56 



PLANT STRUCTURES 



finally deadened spots on the leaves, where the tissues have 

 been killed. 



From tliis internal mycelium numerous sporophores 

 arise, coming to the surface of the host and securing the 

 scattering of their 

 spores, which fall 

 upon other leaves 

 and germinate, the 

 new mycelia pene- 

 trating among the 

 tissues and begin- 

 ning their ravages. 

 The sporophores, af- 

 ter rising above the 

 surface of the leaf, 



branch freely ; and many of them rising near together, 

 they form little velvety patches on the surface, suggesting 

 the name " downy mildew." 



Fig. 39. A branch of Peronospoi'a in contact with 

 two cells of a host plant, and sending into them 

 its large haustoria.— After DeBary. 



Fig. 40. Peronospm^a, one of the Phycomycetes, showing at a an oogonium (o) con- 

 taining an egg, and an antheridinm in) in contact; at b the antheridial tube pene- 

 trating the oogonium and discharging the contents of the antheridium into the 

 egg; at c the oogonium containing the oospore or resting spore.— After DeBary. 



In certain conditions special branches arise from the 

 mycelium, which organize antheridia and oogonia, and 

 remain within the host (Fig. 40). The oogonium is of the 

 usual spherical form, organizing a single Qgg. The an- 



