Yes,
the 700 known yeasts really are fungi, although their colonies
often look more like those of bacteria.
Most
fungi explore their surroundings by producing miles of fine,
branching filaments called hyphae, but most yeasts have
become more or less unicellular, with rounded cells. This
is often an adaptation to living in a liquid medium of high
osmotic pressure. This usually means media with a high sugar
content, such as is found in the nectaries of flowers or
on the surface of fruits, where if they present the least
possible surface area (as close to spherical as possible),
it makes it easier for them to control the movement of dissolved
substances in and out of their cells.
The
cells of most yeasts can be regarded as asexual propagules,
and they produce more of the same by a variety of methods
similar to those found in moulds.
Some
yeasts are related to ascomycetes, others to basidiomycetes,
and even zygomycetes sometimes take on a yeast-like appearance.
Some
yeasts make hyphae as well as unicells, and some are even
exclusively hyphal, being recognizable as yeasts only by
biochemical characters.
Yeasts
are, of course, among the most important fungi, because
they raise bread, ferment sugars to make beer, wine, and
spirits, and represent a concentrated food and a source
of B vitamins.
