42 PROTISTS AND DISEASE 



Cystopus candidus. This fungus, which is parasitic in 

 Crucifers, is a common parasite in the weed, shepherd's 

 purse ; it attacks also crucifers of economic value : cabbage, 

 radish, horse-radish, cress, &c. The conidia form beneath 

 the epidermis causing the appearance known as " white 

 rust." They are really sporangia, which, on germination 

 under water, produce a number of bi-flagellate zoospores. 

 These in turn lose their flagella and germinate as shown in 

 Fig. 8, d. 



Details of structure in Oospores. In Pyihium the oospore 

 or fertilised ovum, Fig. 7, /, secretes a stout cell- wall inside 

 that of the original oogonium ; there is but little periplasm, 

 and no thick exospore is produced. In Cystopus candidus 

 a thick exospore is formed from the periplasm. After a 

 long resting period the oospore germinates in water as is 

 shown in Fig. 8, g, h : the innermost wall of the spore bulges 

 with its contents, becoming a sporangium, from which 

 zoospores escape. 



Nuclear processes in Cystopus. In C. Candidus the 

 gametic nuclei behave as in Pythium, and there is an 

 additional feature in the presence in the ovum of a deeply- 

 staining body, the coenocentre, Fig. 8, i, which, as Wager has 

 suggested, may be the equivalent of a vegetative chromidium. 



In C. bliti the nuclei of the gametes subdivide mitotically 

 and conjugate in pairs. 



A second simultaneous division of nuclei occurs in the 

 oosphere before fertilisation. 



