FUNGI: MUCOR AND SAPROLEGNIA.
Mucor.
=387.= In the chapter on growth, and in our study of protoplasm,
we have become familiar with the vegetative condition of mucor. We now
wish to learn how the plant multiplies and reproduces itself. For this
study we may take one of the mucors. Any one of several species will
answer. This plant may be grown by placing partially decayed fruits,
lemons, or oranges, from which the greater part of the juice has been
removed, in a moist chamber; or often it occurs on animal excrement
when placed under similar conditions. In growing the mucor in this way
we are likely to obtain Mucor mucedo, or another plant sometimes known
as Mucor stolonifer, or Rhizopus nigricans, which is illustrated in
fig. 191. This latter one is sometimes very injurious to stored fruits
or vegetables, especially sweet potatoes or rutabagas. Fig. 190 is from
a photograph of this fungus on a banana.
=388. Asexual reproduction.=—On the decaying surface of the
vegetable matter where the mucor is growing there will be seen numerous
small rounded bodies borne on very slender stalks. These heads contain
the gonidia, and if we sow some of them in nutrient gelatine or agar
in a Petrie dish the material can be taken out very readily for
examination under the microscope. Or we may place glass slips close
to the growing fungus in the moist chamber, so that the fungus will
develop on them, though cultures in a nutrient medium are much better.
Or we may take the material directly from the substance on which it is
growing. After mounting a small quantity of the mycelium bearing these
heads, if we have been careful to take it where the heads appear
quite young, it may be possible to study the early stages of their
development. We shall probably note at once that the stalks or upright
threads which support the heads are stouter than the threads of the
mycelium.
[Illustration: Fig. 190. Portion of banana with a mould (Rhizopus
nigricans) growing on one end.]
These upright threads soon have formed near the end a cross wall which
separates the protoplasm in the end from the remainder. This end cell
now enlarges into a vesicle of considerable size, the head as it
appears, but to which is applied the name of _sporangium_ (sometimes
called gonidangium), because it encloses the _gonidia_.
At the same time that this end cell is enlarging the cross wall is
arching up into the interior. This forms the _columella_. All the
protoplasm in the sporangium now divides into gonidia. These are
small-rounded or oval bodies. The wall of the sporangium becomes
dissolved, except a small collar around the stalk which remains
attached below the columella (fig. 192). By this means the gonidia are
freed. These gonidia germinate and produce the mycelium again.
[Illustration: Fig. 191. Group of sporangia of a mucor (Rhizopus
nigricans) showing rhizoids and the stolon extending from an older
group.]
=389. Sexual stage.=—This stage is not so frequently found, but
may sometimes be obtained by growing the fungus on bread.
Conjugation takes place in this way. Two threads of the mycelium which
lie near each other put out each a short branch which is clavate in
form. The ends of these branches meet, and in each a septum is formed
which cuts off a portion of the protoplasm in the end from that of the
rest of the mycelium. The meeting walls of the branches now dissolve
and the protoplasm of each gamete fuses into one mass. A thick wall
is now formed around this mass, and the outer layer becomes rough and
brown. This is the _zygote_ or _zygospore_. The mycelium dies and it
becomes free often with the suspensors, as the stalks of these sexual
branches are called, still attached. This zygospore passes through
a period of rest, when with the entrance of favorable conditions of
growth it germinates, and usually produces directly a sporangium with
gonidia. This completes the normal life cycle of the plant.
=390. Gemmæ.=—Gemmæ, as they are sometimes called, are often
formed on the mycelium. A short cell with a stout wall is formed on the
side of a thread of the mycelium. In other cases large portions of the
threads of the mycelium may separate into chains of cells. Both these
kinds of cells are capable of growing and forming the mycelium again.
They are sometimes called _chlamydospores_.
[Illustration: Fig. 194.
A mucor (Rhizopus nigricans); at left nearly mature sporangium with
columella showing within; in the middle is ruptured sporangium with
some of the gonidia clinging to the columella; at right two ruptured
sporangia with everted columella.]
=390=_a_. The Mucorineæ according to their manner of zygospore
formation are of two kinds: 1st, the _homothallic_ (monœcious), in
which all of the colonies of thalli developed from different spores
are the same, and both gametes may be developed from the mycelium
from a single spore, as in Sporodinia grandis, a mould common on old
mushrooms; 2d, the _heterothallic_ (diœcious), in which certain plants
are of a male nature and small in comparison with those of perhaps a
female nature which are larger or more vigorous. When grown separately
each of these two kinds of thalli, or colonies of mycelium, produce
their own kind but only sporangia. If the two kinds are brought
together, however, branches from one conjugate with branches from
the other and zygospores are produced, as in Rhizopus nigricans, the
common bread or fruit mould. This is one reason why we rarely find this
fungus forming zygospores. (See Blakeslee, Sexual Reproduction in the
Mucorineæ, Proc. Am. Acad. Arts and Sci., =40=, 205-319, pl. 1-4,
1904.)
Water Moulds (Saprolegnia).
=391.= The water moulds are very interesting plants to study
because they are so easy to obtain, and it is so easy to observe a type
of gonidium here to which we have referred in our studies of the algæ,
the motile gonidium, or zoogonidium. (See appendix for directions for
cultivating this mould.)
