last expelled as minute ovoid bodies as 

 illustrated at G. At first these small secondary spores or 

 zoospores remain immovable at the mouth of the burst 

 sporangium ; soon, however, they begin to slightly oscillate, 

 and two excessively attenuated hairlike cilia are de- 

 veloped from beneath as at H. At a special moment the 

 foremost cilium is distended in a straight line as shown, 

 whilst the hindermost cilium at the same time suddenly 

 quivers, and the zoospore sails away over any moist 

 surface, as if endowed with animal life. Each zoospore 

 exhibits within one or more lustrous, perhaps contractile, 

 vacuoles. 



The phenomena just described can only be seen when a 

 zoosporangium of -the white-rust fungus, has been placed 

 in water upon a glass slide, and viewed under a cover- 

 glass with a high power of the microscope. In dry air 

 no differentiation of the contents of the conidium takes 

 place. It is certain that rain, dew, or moisture of some 

 sort is essential for the bursting of the sporangia and the 

 expulsion of the zoospores. The bursting, as seen in 

 water, under the microscope takes place in an hour or 

 two after immersion ; the conidia retain the power of pro- 

 ducing zoospores for about a month. The zoospores are 

 able to swim about for several hours ; their cilia then 

 vanish, the zoospore retakes a globular tailless form, 

 bursts as at J, produces a germ tube, and this germ tube 

 is then a spawn thread of white rust capable of pro- 

 ducing a new series of clubs capped with zoospore- 

 bearing sporangia. 



It is obvious from the above description that the white- 



