number differ very strongly from all green plants both in
their habits and in their structure and reproduction. It is a
much-disputed point whether sexual reproduction occurs in any of them,
and it is highly probable that in the great majority, at any rate, the
reproduction is purely non-sexual.

Probably to be reckoned with the _Mycomycetes_, but of doubtful
affinities, are the small unicellular fungi that are the main causes
of alcoholic fermentation; these are the yeast fungi (_Saccharomycetes_).
They cause the fermentation of beer and wine, as well as the incipient
fermentation in bread, causing it to "rise" by the giving off of
bubbles of carbonic acid gas during the process.

If a little common yeast is put into water containing starch or sugar,
and kept in a warm place, in a short time bubbles of gas will make
their appearance, and after a little longer time alcohol may be
detected by proper tests; in short, alcoholic fermentation is taking
place in the solution.

If a little of the fermenting liquid is examined microscopically, it
will be found to contain great numbers of very small, oval cells,
with thin cell walls and colorless contents. A careful examination
with a strong lens (magnifying from 500-1000 diameters) shows that
the protoplasm, in which are granules of varying size, does not fill
the cell completely, but that there are one or more large vacuoles
or spaces filled with colorless cell sap. No nucleus is visible in
the living cell, but it has been shown that a nucleus is present.

If growth is active, many of the cells will be seen dividing. The
process is somewhat different from ordinary fission and is called
budding (Fig. 37, _B_). A small protuberance appears at the bud or
at the side of the cell, and enlarges rapidly, assuming the form of
the mother cell, from which it becomes completely separated by the
constriction of the base, and may fall off at once, or, as is more
frequently the case, may remain attached for a time, giving rise
itself to other buds, so that not infrequently groups of half a
dozen or more cells are met with (Fig. 37, _B_, _C_).

[Illustration: FIG. 37.--_A_, single cells of yeast. _B_, _C_, similar
cells, showing the process of budding, x 750.]

That the yeast cells are the principal agents of alcoholic
fermentation may be shown in much the same way that bacteria are shown
to cause ordinary decomposition. Liquids from which they are ex