Fungi imperfecti (Incompletely known Fungi).
1. The =Saccharomyces-forms= are Fungi which are only known in their
yeast-conidial form. They are _conidia of higher Fungi_ which can
multiply to an unlimited extent by budding in nutritive solutions, and
in this way maintain their _definite_ size and shape. The budding takes
place _only at the ends_ of the conidia. The wall of the conidium forms
at one or at both ends a small wart-like outgrowth, which gradually
becomes larger, and is finally separated from its mother-cell as an
independent cell, surrounded by a closed cell-wall (Fig. 182 _a_, _b_).
[Illustration: FIG. 182.--Beer-yeast (_Saccharomyces cerevisiæ_):
_a-b_ (× 400); _c-f_ (× 750); _c_ a cell in the process of
forming spores; _d_ a cell with four ripe spores; _e_ the
spores liberated by the dissolution of the cell-wall; _f_ three
germinating spores; _g_ mycelium-like cell-chains. (× 1000: after
Em. Chr. Hansen.)]
Under very favourable conditions multiplication occurs so rapidly that
the daughter-cells themselves commence to form buds, before they have
separated from their mother-cell, with the result that pearl-like
chains of cells are produced. When the yeast-cells have only limited
nutriment, with an abundant supply of air, at a suitable temperature,
an endogenous formation of _spores_ takes place. The protoplasm of the
cells divides into 1–4 (rarely a greater number) masses (Fig. 182 _c_,
_d_, _e_) which surround themselves with a thick cell-wall, and in this
state can withstand adverse conditions and periods of dryness lasting
for several months.
The _sporangia are not asci_ since they have no definite form, and
a definite number, form and size of spores is not found. The spores
in the different species and kinds occupy varying periods for their
development, although exposed to the same temperature, a fact of
importance in determining one from another. On germination the wall of
the mother-cell is destroyed, and each spore gives rise to a new cell,
multiplication taking place by budding (Fig. 182 _f_). The majority of
Yeast-Fungi are able to produce alcoholic fermentation in saccharine
fluids.
The most important of these Fungi is the Beer-yeast (_Saccharomyces
cerevisiæ_) with ovate, ellipsoidal or spherical cells (Fig. 182). It
is a plant which has been cultivated from time immemorial, on account
of its property of producing alcoholic fermentation in sugar-containing
extracts (wort), derived from germinating barley (malt). Carbonic
acid is also set free during this process. The “surface-yeast” (Fig.
182 _a_), which produces ordinary beer when the brewing takes place
at higher temperatures, has cell-chains; “sedimentary yeast” (Fig.
182 _b_), used in the brewing of Bavarian beer, has spherical cells,
solitary, or united in pairs. Both these and the following Yeast-Fungi
include, according to Hansen, several species and kinds.
[Illustration: FIG. 183.--_Saccharomyces mycoderma._]
The “Ferment of Wine” (_Saccharomyces ellipsoideus_) produces wine in
the juice of grapes. Uncultivated yeast-cells are always present on
grapes; an addition of this species to the “must” is not necessary to
secure fermentation. A large number of other “uncultivated” yeast-cells
appear in breweries mixed with the cultivated ones, and cause different
tastes to the beer (_S. pastorianus_, etc.). _S. ludwigii_, found,
for instance, on the slimy discharge from Oaks, produces abundant
cell-chains on cultivation. _S. apiculatus_ is very frequently met with
on all kinds of sweet fruits, it has orange-like cells. _S. mycoderma_
has cylindrical cells, often united together in chains (Fig. 183): it
forms a whitish-gray mass (“fleur de vin”) on wine, beer, fruit-juice,
etc., standing in bottles uncorked or not entirely filled. It is
thought that this Fungus causes decomposition and oxydises the fluid
in which it is found, but it cannot produce alcoholic fermentation
in saccharine liquids, and it does not form endospores; hence it is
uncertain whether it is true _Saccharomyces_.
[Illustration: FIG. 184.--_Oidium lactis_: _a_ branched hypha
commonly met with; _b_ a hypha lying in milk and producing aerial
hyphæ which give rise to oidia; _c_ a branch giving rise to
oidia, the oldest (outermost) oidia are becoming detached from
one another; _d_ a chain of divided cells; _e_ germinating oidia
in different stages (slightly more magnified than the other
figures).]
The “Dry-yeast” used in baking white bread is “surface-yeast.” In
_leaven_, a kneaded mixture of meal, barm and water, which is used for
the manufacture of black bread, _Saccharomyces minor_ is present, and
a species allied to this produces alcoholic fermentation in dough with
the evolution of carbonic acid, which causes the dough to “rise.”
2. =Oidium-forms.= Of many Fungi only the Oidium-forms are known,
which multiply in endless series without employing any higher form of
reproduction. _Oidium lactis_ (Fig. 184) is an imperfectly developed
form which frequently appears on sour milk and cheese. It can produce a
feeble alcoholic fermentation in saccharine liquids. Thrush or aphthæ
(_O. albicans_) appears as white spots in the mouths of children.
Several similar _Oidium-forms_ are parasites on the skin and hair
of human beings, and produce skin diseases, such as scurvy (_O.
schoenleinii_) and ringworm (_O. tonsurans_).
3. =Mycorhiza.= These Fungi, which have been found on the roots of many
trees and heath-plants, particularly Cupuliferæ and Ericaceæ, consist
of septate hyphæ, and belong partly to the Hymenomycetes, partly to
the Gasteromycetes. It has been shown that the Mycorhiza enters into a
symbiotic relationship with the roots of higher plants.