CYSTOPUS, OR WHITE RUST. 137 



It is these conidia, crowded together and lying under the 

 surface of the epidermis, which produce the white pustules already 

 described. When large numbers of conidia have been thus pro- 

 duced, the epidermis of the host-plant bursts in an irregular manner 

 and the conidia are set free. The detachment of the conidia, one 

 from the other, is effected in the following manner : — The primary 

 septum or constriction separating two conidia becomes divided by 

 a central gelatinous lamella into two plates belonging to the 

 respective conidia, and by the solution of this gelatinous lamella, 

 the conidia are set free. 



Each conidium is filled with finely granular protoplasm, and 

 when it becomes matured in damp air or water, a division of the 

 protoplasm into five or eight portions takes place, just as in the 

 case of the potato fungus. These secondary spores — or, rather, in 

 this case zoospores — are presently discharged from their conidium 

 or spore-case, and after remaining immoveable for a short time, 

 two vibratile cilia are developed from beneath, and the zoospore 

 sails away over any moist surface as if endowed with animal life 

 (PI. XV., Figs. 3-7). 



Moisture of some sort, as rain or dew, seems essential for the 

 bursting of the sporangia and the expulsion of the zoospores ; in 

 dry air no difTerentiation of the contents of the conidium ever 

 takes place. Thus, when the conidia are fully developed, the first 

 shower of rain will cause them to burst into activity, till the whole 

 surface of the plant and every drop of water upon it is swarming 

 with zoospores. It is thus obvious that the fungus may pass from 

 leaf to leaf and from plant to plant and from weeds to food- 

 plants by means of these active zoospores, which seem to retain 

 their activity for several hours. The zoospores are also undoubt- 

 edly carried about in damp air and in currents of wind ; they are 

 also widely distributed by means of insects, birds, and small 

 animals. Fortunately, the mycelium, conidia, and zoospores are 

 not possessed of great vitality, and they easily perish by frost, 

 drought, or even excess of moisture. But the question then 

 arises, " Why is not this fungus exterminated during the winter 

 season?" and the answer is, that this fungus, like the Feronospora, 

 possesses another method of reproduction — a sexual one — which 

 results in the formation of the winter or resting-spores. 



