MOULDS. 165 



vigorous green of the leaves^ prior to tlie appear- 

 ance of tlie mouldy is not at all an uncommon 

 occurrence : this phenomenon lias been noticed in 

 the ears of corn^ in which every grain was soon 

 afterwards filled with spores of bunt. 



This species was at one time believed to be para- 

 sitic on the '' white rust/^ from which circumstance 

 its specific name was derived. More precise ex- 

 amination proves that it sometimes occurs where no 

 white rust is present^ and therefore its parasitism is 

 imaginary. The suckers in this species are large 

 and penetrate the cells^ often entirely filling them. 

 This feature is sufficient to distinguish the mycelium 

 of the mould from that of the ^' rust.''' The turnip 

 mould occurs on many cruciferous plant s^ and 

 especially on the shepherd's-purse^ as well as upon 

 the different varieties of cabbage and its more 

 immediate allies. 



Onion Mould. — Another disease, produced by 

 fungi of the same genus, makes its appearance 

 upon young onion plants in the spring. The mould 

 is called Peronospora SMeideniana, and has many 

 features in common with those already described. 

 In this instance the threads are greyish and erect, 

 with alternate branches, not divided by transverse 

 septa, and the spores are obovate, attenuated 

 towards their base (fig. 263). This mould, in some 

 years, is very common and destructive, by prevent- 

 ing the young plants which are attacked from 

 coming to perfection. It is not confined to the 



