136 MICEOSCOPIC FUNGI. 



large number of oospores^ each containing a liiin- 

 di-ed zoospores. It can scarcely be considered 

 marvellous tliat the white rust should be so com- 

 mon on plants favourable to its development^ the 

 marvel being rather that any plant should escape. 



Until recently it was doubtful whether more than 

 one or two species of Cijstopus (white rust) were 

 known. It is now certain that we have three in 

 Great Britain^ and three or four others are found 

 elsewhere. Of the British species one is found on 

 many cruciferous plants^ as the shepherd^s-pm'se^ 

 garden-cresSj mustard^ radish^ and plants of the 

 cabbage kind. This is the Cijsiojjus candidus. 

 Another occurs on the goat^s-beard^ salsify^ and 

 scorzonera^ which is called Cystopus cuhicus. Both 

 have great external resemblances^ but both possess 

 specific internal differences. In the Goat^s-beard 

 rust (plate X. fig. 201) the terminal conidia in the 

 bunches or fascicles of conidia which are produced 

 within the pustules are spheroidal^ large^ and of a 

 yellow-brown tint_, whilst the residue are cylindricab 

 smaller (plate X. fig. 202), and more or less com- 

 pressed. In the crucifer rust the conidia are all 

 equal in the pustules and globose. The oospores 

 in the former of these are subglobose and the warts 

 on their surface are solid ; whilst in the latter the 

 oospores are truly globose,, and the warts on the 

 surface are hoUow (plate X. fig. 210). The third 

 species is the Sandspurry white rust {Cystopus 

 Lepigoni), which was found on the common sand- 



