I] SAPROPHYTISM ii 



In its simplest possible form it may be represented by the equation: 

 QH,,Os=2QH„0 + 2CO, 



Only certain monosaccharides with the formula CsHijOb (glucose, fructose) 

 are capable of undergoing alcoholic fermentation; polysaccharides (cane- 

 sugar, lactose, maltose) must undergo a preliminary hydrolysis resulting in 

 the production of appropriate monosaccharides before alcoholic fermentation 

 can take place. 



The majority of the fungi capable of inducing these changes are yeasts 

 (Saccharomycetaceae) acting either alone or in conjunction with appropriate 

 bacteria; they are very commonly present as epiphytes on the skin of ripe 

 fruits and feed on the drops of sugary solution that may be exuded or escape 

 where the skin is broken. At other times, even in the winter, they may be 

 found in the neighbouring soil, but they are very rarely present upon unripe 

 fruits, presumably because any cells that happen to be carried there soon die. 



A number of yeasts are made use of economically in baking, where their 

 value depends on the formation of carbon dioxide, causing the dough to "rise," 

 as well as in brewing and the other processes concerned with the production 

 of alcohol. The characteristic yeast of wine which ferments glucose (or grape- 

 sugar)is found in abundance at vintage time on the grapes and their stalks, and 

 the cider yeasts on apples ; the yeast of beer on the other hand, which acts 

 on the sugar formed in germinating barley, is not known in the wild state. 



, In the production of a number of alcoholic beverages the yeast acts sym- 

 biotically with one or more bacteria ; this is the case in the group of organisms 

 included in the "ginger-beer plant "^ which, added to commercial ginger, 

 sugar and water, causes the formation of ginger-beer. The "plant" has the 

 appearance of lumpy irregular masses rather like pieces of soaked tapioca 

 or sago; its essential constituents, as Marshall Ward demonstrated in 1892, 

 are the yeast Saccharomyces pyreformis and the bacterium B. vermiforme; 

 the bacterium is able to utilize the products of the metabolism of the yeast, 

 and can do so most successfully at their first formation, that is, in the 

 neighbourhood of the living yeast cell; the yeast benefits by the removal of 

 these substances, the accumulation of which would inhibit its development, 

 and is able, in the presence of the bacterium (and of appropriate food 

 materials), to continue its activity for weeks as shown by the evolution of 

 carbon dioxide. The relation between the two organisms is thus a symbiosis 

 in which each constituent gains by its association with the other. 



A similar combination appears to exist in the materials used for two 

 different fermentations of lactose or milksugar which underlie the productions 

 of the beverages known as kephir and koumiss. Kephir is prepared from 



^ The ginger-beer plant is also known as Californian bees and by other popular names; both 

 lately and after the Crimean War a tradition arose that it had been brought to this country by 

 soldiers from overseas. Cf. J. Ramsbottom, Trans. Brit. Myc. Soc. 1920, p. 85. 



