138 MICROSCOPIC r-TTar. 



CHAPTER XI. 



MOULDS. 



THIRTY years since, and some of these little 

 pests were altogetlier 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 num^ous, 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 Hving plants. Upon the potato 

 plant, again, no less than ten different kinds of 

 fungi have been described; whilst upon other and 

 more fortunate plants only one or two parasites of 

 this natm^e estabhsh themselves. 



It will be sufficient for our present purpose to 



