CHAPTER II 



ASCOMYCETES 



The Ascomycetes include over 15,000 species, all of which, excepting only 

 the yeasts, possess a well-developed mycelium of richly branched and septate 

 hyphae. The cells of the mycelium may be uninucleate, as in the Erysi- 

 phaceae and species of Chaetomium and Sordaria, or they may contain 

 a few or several nuclei; energetic growth and a rapid succession of nuclear 

 divisions often cause the nuclei of multinucleate elements to be arranged 

 in pairs. 



Multiplication may take place by means of conidia, oidia, or chlamydo- 

 spores, but the characteristic method of reproduction is by ascospores or 

 spores produced in the interior of a mother-cell or ascus. 



The Ascospores. In the large majority of species the ascospores are 

 elliptical in outline, but they may be spherical or globose, as in Ascodesmis 

 nigricans, long and narrow (filiform) as in species of Cordyceps or Claviceps, 

 or of intermediate form. They contain a densely granular or reticulate 

 cytoplasm in which one or more oil drops are usually present. The epispore 

 may be smooth or variously sculptured ; in several cases it is verrucose, in 

 others reticulate; the latter arrangement is particularly well seen in Asco- 

 bolus furfuraceus. 



All ascospores are colourless when first formed; they may remain so 

 when ripe, or may assume a variety of colours. 



The young spore is unicellular and uninucleate 1 . Before it is set free the 

 nucleus may divide, and this division is frequently accompanied by wall- 

 formation so that, like the asexual spore of Pcllia, the ascospore may be 

 regarded as undergoing premature germination. Divisions may take place 

 in three dimensions, so that the spore is muriform ( fig. 1 ), or the septa may 

 be all in one plane so that it consists of a row of cells (fig. 2 a). 



The multicellular spore thus produced may break up into its constituent 

 cells, which proceed with their development independently, or it may function 

 as a single structure. 



In examining any of these characters or in measuring the size of the 

 spore it is essential to deal with fully mature examples since the distinctive 

 sculpturing, pigmentation, etc., may not appear till late in development. 

 Spores which have escaped naturally from the ascus may be used with safety. 



1 Lewton Brain [Ann. Bot. 1901) describes the spores of Cordyceps ophioglossoides as multi- 

 nucleate from their first inception. 



