THE DISCOMYCETES OF EASTERN IOWA. 237 



for some time as though under tension, when suddenly the 

 ascus breaks at a certain point called the point of dehiscence 

 and the spores are ejected in a mass. 



The position and form of the rupture of the ascus for the 

 exit of the spores varies in different species. In some species 

 it is a simple slit-like opening across the end of the ascus, 

 while in others it is in the form of a circle near its end so that 

 the whole top is entirely removed or remains attached at one 

 side in the form of a lid. That the operculum should remain 

 attached at one side seems to be accidental. The spore 

 pressure from within would be likely to be more on one side 

 than the other, and the operculum, having broken loose on 

 that side would naturally remain attached at the other, as it 

 does in many species. Boudier has placed a great deal of 

 importance upon the manner of dehiscence of the ascus and 

 in one of his papers he suggests that this be used as a basis of 

 classification for separating the discomycetes into two large 

 groups, the operculate and the non-operculate forms. 



In many of the species of this group the asci emerge 

 beyond the surface of the hymenium at maturity, this being 

 brought about, in part at least, by the elongation of the mature 

 ascus. This is characteristic of the family Ascobolacccr, in 

 many of which the spores become purple at maturity. 

 Measurements show that the asci which contain purple spores 

 are much longer than those in which the spores are not yet 

 mature. This may be in part due to the stretching of the 

 ascus from the pressure within, as is indicated by the fact 

 that the asci tend to retract after the spores have been 

 ejected. 



The emergence of the asci in this group led to the old idea 

 that the asci themselves were ejected with their spores. This 

 was commonly thought to be the case by early writers on 

 account of the disappearance of the asci after the ejection of 

 their dark colored contents. Prof. Underwood, of Columbia 

 University, in his book entitled "Moulds, Mildews and Mush- 

 rooms," page 56, describes it in the following words: " The 



