PRINCIPAL GROUPS OF PLANTS. 



47 



three membranes, two belonging to the spore proper and the third 

 being formed by the united hyphse. As would be expected, these 

 spores are quite resistant, being able to withstand unfavorable 

 conditions, and germinate (Fig. 23, C) only after a period of rest. 

 ASCOMYCETES. — The Ascomycetes are distinguished for 

 the most part, like the other higher Fungi, in having a septate 

 mycelium, i.e., one cellular in structure, and in producing asci 



Fig. 25. Species of Saccharomyces (Yeasts). A, 5. cerevisioe or beer yeast; B, 5. 

 Pastorianus; C, 5. glomeralus; D. 5. Piculalus: a, vegetative cells reproducing by budding; 

 b, formation of ascospores. — After Reesz. 



(sacs), which latter are formed at the ends of the branches of 

 the mycelia. Two main sub-groups are recognized, the one 

 producing an indefinite number of spores m asci which are not 

 well developed, and known as the Heaiiasci ; the other producing 

 a definite number of spores, which number is characteristic for 

 each species, in a well-developed ascus, and known as the Euasci. 

 In the latter group the spores arise by successive divisions of the 

 primary nucleus into two, as shown in Peniza condiicns (Fig. 24). 

 Yeasts.— The simplest of the Ascomycetes is the sub-group 

 known as the Saccharomyces, or Yeasts. The Yeasts do not 

 produce a mycelium, but the plant body consists of a single cell, 

 or a chain of cells, and multiplies by a peculiar process known 

 as '' yeast budding" (Fig. 25, a). From either end of the cell 

 a wart-like process develops, which enlarges until about the size 



