CLASS ASCOMYCETEAE 271 



meiotic division in the ascus, in which case the four spores of similar phase 

 are placed in succession followed by the four of the other phase. Segrega- 

 tion may occur in the second meiotic division in which case the nuclei are 

 distributed otherwise so that two of the four at either end of the row of 

 eight ascospores will be of one sexual phase and the other two will be of 

 the other phase. 



Lindegren (1932) reported the results of experiments with Neurospora 

 crassa Shear and Dodge, a species in which each ascus contains eight 

 uninucleate ascospores. He picked out the individual ascospores in order 

 one by one from the ascus and obtained cultures which were then mated 

 to determine their sexual phase. He showed that in about 85 per cent of 

 the asci the two strains are segregated in the first nuclear division and in 

 about 15 per cent in the second. No evidence was obtained to indicate 

 that this segregation ever occurs in the third nuclear division. In M. 

 sitophila, with eight ascospores per ascus, and N. tetrasperma, with 

 normally four binucleate ascospores, the segregation also may occur in 

 either the first or second division, more often in the first. 



From the foregoing it seems possible that the frequent failure to obtain 

 perithecial development in pure cultures from a single ascospore may in 

 some cases be due to the occurrence of two sexual phases in that species. 



The groups of families included in the collective term "Pyreno- 

 mycetes" differ from those collectively called " Discomycetes " in the 

 nature of the spore fruit. In the latter group, described in the preceding 

 chapter, the spore fruit is an apothecium or a modification of it, in which 

 the asci are in a more or less extensive hymenium which is eventually, in 

 the typical forms, exposed to the air with numerous paraphyses separating 

 and supporting the asci. In the Pyrenomycetes the spore fruits are mostly 

 much smaller, thicker-walled, and rarely opening wide, the ascospores 

 escaping through a small opening, the ostiole, or by the rupture of the 

 whole structure. Formerly these small closed or ostiolate structures were 

 all called perithecia, in contrast to the open apothecia of the Discomy- 

 cetes. More recent studies in the last forty years, especially by von Hohnel 

 (1918), Theissen (1913), Arnaud (1925), Nannfeldt (1932)^ and others, 

 have shown that three distinct structural types have been included under 

 the name perithecium. Of the thousands of species with so-called peri- 

 thecia a great many still remain to be studied carefully to determine to 

 which of these three types they belong. 



The first group, called by Nannfeldt the Ascohymeniales, includes the 

 apothecial orders Pezizales, Lecanorales, and others included in Chapter 

 9, and three orders with true perithecia, Sphaeriales, Hypocreales, and 

 Pyrenulales, discussed in this chapter. The second group, called by Nann- 

 feldt the Ascoloculares, includes what the author considers to constitute 

 the three orders Pseudosphaeriales, Dothideales, and Hemisphaeriales, 



