_RUSTS._
UNFORTUNATELY, this group of fungi contains species but too well known
for their ravages amongst graminaceous plants, especially the cereals.
“Corn rust,” as it is generally called, has a reputation little better
than mildew, and it really deserves no better, for it is only another
form of that pest of the farm, from the mycelium of which the
corn-mildew is at length developed. There are two species very closely
allied (doubtless only forms of the same species with different spores)
which attack the leaves and culms of growing corn, and, bursting through
the cuticle in the manner represented in Plate VII. fig. 141
(magnified), give a peculiar rusty appearance to the plant, as
represented in Plate VII. fig. 140. One of these corn-rusts is
botanically named _Trichobasis rubigo-vera_ (Lev.), or the “true rust
_Trichobasis_;” the latter, which is the generic name, being a compound
of two Greek words (_thrix_, a hair, and _basis_, a foundation), on
account of the spores being at first furnished at their base with a
short, thread-like peduncle, which at length falls away (Plate VII. fig.
142). The other Corn rust is _Trichobasis linearis_, or “line-like
_Trichobasis_,” because the sori or pustules are linear, or lengthened
out like a line; the spores nearly double the length of those of the
other Corn rust (Plate VII. fig. 144), and not so bright in colour. By
intermediate forms these two rusts pass insensibly the one into the
other, so that it is sometimes difficult to distinguish them. Both have
the spores clustered together in the pustules, at first (Plate VII. fig.
143) attached by their peduncles, but they soon become free, and are
scattered like rust-powder over the plant. Adverting to the attack of
rust upon the wheat crops to an unusual extent a few years since, the
Rev. M. J. Berkeley wrote as follows:—
We have seen rust more prevalent in white wheats, especially in
the variety called Russian white, which has red smooth chaff,
than we ever remember it. It is, however, confined to particular
spots; and while in one field not a single leaf is free,
insomuch that a person walking through the wheat is completely
painted with the spores, of a fine rust-red; an adjoining field,
separated only by a hedge, has not a rusty leaf. So long,
however, as the rust is confined to the leaf, it is, we believe,
perfectly harmless. The grain swells in spite of it, and the
only effect is that the flag dies a little earlier, which is not
undesirable when it is too luxuriant. If, however, it gets to
any extent upon the chaff, much more if it attacks the seed
itself, as is sometimes the case, it is very mischievous.
[Illustration:
Plate VI.
W. West imp.
]
There is, indeed, a notion that rust is merely a form of mildew,
and this may be true; but if so, it is a harmless form. The true
rust (_T. rubigo-vera_), if a form of mildew, preserves its own
characters to the end, and merely presents a second form of
fruit, a circumstance which is exceedingly common amongst the
lower fungi. It may exist in company with the mildew, but it
also more frequently is developed without a trace of mildew
intermixed. As far, then, as its immediate effects upon the
plant go, it may be treated as a distinct parasite, though
advanced botanists may look upon it as offering a case of what
they call dualism, which may be easily understood on a
comparison with such plants as marigold and orache, which
produce seeds of two different forms. Its ultimate effects may,
indeed, be injurious, as, if it be really a second form of
mildew, it is capable of propagating that pest. The case is
quite different with _Trichobasis linearis_, which is in fact
merely the young state of the mildew. When once that supposed
species makes its appearance, it is quite certain that there
will be mildew to a greater or less extent. While the spores of
the one remain unaltered, though intermixed with the true
bipartite spores of the mildew, the other exhibits every
intermediate state of form and colour.
M. Tulasne, to whom we have already alluded as a high authority on this
subject, is of opinion that all the species enumerated in this group or
genus, are mere conditions of other species, and therefore excludes it
altogether. Another genus (_Lecythea_), for the same reason, has shared
the same fate.
