The segments of the hyphae in this group usually contain several
nuclei. At the time of sporangial formation the protoplasm with
numerous nuclei streams into the swollen end of the sporangiophore and
there becomes cut off by a cell-wall to form the sporangium. The
protoplasm then becomes cut up by a series of clefts into a number of
smaller and smaller pieces which are unicellular in _Pilobolus_,
multicellular in _Sporodinia_. These then become surrounded by a
cell-wall and form the spores. This mode of spore-formation is totally
different from that in the ascus; hence one of the difficulties of the
acceptance of Brefeld's view of the homology of ascus and sporangium.
The cytology of zygospore-formation is not known in detail; the
so-called gametes which fuse are multinucleate and are no doubt of the
nature of gametangia. The fate of these nuclei is doubtful, probably
they fuse in pairs (fig. 6).
Blakeslee has lately made some very important observations of the
Zygomycetes. It is well known that while in some forms, e.g.
_Spordinia_, zygospores are easily obtained, in others, e.g. most
species of _Mucor_, they are very erratic in their appearance. This
has now been explained by Blakeslee, who finds that the Mucorinae can
be divided into two groups, termed homothallic and heterothallic
respectively. In the first group zygospores can arise by the union of
branches from the _same_ mycelium and so can be produced by the growth
from a single spore; this group includes _Spordinia grandis_,
_Spinellus fusiger_, some species of _Mucor_, &c. The majority of
forms, however, fall into the heterothallic group, in which the
association of branches from two mycelia _different in nature_ is
necessary for the formation of zygospores. These structures cannot
then be produced from the product of a single spore nor even from the
thalli derived from _any_ two spores. The two kinds of thalli
Blakeslee considers to have a differentiation of the nature of sex and
he distinguishes them as ( + ) and (-) forms; the former being usually
distinguished by a somewhat greater luxuriance of growth.
The classification of the Mucorini depends on the prevalence and
characters of the conidia, and of the sporangia and zygospores--e.g.
the presence or absence of a columella in the former, the formation of
an investment round the latter. Most genera are saprophytes, but
some--_Chaetocladium_, _Piptocephalis_--are parasites on other
Mucorini, and one or two are associated casually with the rotting of
tomatoes and other fruits, bulbs, &c., the fleshy parts of which are
rapidly destroyed if once the hyphae gain entrance. Even more
important is the question of mycosis in man and other animals,
referred to species of _Mucor_, and investigated by Lucet and
Costantin. Klebs has concluded that transpiration is the important
factor in determining the formation of sporangia, while
zygote-development depends on totally different conditions; these
results have been called in question by Falck.
The _Entomophthoraceae_ contain three genera, _Empusa_,
_Entomophthora_ and _Basidiobolus_. The two first genera consist of
forms which are parasitic on insects. _Empusa Muscae_ causes the
well-known epidemic in house-flies during the autumn; the dead,
affected flies are often found attached to the window surrounded by a
white halo of conidia. _B. ranarum_ is found in the alimentary canal
of the frog and growing on its excrement. In these three genera the
conidia are cast off with a jerk somewhat in the same way as the
sporangium of _Pilobolus_.
B. HIGHER FUNGI.--Now that Brefeld's view of the origin of these forms
from the Zygomycetes has been overthrown, the relationship of the higher
and lower forms of fungi is left in obscurity. The term _Eumycetes_ is
sometimes applied to this group to distinguish them from the
Phycomycetes, but as the same name is also applied to the fungi as a
whole to differentiate them from the Mycetozoa and Bacteria, the term
had best be dropped. The Higher Fungi fall into three groups: the
_Ustilaginales_, of doubtful position, and the two very sharply marked
groups _Basidiales_ and _Ascomycetes_.
[Illustration: From Vine's _Students' Text Book of Botany_, by
permission of Swan Sonnenschein & Co.
FIG. 7.--Germinating resting-gonidia. A, of _Ustilago receptaculorum_;
B, of _Tilletia Caries_.
sp, The gonidium.
pm, The promycelium.
d, The sporidia: in B the sporidia have coalesced in pairs at v.]
