130 



PRACTICAL BOTANY. 



notice that the aerial hyphse turn towards the light ; they 

 are positively heliotropic. 



References for Reading. Goebel's "Classification of Plants," pp. 

 90-92 ; Vines' " Text-Book of Botany," pp. 287-289 ; Carpenter's 

 " The Microscope," pp. 569-571 ; Bennett and Murray's " Crypto- 

 gamic Botany," pp. 335-343 ; Strasburger and Hillhouse's " Practical 

 Botany," pp. 255, 256 f; Parker's "Practical Biology," pp. 158-168; 

 Bidgood's "Elementary Biology," pp. 72-79; Bower's "Practical 

 Botany," pp. 497-502 ; Huxley and Martin's " Practical Biology," pp. 

 420-429 ; Cooke's "Introduction to the Study of Fungi," pp. 226-229. 



The Mildews and White Rusts (Peronosporece). These 

 are especially important members of the class on account 



of their destructive ef- 

 fects on valuable food 

 plants. They may be 

 found on the Grape as 

 mildew, Peronospora viti- 

 cola. One species, Phy- 

 tophthora or Peronospora 

 infestans, causes a Potato 

 disease; another species, 

 Cystopus candidus, grows 

 on Lettuce, Radish, and 

 Shepherd's Purse ; an- 

 other on Spinach, Pero- 

 nospora effusa; another, 



FIG. 71. Diseased Potato leaf. A Po- n . *i7v 



tato leaf infected with downy mildew, Cystopm Ultl, grows On 



Peronospora infestans. The figure Pigweed ill late autumn, 



shows portions of the leaf covered rr^^ ^ j j/u 



with a dense mass of the Fungus which The ra P ld spread ot these 



when mature turns the leaf dark 

 brown or black. (After Marshall.) 



diseases IS Caused by 

 ,, ,. . ,, 



the dissemination of the 

 spores by the wind. The spores fall on the plants, and 

 develop, sending their rhizoids through the stomata or 

 the thin, moist epidermis of the leaves, developing hau- 

 storia a special form of mycelium growth in the 