It must not be forgotten that twenty-eight species of rust belonging to
this genus are now recorded as occurring in Great Britain. Some of these
have yellow or yellowish spores; the rest have the spores of a decidedly
brown colour. One of the most attractive of the yellow-spored species is
that which appears on the groundsel (_T. Senecionis_), and a good notion
of its external appearance may be gained from our figure (Plate VII.
fig. 145). It is not always to be found without searching closely, for
in some localities we have found it very sparingly, whilst elsewhere—as,
for instance, on Wandsworth Common—we have met with it as soon or as
late as there were groundsel leaves for it to grow upon. No spot or
change in the appearance of the upper surface of the leaves indicates
the presence of the parasite beneath; this is, however, often betrayed
by the golden-yellow streaks which appear on the stem. Many a time and
oft the boys in the semi-rural districts about town have marvelled and
questioned each other concerning the reason why, in gathering
“groundsel” for the little bird at home, we should be so particular, and
pass so many promising plants, plucking here and there a leaf, and
seldom collecting one which they consider eligible for the purpose.
Collectors of minute fungi must expect to overhear occasionally even
hints touching their sanity from those who, without the remotest idea of
their mission, think they must be slightly “wrong in the head” to gaze
so narrowly and intently, amongst nettles, groundsel, grass, or dry
leaves, and only take an occasional fragment of a rotten stick, or two
or three sickly leaves, carefully deposit them in their wallet, hat, or
pocket, and then “move on.”
It will be unnecessary to repeat what has been stated in a former
chapter on spermogones. It will be remembered that these organs are
found developed in connection with some of the species of the present
genus, as well as with the cluster-cups, or _Æcidiacei_. They consist
externally of small conical elevations, pierced at the apex, which
contain minute, cellular, linear bodies called spermatia, invested with
a kind of mucous substance, that over-flows with them from the orifice
of the spermogone, like lava from the crater of a miniature volcano. The
rust in company with which they have been found most plentifully, is
that which covers the under surface of the leaves of the commonest of
all thistles (_Cnicus arvensis_). The external form of one of these
spermogones is figured Plate VII. fig. 153. The rust possesses, when
fresh, a peculiar odour, which is said to resemble that of
orange-flowers; whence was derived its name of “sweet-smelling rust”
(_Trichobasis suaveolens_, Lev.). M. Tulasne writes:—“With respect to
this species, it is, in my opinion, but the first form of a _Puccinia_,
analogous to _P. Compositarum_, D.C.: the spermogones with which it is
mixed being very abundant, it ought to be placed with that _Puccinia_. I
should, perhaps, retain some doubts upon the legitimacy of this
relationship, if it had not been my fortune to meet on another species
of the same kind (_Puccinia Anemones_, P.) spermogones perfectly
characterized, and in which the colour, habit, and position would not
allow me to hesitate an instant in attributing them to the _Puccinia_.”
In this species of rust the whole under surface of the leaf is covered
with a purplish-brown dusty coating of the spores, from the numerous
pustules which are produced (Plate VII. fig. 151). The plants, when
attacked, have a paler and more sickly appearance; the leaves have a
tendency to fold backwards at their margins, and thenceforth their
growth seems to be determined. Sowerby, in his “British Fungi,”
says:—“Two or three sorts of flies are occasionally found dead on this
plant at the time of the fungus being upon it, which is after wet
weather in the summer, or early in autumn; being apparently tempted by
its flavour, they over-eat themselves, or else are destroyed by some
poison.” This rust has spores resembling, in general characters, those
of the yellow-spored series Plate VII. fig. 152).
We have not thought it necessary to give figures of many species, partly
on account of the uncertainty existing in many minds whether they ought
to be regarded as species, and whether they will long claim a place in
the British Flora; and partly on account of the similarity which exists
between them, at least so far as they are of interest to the
microscopist only.