I. _Ustilaginales._--This includes two families Ustilaginaceae (smuts)
and Tilletiaceae (bunts). The bunts and smuts which damage our grain
and fodder plants comprise about 400 species of internal parasites,
found in all countries on herbaceous plants, and especially on
Monocotyledons. They are remarkable for their dark spores developed in
gall-like excrescences on the leaves, stems, &c., or in the fruits of
the host. The discovery of the yeast-conidia of these fungi, and their
thorough investigation by Brefeld, have thrown new lights on the
group, as also have the results elucidating the nature of the ordinary
dark spores--smuts, bunt, &c.--which by their mode of origin and
development are chlamydospores. When the latter germinate a slender
"promycelium" is put out; in _Ustilago_ and its allies this is
transversely septate, and bears lateral conidia (sporidia); in
_Tilletia_ and its allies non-septate, and bears a terminal tuft of
conidia (sporidia) (fig. 7). Brefeld regarded the promycelium as a
kind of _basidium_, bearing lateral or terminal conidia (comparable to
_basidiospores_), but since the number of basidiospores is not fixed,
and the basidium has not yet assumed very definite morphological
characters, Brefeld termed the group _Hemibasidii_, and regarded them
as a half-way stage in the evolution of the true Basidiomycetes from
Phycomycetes, the _Tilletia_ type leading to the true basidium
(Autobasidium), the _Ustilago_ type to the protobasidium, with lateral
spores; but this view is based on very poor evidence, so that it is
best to place these forms as a separate group, the _Ustilaginales_.
The yeast-conidia, which bud off from the conidia or their resulting
mycelium when sown in nutrient solutions, are developed in successive
crops by budding exactly as in the yeast plant, but they cannot
ferment sugar solutions. It is the rapid spread of these yeast-conidia
in manure and soil waters which makes it so difficult to get rid of
smuts, &c., in the fields, and they, like the ordinary conidia,
readily infect the seedling wheat, oats, barley or other cereals.
Infection in these cases occurs in the seedling at the place where
root and shoot meet, and the infecting hypha having entered the plant
goes on living in it and growing up with it as if it had no parasitic
action at all. When the flowers form, however, the mycelium sends
hyphae into the young ovaries and rapidly replaces the stores of sugar
and starch, &c., which would have gone to make the grain, by the
soot-like mass of spores so well known as smut, &c. These spores
adhere to the grain, and unless destroyed, by "steeping" or other
treatment, are sown with it, and again produce sporidia and
yeast-conidia which infect the seedlings. In other species the
infection occurs through the style of the flower, but the fungus after
reaching the ovule develops no further during that year but remains
dormant in the embryo of the seed. On germination, however, the fungus
behaves in the same way as one which has entered in the seedling
stage. The cytology of these forms is very little known; Dangeard
states that there is a fusion of two nuclei in the chlamydospore, but
this requires confirmation. Apart from this observation there is no
other trace of sexuality in the group.
II. _Ascomycetes._--This, except in the case of a few of the simpler
forms, is a very sharply marked group characterized by a special type
of sporangium, the ascus. In the development of the ascus we find two
nuclei at the base which fuse together to form the single nucleus of
the young _ascus_. The single nucleus divides by three successive
divisions to form eight nuclei lying free in the protoplasm of the
ascus. Then by a special method, described first by Harper, a mass of
protoplasm is cut out round each nucleus; thus eight uninucleate
ascospores are formed by free-cell formation. The protoplasm remaining
over is termed _epiplasm_ and often contains glycogen (fig. 8). In
some cases nuclear division is carried further before spore-formation
occurs, and the number of spores is then 16, 32 and 64, &c.; in a few
cases the number of spores is less than eight by abortion of some of
the eight nuclei. The ascus is thus one of the most sharply
characterized structures among the fungi.
In some forms we find definite male and female sexual organs
(_Sphaerotheca_, _Pyronema_, &c.), in others the antheridium is
abortive or absent, but the ascogonium (oogonium) is still present and
the female nuclei fuse in pairs (_Lachnea stercorea_, _Humaria
granulata_, _Ascobolus furfuraceus_); while in other forms ascogonium
and antheridium are both absent and fusion occurs between vegetative
nuclei (_Humaria rutilans_, and probably the majority of other forms).
In other cases the sexual fusion is apparently absent altogether, as
in _Exoascus_. In the first case (fig. 9) we have a true sexual
process, while in the second and third cases we have a _reduced_
sexual process in which the fusion of other nuclei has replaced the
fusion of the normal male and female nuclei. It is to be noted that
all the forms exhibit the fusion of nuclei in the ascus, so that those
with the normal or reduced sexual process described above have two
nuclear fusions in their life-history. The advantage or significance
of the second (ascus) fusion is not clearly understood.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.
FIG. 8.--Development of the Ascus.