The mycelium is bladder-like or branched. Zoospores. Sexual
reproduction by oospores, which are produced in oogonia. The latter are
fertilised, in some forms, by the antheridium.
Order 1. =Ancylistaceæ.= The entire bladder-like mycelium
is used for the construction of zoosporangia, oogonia, or
antheridia. _Lagenedium_ is parasitic on _Spirogyra_, etc.
Order 2. =Peronosporaceæ.= Almost entirely _parasites_. The
unicellular, often very long and abundantly branched mycelium lives
in the intercellular spaces of living plants, especially in the
green portions, and these are more or less destroyed and deformed in
consequence. Special small branches (_suction-organs_, “_haustoria_”)
are pushed into the cells in order to abstract nourishment from them.
Both oospores and conidia germinate either immediately, or they
develope into sporangia with swarmspores, having always two cilia. Only
one oospore is formed in each oogonium; its contents (Fig. 89) divide
into a centrally placed egg-cell and the “periplasm” surrounding it;
this is of a paler colour and on the maturity of the oospore forms its
thick, brown, external covering.
[Illustration: FIG. 88.--_Polyphagus euglenæ. A_ with smooth,
_B_ with thorny oospores; _m_ and _f_ the two conjugating cells.]
[Illustration: FIG. 89.--_Peronospora alsinearum._ Mycelium with
egg-cell and antheridium.]
[Illustration: FIG. 90.--_Phytophthora infestans_ (strongly
magnified). Cross section through a small portion of a
Potato-leaf (the under side turned upwards): _a_ the mycelium; _b
b_ two conidiophores projecting through a stoma; _c_ conidia; _e_
the spongy tissue of the leaf; _g_ the epidermis.]
_The Potato-fungus_ (_Phytophthora infestans_) is of great interest.
Its thallus winters in the Potato-tuber; other organs for passing the
winter, such as oospores, are not known. When the tuber germinates, the
Fungus-hyphæ penetrate the young shoot and keep pace with the aerial
growth and development of the plant. The conidiophores emerge through
the stomata, especially on the under side of the leaves; they branch
like a tree (Fig. 90), and appear to the naked eye as a fine mould on
the surface of the plant. The disease soon makes itself known by the
brown colouring of those parts of the plant which are attacked, and by
their withering. An ovoid conidium arises at first by the formation of
a dividing wall at the apex of each branch of the conidiophore (Fig. 90
_c c_), and immediately underneath it another is formed, which pushes
the first to one side, and so on. These conidia sometimes germinate
directly, and form a mycelium, but most frequently their protoplasm
divides into many small masses, each of which becomes a pear-shaped
zoospore provided with two cilia (Fig. 91). Water is required for
their germination, and when the ripe conidia are placed in a drop of
water the swarm-cells are formed in the course of about five hours.
They swarm about in rain and dewdrops in the Potato-fields, and are
carried with the water to the Potato-plants and to the tubers in
the soil. The wind also very easily conveys the conidia to healthy
Potato-fields and infects them. The enormous quantity of conidia and
swarm-cells that may be formed in the course of a summer explains the
rapid spreading of the disease; and the preceding makes it clear why
wet summers are favourable to its existence. When the swarm-cells
germinate, they round off, and then surround themselves with a
cell-wall which grows out into the germ-tube, and _pierces through the
epidermis_ of the host-plant (Fig. 92). Having entered the host, a new
mycelium is formed. The potato disease, since 1845, has been rampant in
Europe; it has, no doubt, been introduced from America, which, it must
be remembered, is the home of the Potato-plant.
[Illustration: FIG. 91.--_Phytophthora infestans_: _a-c_ conidia
detached; in _c_ the swarm-cells are leaving the mother-cell; _d_
two free-swimming swarm-cells.]
[Illustration: FIG. 92.--_Phytophthora infestans._ Cross section
through a portion of a Potato-stalk. Two germinating conidia
(_a_, _b_) piercing the epidermis, and the mycelium penetrating
the cells.]
The conidia exhibit various characters which are employed for
the separation of the genera. _Pythium_ is the most simple
form. The contents of the terminally-formed conidia emerge as
a spherical mass and divide into swarmspores. _P. de Baryanum_
lives in the seedlings of many different Flowering-plants,
which it completely destroys.--_Phytophthora_ is distinguished
by the circumstance that the sparsely-branched conidiophores
bear, sympodially, chains of conidia. Besides the Potato-fungus
(see above), _Ph. fagi_ belongs to this group; it developes
oospores very abundantly, and does great harm to seedlings of
the Beech, Sycamore, and Pine trees.--_Peronospora_ generally
has conidiophores which are repeatedly forked, and bear a
conidium on each of the most extreme ramifications. Many do
great harm to their host-plants. _P. viticola_, on Vines, and
_P. nivea_, on umbelliferous plants, have swarmspores, which are
absent in the following species of this genus: _P. sparsa_, on
Roses; _P. gangliformis_, on composites; _P. alsinearum_, on
Stitchwort; _P. parasitica_, on cruciferous plants; _P. viciæ_,
on Vetches and Peas; _P. schachtii_, on Beets; _P. violacea_,
on the flowers of _Scabiosa_; _P. radii_, on the ray-florets of
_Matricaria_.--_Cystopus_ (_Albugo_) has the conidia developed
in chains, which form a cohesive white layer underneath the
epidermis of the host-plant. _Cystopus candidus_, on cruciferous
plants, especially Shepherd’s Purse and _Brassica_; the
germination commences on the cotyledons, and from this point the
mycelium developes together with the host-plant; _C. cubicus_,
on the leaves of Compositæ.
[Illustration: FIG. 93.--A fly overgrown with _Saprolegnia_.]
[Illustration: FIG. 94.--Formation of swarmspores in a
_Saprolegnia_: a germinating swarmspores.]
Order 3. =Saprolegniaceæ=, _Water-Fungi_ which live as saprophytes on
organic remains lying in water, for instance, on dead flies (Fig. 93),
worms, remains of plants; but they may also make their appearance on
living animals, being frequently found, for example, on the young trout
in rearing establishments.
[Illustration: FIG. 95.--Oogonium with two antheridia, _Achlya
racemosa_.]
The thallus is a single, long and branched cell. It has one portion
which serves as root, and lives in the substratum, where it ramifies
abundantly for the purpose of absorbing nourishment; and another
portion projecting freely in the water, and sending out hyphæ on all
sides (Fig. 93). The asexual reproduction takes place by swarmspores
(Fig. 94), which are developed in large sporangia; these swarmspores
generally possess two cilia, and on germination grow into new plants.
The entire protoplasm in the oogonium is formed into one or more
oospheres, without any surrounding “periplasm.” The oospheres may not
be fertilised (p. 100), and then develope parthenogenetically.
Genera: _Saprolegnia_, whose swarmspores disperse immediately
after having left the sporangium. _S. ferax_ is the
cause of a disease in fish (“Salmon disease”) and in the
crayfish.--_Achlya_, whose swarmspores accumulate in a hollow
ball before the mouth of the sporangium.--_Leptomitus_ has
strongly indented hyphæ, causing a “linked” appearance.
_L. lacteus_ is frequent in the waste matter from sugar
factories.--_Monoblepharis_ deviates from the others by the
greater development of its fertilising process; the oosphere,
situated in an open oogonium, becoming fertilised by self-motile
spermatozoids, which are provided with a cilium at the posterior
end.