2 TEXT-BOOK OF FUNGI 



appears to furnish incontrovertible evidence in favour of 

 De Bary's view of the existence of true sexuality in the 

 Ascomycetes, as opposed to the idea on this point enter- 

 tained by Brefeld, who denies sexuality to all groups 

 excepting the Oomycetes and Zygomycetes. 



On the other hand, speaking broadly, the sexual mode 

 of reproduction is on the wane in the fungi, and instances 

 are not wanting where, even in allied forms, one species 

 may have sexual organs of functional value, whereas in 

 others these organs are sterile, more or less rudimentary, 

 or absent. It is even possible that sexual reproduction 

 may not be constant in the same species. This gradual 

 degeneration of the sexual process, which commences in 

 the Oomycetes, and, so far as we know, has entirely dis- 

 appeared in the Basidiomycetes, is probably due to an 

 adaptation to terrestrial conditions. These altered con- 

 ditions also account for the change from the sporangium 

 containing zoospores in Peronospora, into wind-borne 

 conidia. 



If we accept as evidence of sexual fertilisation in fungi 

 the fusion of a male and female nucleus derived from more 

 or less specialised cells respectively, to form the first 

 nucleus of a new generation, as is known to occur in the 

 higher plants and in animals, then considerable additions 

 have been made to the number of sexual forms, not only 

 in groups where sexuality was previously known, but also 

 in others where its existence was denied, or conclusive 

 evidence was not forthcoming. 



In the Saprolegnieae, according to De Bary, Ward, 

 and Hartog, fertilisation does not occur, the tube of the 

 antheridium not opening into the oosphere, although it 

 adheres to it. Quite recently, however, Trow has demon- 



