120 MORPHOLOGY AND LIFE-HISTOEY OF YEASTS. 



Up to the present the composition of the deposit in wine 

 and the " wine yeast," in the sense used by tartaric acid manu- 

 facturers, has not been so thoroughly examined as is the case 

 with the sedimental yeast of breweries. Next to yeast, the 

 chief constituent is cream of tartar, which is gradually pre- 

 cipitated in the crystalline form by the increasing amount of 

 alcohol in the fermenting wine. 



When exposed to very unfavourable conditions, the yeast 

 cell assumes a true involution form. Cells of this kind, re- 

 sembling amcebaj in contour, are found, for example, in very 

 old gypsum-plate cultures, or similar cultures on wort gelatin. 

 They have been drawn by P. Lindner (VI. and X.). Many 

 cells that have become exposed to unfavourable conditions in 

 sedimental yeast are able to withstand the same and survive 

 uninjured, because they have previously laid up a considerable 

 store of material and greatly strengthened their membrane. 

 Under these circumstances they become permanent cells, which 

 may be regarded as gemmae (§ 223). These forms will be further 

 discussed in §§ 246, 249, and 251. 



§ 246.— Film Formation. 



Yeast cultures, started in the manner foreshadowed at the 

 commencement of the preceding paragraph, are able to exhibit 

 another form of development. If the samples have been taken 

 with every care, all infection being prevented, and the vessels 

 again closed with their germ-proof plugs of cotton-wool — which, 

 however, do not exclude air — all that is then necessary is to 

 store them in a quiet corner of the laboratory for some time 

 (several weeks or even months), to find that film has developed 

 on the surface of the liquid. The structure and physiological 

 behaviour of this film will now be described. 



This form of development originates in cells, which, thanks 

 either to the presence of some fatty excretion adhering to the 

 outside of the membrane, or to some albuminous or resinous 

 deposit acquired from the nutrient medium, are able to float on 

 the surface of the liquid when the latter becomes quiescent after 

 fermentation has terminated. A good opportunity for these 

 cells to remain afloat is afforded at the places where the surface 

 of the liquid is in contact with the walls of the (round) culture 

 vessel. Hence, it is in such positions that the development of 

 the film will be earliest observed, in the form of a whitish ring. 

 In many instances this ring is by no means complete at first, 

 but appears in the form of a series of patches, i.e. colonies of 

 cells, which gradually increase and then meet, coalescing to a 

 closed ring. In other cases the annular formation is less fully 

 developed, and the growth of the film proceeds from small 



