450 



BOTANY 



PART II 



FIG. 395. Saccharomyces cerevisiae. A, Yeast-cell ; B, C> 

 yeast -cell budding; D, ascus with four spores, 

 (x 1125. After GUILLIERMOND.) 



by means of long tubes before spore-formation. These nuclear fusions possibly 



correspond to those in the young asci of other Ascomycetes. 



Physiologically these fungi are remarkable for their power of exciting, by means 



of an enzyme (zymase), the fermentation of saccharine solutions, alcohol and 



carbon dioxide being produced (cf. p. 274). The beer yeast (Saccharomyces 



cerevisiae} is only known in the 

 cultivated form ; the wine yeast (S. 

 eUipsoideus}, on the other hand, 

 occurs regularly in the soil of vine- 

 yards in the spore-form ; the latter 

 is therefore always present on the 

 grapes and need not be added to 

 the grape-juice. Other genera, in 

 some of which a mycelium is de- 

 veloped, belong to this order. 



No evidence is at present forth- 

 coming to show that the Yeasts are to be regarded as developmental forms of other 



fungi. In various members of the 



Exoasci and Ustilagineae, however, 



yeast-like conidia which reproduce 



by budding are known. Possibly 



the Saccharomycetes are reduced 



Ascomycetes, or they may represent 



an independent group of very 



simple fungi at the base of the 



Ascomycetes. 



Owing to their richness in 



readily digestible substances, 



especially proteids and glycogen, 



but also fats, yeast has a consider- 

 able food- value. It is purified, dried 



at 125 C., and sold for this purpose. 



Order 8. Laboulbeniaceae ( 74 ) 



The Laboulbeniaceae are a group 

 of minute fungi occupying an 

 isolated position ; our knowledge 

 of them is largely due to the work 

 of THAXTER. Their thallus con- 

 sists of two to a number of cells, 

 and is attached to the body of the 

 insect, most commonly a beetle, 

 on which it is parasitic by means FIG. 396. Stigmatomyces Baerii. Description in text. 



A, Spore. B-F, Successive developmental stages. D. 

 With spermatia escaping from the antheridia an. E. 

 With antheridia above and the lateral female organ, 

 F, Perithecium with developing asci. G, Ripe ascus 

 (After THAXTER.) 



of a pointed process of the lowest 



cell inserted into the chitinous 



integument of the insect, or by 



means of rhizoids which penetrate 



more deeply. Stigmatomyces Baerii 



which occurs on house-flies in Europe may be taken as an example. The 



bicellular spore (Fig. 396 A), which has a mucilaginous outer coat, becomes 



attached by its lower end (}, and divisions occur in both cells (C). From 



