186 CYSTOPUS, OR WHITE RUST. 



But except for the characteristic of parasitism common to all, 

 Cystopus seems to have little connection with the genera it is thus 

 associated with ; and it seems preferable, at all events for the 

 present, to accept De Bary's classification, in which resemblance 

 of life history is made the principal feature. By this means Cys- 

 topus and Pero?iospora become associated together as different 

 genera of the same order, and closely allied to the Saproleguiece. 



Cystopus, or White Rust, is one of the commonest of parasitic 

 fungi, and may be seen in the summer on numerous Cruciferous 

 plants, and unfortunately also in many kitchen-gardens, where it 

 attacks the cauliflowers and cabbages. The general appearance of 

 a plant affected by the fungus is, to use the comparison of many 

 observers, as though it had been splashed with whitewash. But 

 when the plant is closely examined, the splashes are seen to be 

 arranged in a somewhat concentric or spiral manner, those, how- 

 ever, on the leaf-stalks and flower-stems being disposed more 

 irregularly (PI. XVI., Fig. 21). 



Experienced observers can detect the presence of the fungus 

 long before the white splashes or pustules are visible, by the 

 swollen and distorted appearance of the leaves and stems, caused 

 by the presence of the spawn or mycelium, which invades every 

 part of the plant above ground. The mycelium consists of unpar- 

 titioned, irregularly-branched tubes, having thick, gelatinous walls, 

 and containing colourless and almost homogeneous protoplasm. 

 It grows exclusively in the intercellular canals of the parenchyma, 

 and is furnished with numerous suckers, which become affixed to 

 the constituent cells of the host. These suckers are a character- 

 istic feature of the mycelium of Cystopus (PI. XV., Fig. i). 



From this mycelium arise bundles of club-shaped tubes, each 

 of which produces from its summit the reproductive cells called 

 conidia (PI. XV., Figs. 1,2). The growing conidium, to use the 

 expression of Tulasne, is nothing less than the obtuse summit of 

 the club-shaped tube, which, by a partition, separates itself from 

 the lower part, becomes constricted till it is attached only by a 

 short, narrow neck, and eventually falls off. The process is, how- 

 ever, so frequently repeated that, as a result, each club supports a 

 short chain of conidia attached to one another by joints, and this 

 constitutes another distinguishing feature. 



