130 BACTERIA AND THE CARBON DIOXIDE CYCLE 



on their skins numerous yeast cells, which multiply rapidly on burst or 

 broken grapes and are carried about by insects from one plant to another. 

 After the grapes have been gathered, millions of yeast cells remain in 

 the soil, where they pass the winter until next year's crop supplies 

 them with fresh food. 



When the fermentation is to be made with a pure yeast, it is not 

 necessary to kill the Saccharomyces already present in the grape juice 

 by heat ; it is sufficient to add a large quantity of the pure culture, 

 the organisms in which usually easily overcome the less numerous ' wild ' 

 yeasts. 



The term ' species ' has the same meaning and value among the 

 Saccharomyces as among the bacteria, but it must not be forgotten that 

 the yeast fungi are among the oldest of cultivated plants, and that in the 

 course of ages innumerable varieties and races have arisen. The various 

 yeasts of breweries and distilleries must all be looked upon as varieties 

 of the one species ^. cerevisiae, and a few others, those used in wine- 

 making, as races of S. ellipsoideus (Fig. 35, a, c, d), a somewhat smaller, 

 thinner form than S. cerevisiae. 



The peculiar mode of propagation of the Sacchdromycetes characterizes 

 them as a definite and independent group of organisms, whose systematic 

 value is the same as that of any other order of fungi. This independence 

 would never have been challenged had not a similar budding-off of cells 

 been observed in some other fungi (106). Spores of the smut- fungus 

 (Ustilago) if sown in a decoction of horse-dung germinate and protrude 

 a small few-celled pro-mycelium, which then buds off laterally rounded 

 cells just in the same way as yeasts do. The free-lying rounded cells 

 multiply still further by budding, and give rise to cell groups and colonies 

 indistinguishable, as far as appearance goes, from true yeasts. They are, 

 however, quite unable to set up alcoholic fermentation. 



Again, the filamentous hyphae of some of the mucorine fungi, if 

 grown submerged in saccharine solutions, produce cells which sprout and 

 multiply like yeasts, and even cause a small quantity of alcohol to be 

 formed (Mucor racemosus, M. erectus, M. circinelloides). Finally, there 

 are ascomycetes (Exoascus) which give rise to budding cells, and whose 

 mode of sporulation (ascospores) is suggestive of the sporulajion of yeasts. 

 On these various and insufficient grounds the independence of the Saccharo- 

 mycetes as a group has been questioned, and the suggestion has been made 

 that they are nothing but derivatives of higher groups that have lost 

 the power of repeating the cycle of forms which their ancestors went 

 through. But none of the Saccharomycetes ever show any indications of 

 a higher type of growth ; they produce vegetative cells, cell groups and 

 spores, and never anything else. The announcements that have been 

 made from time to time, that true yeasts have been cultivated from higher 



