Brown. — Some Studies on Yeast . 
225 
perfectly useless for the economics of the cell, and immensely dispropor- 
tionate to any ‘ toll which the mill requires to keep it in working 
order. 
The explanation of this apparent paradox appears to lie in the fact 
that the conditions under which we generally cultivate yeast, that is to say, 
in comparatively large masses of liquid containing but a very limited supply 
of oxygen, are eminently artificial. They have been imposed by the 
exigencies of industries in which the fermentative faculty of the yeast-cells 
has been purposely enhanced, whilst the reproductive faculties have been 
restricted. These artificial conditions differ in toto coelo from the natural con- 
ditions under which the specific physiological characters of the Saccharomy- 
cetes have been evolved. If we wish to study the question in a philosophical 
manner, it is to the vineyard and orchard we must have recourse, rather 
than to the vats of the wine-maker, the brewer, and distiller. 
The natural habitat of the various forms of yeasts is the outer skin of 
fruits, and especially those succulent fruits which contain abundance 
of sugar when ripe . 1 They may, for instance, be detected somewhat 
sparsely scattered over the skin of a ripe grape, awaiting their opportunity 
of gaining access to the stores of nutrient material from -which they are 
separated by a thin semi-permeable membrane. This opportunity arises 
the moment the skin of the grape is ruptured, and such a rupture may be 
brought about by a variety of natural causes, such as the undue swelling of 
the ripe fruit after rain, the attack of birds, insects, or fungi, or by the 
accidental crushing of the berry by the foot of a passing animal. 
The extraneous yeasts and their ascospores now find themselves in 
a medium rich in all the nutrient substances they require, and one to which 
atmospheric oxygen can gain ready and continuous access, owing to the 
small scale of the natural operations. 
Hence, as one may readily see in the vineyard at vintage time, 
extremely rapid cell- multiplication takes place, which has not the same 
check imposed upon it as it has when the cultivations are made on a large 
scale in the wine-makers’ vats. This rapid and constant building up of new 
cells, under the continued action of oxygen, requires a constant source of 
extraneous energy, which is furnished by the ancillary fermentative function 
of the cells. Moreover, since the cell-growth is going on in a medium of 
high specific heat, and under conditions in which there is a natural tendency 
for rapid equalization of temperature with the surroundings, it is manifestly 
to the advantage of the organism to have some other and more intense 
source of energy than that supplied by the respiratory processes. By 
a study of these natural cases, it becomes easier to understand the true 
1 The origin of the wine-yeasts is obvious. That of the various races of brewers’ yeast is not 
known with certainty, any more than the origin of some of our domestic animals and cereals. 