During the autumn of last year, whilst on a botanical excursion through
a portion of Epping Forest, the “great bog” became a centre of some
interest. Bogs are generally attractive spots to those who are in search
of microscopic organisms. On this occasion the chief objects of interest
were the small brown pustules (Plate VIII. fig. 168) with which the
upper surface of a large number of the leaves of the pennywort
(_Hydrocotyle vulgaris_) were sprinkled. These pustules were brown,
orbicular, regular, and in habit seemed to resemble rather those of most
of the _Pucciniæ_ than of a _Trichobasis_. The large, vigorous, and
healthy leaves were less affected. Microscopical examination, at first
incomplete, led us to the conclusion that it was a species of _Uredo_,
which had been met with in France (_Uredo Hydrocotyles_, Mont.); but a
re-examination, to which we were prompted by Mr. Currey, led to the
conclusion that it belonged rather to the present genus; but it can
scarcely be associated with any species already described,
notwithstanding its apparent affinity with the brown rust found on
umbelliferous plants, in which the pustules are invariably developed on
the _under_ surface. Under these circumstances, we have called it the
Pennywort rust (_Trichobasis Hydrocotyles_), whilst still doubtful
whether it is not the same fungus as that described by Montagne, with
whose description it agrees in everything, except what is implied by the
generic name. It should not be forgotten, that the work in which
Montagne first described this species was published when the genus
_Uredo_ embraced the present genus, which was separated from it by
Leveille twelve years later; and it is possible that this species was
included in a more recent work by the same author, in error and without
re-examination, under the old name. An examination of authentic
specimens of Montagne’s plant would settle the point; but we know of no
published collection which contains it. The characteristic difference
between the two genera lies in the presence of a peduncle in the early
stage of _Trichobasis_ Plate VIII. fig. 169), and its absence in all
stages of _Uredo_. Without wandering further into a subject which has
not the merit of being very popular, let us away to some green lane in
search of violets, and having found them, take a little of the brown
dust from one of the small pustules on the leaves, upon the point of a
penknife; place this, with a drop of water, upon a glass slide, and make
a record of what we observe.
The field is covered with the myriad spores of a rust of a nearly
spherical shape, brownish in colour, and here and there one with a short
transparent colourless stalk or pedicel. This is the violet rust
(_Trichobasis Violarum_, B.), very common all through the summer and
autumn, generally on the under surface of the leaves of violets, in
woods and hedgerows. Should it so happen that the spores when placed
under the microscope are found to be two-celled, it will prove that
instead of a rust, or _Trichobasis_, being under examination, a brand,
or _Puccinia_, has been found, which is almost equally common, and which
may, without such a test, be easily mistaken for a rust. According to
the theory of di-morphism, this is the higher form or complete fruit of
the same fungus, which in its simple-celled state is called _Trichobasis
Violarum_.
A similar circumstance may befall the student in examining the rust of
labiate plants (_Trichobasis Labiatarum_, Lev.), which occurs on
different species of mint, especially the water-mint, about the month of
August. We have found a few of the two-celled spores of the _Puccinia_
imbedded in the pustules of this rust almost constantly, whilst the
one-celled spores are not uncommon in the pustules of what is regarded
as the true mint brand (_Puccinia Menthæ_, P.).
In spring the young leaves of the periwinkle (_Vinca major_) will, in
some situations, become thickened considerably, and ultimately browned
on both faces with the pustules of a rust (_Trichobasis Vincæ_), which
though covered with a conidioid dust are often very tardy in bursting
the epidermis. It is almost impossible, after the pustule is ruptured,
to find one in which the bilocular spores of _Puccinia_ (fig. 132) are
not largely intermingled with the unilocular spores of the “rust” (Plate
VI. fig. 131). Later in the season by two or three months, other leaves
of the same plants will be found occupied by the smaller and more widely
scattered pustules of _Puccinia Vincæ_, in which the unilocular spores
of the “rust” will seldom be found. In this instance the leaves are
scarcely thickened, and the colour of the spores is much darker. The
under surface of the leaves is commonly alone occupied, and
corresponding paler spots on the upper surface indicate the presence of
the parasite beneath.
