646 FUNGI 



the presence of which in yeast gives to it the power of exciting the 

 alcoholic fermentation in .saccharine liquids. When a small drop of 

 yeast is placed under a magnifying power of 400 or 500 diameters, 

 it is seen to consist of a large number of globular or ovoid cells, 

 averaging about g-^o^th of an inch in diameter, for the most part 

 isolated, but sometimes connected in short series ; and each cell 

 is filled with a nearly colourless ' endoplasm,' usually exhibiting 

 one or more vacuoles. When placed in a fermentable fluid con- 

 taining some form of nitrogenous matter in addition to sugar, 1 

 they vegetate in the manner represented in fig. 482. Each cell 

 puts forth one or two projections, which seem to be young cells 

 developed as buds or offsets from their predecessors ; these, in 

 the course of a short time, become complete cells, and again per- 

 form the same process ; and in this manner the single cells of yeast 

 develop themselves, in the course of a few hours, into rows of four, 

 five, or six, which remain in connection with each other whilst the 

 plant is still growing, but which separate if the fermenting process 

 be checked, and return to the isolated condition of those which 

 originally constituted the yeast. Thus it is that the quantity of 

 yeast first introduced into the fermentable fluid is multiplied six 



*fi 



a b 



FIG. 482. Saccliaromyces cerevisitz, or yeast-plant, as developed during the process 

 of fermentation : a, b, c, cl, successive stages of cell-multiplication. 



times or more during the changes in which it takes part. Under 

 certain conditions not yet determined, the yeast-cells multiply in 

 another mode namely, by the breaking up of the endoplasm into 

 segments, usually four in number, around each of which a new ' cell- 

 wall ' forms itself ; and these endogenous spores are ultimately set 

 free by the dissolution of the wall of the parent cell, and soon enlarge 

 and comport themselves as ordinary yeast-cells. The process of the 

 formation of these spores resembles in all essential points the 

 formation of ascospores ; and hence Torala is regarded as a low or 

 degraded type of that order. Many other fungi of like simplicity 

 have the power to act as ' ferments ; ' thus in wine-making the 

 fermentation of the juices of the grapes or other fruit employed is set 

 going by the development of minute fungi whose germs have settled 

 on their skins, these germs not being injured by desiccation, and 

 being readily transported by the atmosphere in the dried-up state. 



1 It appears from the researches of Pasteur that, although the presence of alliu- 

 minous matter (such as is contained in a saccharine wort, or in the juices of fruits) 

 favours the growth and reproduction of yeast, yet that it can live and multiply in a 

 solution of pure sugar, containing ammonium tartrate with small quantities of mineral 

 salts, the decomposition of the ammonia salt affording it the nitrogen it requires for 

 the production of protoplasm, while the sugar and water supply the carbon, oxygen, 

 and hydrogen. 



