i.] MICROSCOPIC PLANTS. 19 



plants of rye, and in five or six days after, these 

 plants were affected with rust, whilst the remainder 

 of the crop was unaffected. Dr. Cooke, in his valu- 

 able book, gives detailed instances of other species 

 in which this polymorphism occurs. 



Fig-. 20 is a representation o'f a small portion of the 

 Potato fungus (Peronospora infcstans), which at times 

 destroys the entire crop in large 

 districts. It is produced by spores 

 which, falling on the Potato plant, 

 develop, and their hyphae enter the 

 stomata or breathing-pores of the 

 leaf, and penetrating its cellular tis- 

 sue, absorb all nutriment from it. 

 From the leaf the hyphae or my- 

 celium traverse the substance of 

 the stem, and finally reach the Fu . 



cells of the tuber the Potato 

 itself. Mr. Worthington G. Smith has recently 

 made some valuable researches into the life-history 

 of this fungus, with the result of adding con- 

 siderably to our knowledge of it. Fig. 21 is a 

 modified reproduction, on a small scale, of one of 

 that gentleman's illustrations, representing the sec- 

 tion of a small portion of potato leaf, a, a are two 

 of the hairs with which the leaf is furnished ; b, b, and 

 c, c, the cellular tissue of the leaf, d is a branch of 

 the fungus emerging from a breathing-pore or stom- 

 ate, and "is no other than a continuation of a thread 

 of spawn or mycelium which lives inside, and at 

 the expense of the assimilated material of the leaf. 

 When this thread emerges into the air, as here shown, 



