5 6 LI LA C MILDE W. 



of the marine algae, or Batrachospermum, one of the 

 fresh-water algae. 



The comparison of Microsphaera with Cystopus is 

 very instructive in showing how practically the same 

 ends have been reached by widely different plants. 

 Both are parasitic, the one living inside the host, the 

 other upon its surface, both deriving nourishment by 

 means of haustoria, in addition to what is absorbed 

 directly through the walls of the filaments. It is some- 

 what doubtful, however, if the haustoria of Micro- 

 sphaera pierce the cells of its host, although those of 

 some closely related species are thought to do so. 2 

 Both bear aerial asexual spores, which are formed 

 by successive abstrictions from vertical mycelial 

 threads, the main difference being that in Cys- 

 topus these must break through the surface tis- 

 sue of the host, and are therefore required to grow in 

 groups in order to exert the necessary force, while in 

 the superficial Microsphaera they are single, and evenly 

 distributed. The conidia of Cystopus germinate by 

 formation of zoospores, while those of Microsphaera 

 grow a mycelial filament at once, a difference due to 

 some obscure cause. Both plants form resting spores, 

 but in Cystopus the protective covering is the thick- 

 ened wall of the spore, in Microsphaera it is a specially 

 developed shell, inclosing a number of spores in sacs. 



There is not much known of the manner in which 

 these fruits pass the winter and give rise in the spring 

 to another growth of mildew. 3 It is plain f-om 



3 Cf. Bessey, Botany, p. 279. 



3 Wolff has studied the germination of the ascospores in Erysiphe. 

 graminis. Bot. Zeit. 1874, p. 183, 