=392. Appearance of the saprolegnia.=—In the course of a few days
we are quite certain to see in some of the cultures delicate whitish
threads, radiating outward from the body of the fly in the water. A few
threads should be examined from day to day to determine the stage of
the fungus.
[Illustration: Fig. 195. Sporangia of saprolegnia, one showing the
escape of the zoogonidia.]
=393. Sporangia of saprolegnia.=—The sporangia of saprolegnia
can be easily detected because they are much stouter than the ordinary
threads of the mycelium. Some of the threads should be mounted in fresh
water. Search for some of those which show that the protoplasm is
divided up into a great number of small areas, as shown in fig. 195.
With the low power we should watch some of the older appearing ones,
and if after a few minutes they do not open, other preparations should
be made.
[Illustration: Fig. 196. Branch of saprolegnia showing oogonia with
oospores, eggs matured parthenogenetically.]
[Illustration: Fig. 197.
Downy mildew of grape (Plasmopora viticola), showing tuft of
gonidiophores bearing gonidia, also intercellular mycelium. (After
Millardet.)]
[Illustration: Fig. 198. Phytophthora infestans showing peculiar
branches; gonidia below.]
=394. Zoogonidia of saprolegnia.=—The sporangium opens at the end, and
the zoogonidia swirl out and swim around for a short time, when they
come to rest. With a good magnifying power the two cilia on the end may
be seen, or we may make them more distinct by treatment with Schultz’s
solution, drawing some under the cover glass. The zoogonidium is oval
and the cilia are at the pointed end. After they have been at rest for
some time they often slip out of the thin wall, and swim again, this
time with the two cilia on the side, and then the zoogonidium is this
time more or less bean-shaped or reniform.
[Illustration: Fig. 199.
Fertilization in saprolegnia, tube of antheridium carrying in the
nucleus of the sperm cell to the egg. In the right-hand figure a
smaller sperm nucleus is about to fuse with the nucleus of the egg.
(After Humphrey and Trow.)]
[Illustration: Fig. 200. Branching hypha of Peronospora alsinearum.]
[Illustration: Fig. 201. Branched hypha of downy mildew of grape
showing peculiar branching (Plasmopara viticola).]
=395. Sexual reproduction of saprolegnia.=—When such cultures are
older we often see large rounded bodies either at the end of a thread,
or of a branch, which contain several smaller rounded bodies as shown
in fig. 196. These are the oogonia (unless the plant is attacked by a
parasite), and the round bodies inside are the egg-cells, if before
fertilization, or the eggs, if after this process has taken place.
Sometimes the slender antheridium can be seen coiled partly around the
oogonium, and one end entering to come in contact with the egg-cell.
But in some species the antheridium is not present, and that is the
case with the species figured at 196. In this case the eggs mature
without fertilization. This maturity of the egg without fertilization
is called _parthenogenesis_, which occurs in other plants also, but is
a rather rare phenomenon.
[Illustration: Fig. 202.
Gonidiophores and gonidia of potato blight (Phytophthora infestans).
_b_, an older stage showing how the branch enlarges where it grows
beyond the older gonidium. (After de Bary.)]
[Illustration: Fig. 203. Gonidia of potato blight forming zoogonidia.
(After de Bary.)]
=396.= In fig. 199 is shown the oogonium and an antheridium,
and the antheridium is carrying in the male nucleus to the egg-cell.
Spermatozoids are not developed here, but a nucleus in the antheridium
reaches the egg-cell. It sinks in the protoplasm of the egg, comes
in contact with the nucleus of the egg, and fuses with it. Thus
fertilization is accomplished.
Downy Mildews.
=397.= The downy mildews make up a group of plants which are
closely related to the water moulds, but they are parasitic on land
plants, and some species produce very serious diseases. The mycelium
grows between the cells of the leaves, stems, etc., of their hosts,
and sends haustoria into the cells to take up nutriment. Gonidia are
formed on threads which grow through the stomates to the outside and
branch as shown in figs. 198-201. The gonidia are borne on the tips
of the branches. The kind of branching bears some relation to the
different genera. Fig. 200 is from Peronospora alsinearum on leaves of
cerastium; figs. 197 and 199 are Plasmopara viticola, the grape mildew,
while figs. 198 and 202 are from Phytophthora infestans which causes a
disease known as potato blight. The gonidia of peronospora germinate
by a germ tube, those of plasmopara first form zoogonidia, while in
phytophthora the gonidium may either germinate forming a thread, or
each gonidium may first form several zoogonidia, as shown in fig. 203.
[Illustration: Fig. 204.
Fertilization in Peronospora alsinearum; tube from antheridium carrying
in the sperm nucleus in figure at the left, female nucleus near; fusion
of the two nuclei shown in the two other figures. (After Berlese.)]
[Illustration: Fig. 205. Ripe oospore of Peronospora alsinearum.]
=398.= In sexual reproduction oogonia and antheridia are developed
on the mycelium within the tissues. Fig. 204 represents the antheridium
entering the oogonium, and the male nucleus fusing with the female
nucleus in fertilization. The sexual organs of Phytophthora infestans
are not sufficiently known.
=399.= Mucor, saprolegnia, peronospora, and their relatives
have few or no septa in the mycelium. In this respect they resemble
certain of the algæ like vaucheria, but they lack chlorophyll. They are
sometimes called the alga-like fungi and belong to a large group called
_Phycomycetes_.