L] 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 of a small portion of the 
Potato fungus (Peronospora infestans), which at times 
destroys the entire crop in large 
districts. It is produced by spores 
which, falling on the Potato plant, 
develop, and their hyphe enter the 
stomata or breathing-pores of the ~ 
leaf, and penetrating its cellular tis- 
sue, absorb all nutriment from it. 
From the leaf the hyphe or my- 
celium traverse the substance of 
the stem, and finally reach the 
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@,aare two 
of the hairs with which the leaf is furnished; 8, 6, and 
¢, ¢, the cellular tissue of the leaf. dis 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, 
FIG. 20. 
