CH.XVI.] WHITE-RUST DISEASE OF CABBAGES, ETC. 87 



white pustules are visible, by the swollen and distorted 

 appearance of the leaves and stems, caused by the presence 

 of the spawn of the parasite within the plant. As in 

 Peronospora, the mycelium of Cystopus traverses the host 

 plant by the intercellular passages. The spawn threads 

 resemble the mycelium of Peronospora parasitica, Pers., 

 in being provided with suckers which become affixed to 

 the constituent cells within the leaves and stems of the 

 host. When the white pustules are examined with a 

 microscope they are found to be not dissimilar in 

 character although different in colour from the pustules 

 belonging to Puccinia mixta, FL, already described, or of 

 the rust fungus of corn, Uredo linearis, Pers., described 

 further on. Instead, however, of simple red Uredo spores 

 or compound blackish Puccinia spores being found within 

 the pustules, chains of almost colourless round or ovate 

 spores or conidia are seen in the white -rust fungus. 

 Chains of conidia or spores belonging to Cystopus candidus., 

 Lev., are illustrated in different stages of growth, enlarged 

 400 diameters, at A, Fig. 32. The fungus grows beneath 

 the epidermis of the plant after the manner of Puccinia 

 mixta, Fl., already described ; the pustules produced by 

 the white -rust fungus are, however, very much larger 

 than the blisters of the Puccinia. The spores or conidia 

 grow in chains, a fact first pointed out by the Eev. M. 

 J. Berkeley, in the Journal of the Royal Horticultural 

 Society, vol. iii., p. 269, 1848. The spores or conidia are 

 formed in Cystopus in the following manner : At first 

 simple clublike growths are produced as at B ; a con- 

 striction forms towards the apex of the club, which 

 speedily takes the form of a joint or septum as at C ; in 

 the process of growth another constriction occurs as at D, 

 which in turn speedily becomes a septum or joint. As 

 this process is repeated each club at length supports a 

 short chain of conidia, each conidium being attached to 

 the conidia next in order by joints as at DD. When 

 large numbers of conidia have been produced in this 



