CHAPTER XIV 



THE CIRCULATION OF CARBON DIOXIDE IN NATURE 



(continued) 



3. The Yeast Fungi and Alcoholic Fermentation. Theory of Fermentation 

 and Anaerobiosis. Concluding Remarks on the Circulation of 

 Nitrogen and Carbon in Nature. 



The most important of all fermentative processes, from an industrial 

 point of view, the alcoholic or vinous fermentation, is the work not of 

 bacteria, but of yeast fungi. Some few bacteria are, as we have seen, able 

 to produce ethyl alcohol, e. g. B. ethaceticus, but in all the great technical 

 fermentations, in the manufacture of beer, wine, and spirits, the ferment 

 organisms are yeasts (Sacckaromycetes,Blastomycetes) (102, 103). The yeasts 

 are unicellular non-motile organisms, not cylindrical or spherical in shape 

 like the bacteria, but spheroidal (Fig. 25). The shape of the cells, to some 

 extent of value as a classificatory character, appears at first sight more 

 irregular than it really is. This is due to the remarkable manner of pro- 

 pagation. The yeast cells multiply not by fission into two equal halves, as 

 do bacteria and the cells of higher plants, but by ' budding.' The cell-wall 

 becomes evaginated at one or more points, and the bud-like projections thus 

 formed are shut off from the mother-cell by a new cell-wall. As the young 

 cells do not at once break away, and even begin to bud themselves before 

 connexion with the parent cells is severed, irregular colonial growths arise. 

 The contrast between this mode of reproduction and the multiplication by 

 fission of the bacteria is very great. Among the bacteria each new 

 generation represents half of the parent generation, which in the very 

 act of multiplication ceases to exist as an individual. In the yeasts a small 

 part only is separated from the parent, which continues to live and bud off 

 further descendants. The process of budding, like that of fission, is very 

 rapid ; a new generation may arise in a couple of hours, so that reproduction 

 is little less prolific in the yeasts than in the bacteria. 



As with bacteria, the successive generations of yeast cells often remain 



