138 microscopic rarer. 



CHAPTER XI. 



MOULDS. 



THIRTY years since, and some of these Utile 

 pests were altogether unknown, whilst others 

 were only recognized and partly understood by a 

 few scientific men. During the period to which 

 we have alluded more than half the present species 

 contained in the genus Peronospora had never been 

 observed, and amongst these the most devastating 

 of its tribe, the associate and undoubted cause of 

 the potato disease. 



Parasitic fungi are far more numerous, both in 

 individuals and species, than most persons are 

 aware, and cultivated plants of all kinds are more 

 or less subject to their ravages. Some are more 

 susceptible than others, of which the corn and 

 grass tribe, or Graminacece, as they are termed 

 by botanists, is an example. Not less than thirty 

 species have been recorded upon plants of this 

 natural order, and of these nearly one-half are 

 found upon the living plants. Upon the potato 

 plant, again, no less than ten different kinds of 

 funo-i have been described; whilst upon other and 

 more fortunate plants only one or two parasites cf 

 this nature establish themselves. 



It will be sufficient for our present purpose to 



