THALLOPHYTES 



49 



or the tips of its branches (Fig. 30). The sporophores reach 

 the surface of the host either by emerging singly through 

 numerous openings (" breathing pores ") in the epidermis 



C 



FIG. 31. Downy mildew: A, oogonium (o) with antheridium (a) in contact; B, 

 tube from antheridium penetrating oogonium ; C, oogonium containing the oospore. 

 After DEBARY. 



(Fig. 30), or by massing together and pushing up the epider- 

 mis until it dries out and ruptures. In this latter case, the 

 first appearance on the surface is a whitish blister. 



Later, the internal mycelium de- 

 velops the sexual branches, which in 

 this case are so different that they 

 can be recognized as oogonia and an- 

 theridia (Fig. 31). The antheridium 

 sends out a tube that pierces the wall 

 of the oogonium and through which 

 the contents of the antheridium are 

 discharged into the oogonium, in 

 which the heavy-walled oospore is 

 formed (Fig. 31). These sex-organs 

 and the oospores are not brought to 

 the surface of the host, as are the 

 sporophores, for when the oospores 

 are ready to germinate during the 

 following spring, the host tissues in- 

 closing them have decayed. 



FIG. 32. Lilac leaf covered 

 with mildew: the shaded 

 regions representing the 

 mycelium with its numer- 

 ous spores (giving the 

 dusty appearance) , and 

 the black dots the spore- 

 cases. 