The garden and field bean is liable in some seasons to become quite
rust-coloured in consequence of the profusion of spores with which the
leaves and stems are covered, from the bean rust (_Trichobasis Fabæ_,
Lev.), which in like manner is considered as the simple stage or form of
the bean brand (_Puccinia Fabæ_, Lk.). If the legumes are also examined,
a few pustules will sometimes be found on them. Beans thoroughly
infected with this rust or brand are seldom of much service in either
field or garden.
Beetroot, or mangold wurzel, is another example of garden and field
produce which is subject to a similar visitation. This rust often has
very red spores when produced on red varieties of beet. During
September, 1863, it (_Trichobasis Betæ_, Lev.) was sent us on the leaves
of the wild beet (_Beta maritima_).
All the species of _Polygonum_ are exceedingly subject to the attacks of
the Persicaria rust (_Trichobasis Polygonorum_, B.), which nearly covers
the leaves, till the entire plants seem to be smothered in Scotch snuff.
If it were allowable to affirm of any plants that they are martyrs to
parasitism, such might be said of the Persicarias and their allies.
[Illustration:
Plate VIII.
W. West imp.
]
Sedges are subject to the attacks of a rust much resembling the corn
rust. When it occurs on some species of _Carex_, the upper surface of
the leaf has corresponding pale spots, and the pustules themselves are
surrounded by a yellowish margin. This species (_Trichobasis caricina_)
is far from uncommon (Plate VIII. figs. 170, 171). The sedge-like plants
belonging to the genus _Luzula_ have also their own species of rust
(_Trichobasis oblongata_), the spores of which are deeper in colour than
in the sedge rust (Plate VII. figs. 158, 159).
Amongst the remaining species of _Trichobasis_ (the reader must pardon
our using the generic name, as we have no equivalent), that found on the
leaves of various composite plants is the most common (_Trichobasis
Cichoracearum_, Lev.). It occurs on some thistles, on the saw-wort,
dandelion, several species of hawkweed, and similar plants. The pustules
are small and more diffused than in the species found on _Cnicus
arvensis_, and they as often appear on the upper as on the lower
surfaces of the leaves.
On umbelliferous plants three species are recorded; one with yellow
spores (_Trichobasis Petroselini_, B.); another with a blistered habit,
and brown, ovate, or oblong spores (_T. Umbellatarum_, Lev.); and a
third with tawny, obovate, or egg-shaped spores (_T. Heraclei_, B.),
which is found solely on the cow-parsnip. The species of _Puccinia_
corresponding to some of these species of _Trichobasis_ are known, but,
in other cases, probability, or speculation if you please, occupies the
place of knowledge.
During the month of September, 1864, it was our good fortune to spend a
week in revisiting the scenes of our boyhood, and exploring the minute
botany of one of the marshy districts of East Norfolk. One day of the
seven, memorable to us for the discovery of three specimens of a large
Boletus (_B. cyanescens_),[7] not found, to our knowledge, since the
days of Sibthorpe, was further enriched by a species of _Trichobasis_,
new to Britain, and apparently uncommon on the Continent. This rust was
found on the leaves of the “grass of Parnassus” (_Parnassia palustris_)
on a narrow strip of marsh near Irstead church. It was sought in vain
elsewhere. The leaves were scarcely changed in appearance, except by the
presence of the pustules. There were no discoloured spots, but the
pustules appeared sometimes plentifully, more often scattered, on both
surfaces of the leaves: they were small, of a bright brown, with oval
spores; the latter were, in their early stages, shortly stalked. We have
called this species _Trichobasis Parnassiæ_. It is possibly the same as
published by Westendorp in his “Herbier Cryptogamique Belge” as _Uredo
Parnassiæ_, but we know of no copy which we can consult, and have failed
in discovering any other species to which we can refer it. It is
certainly a _Trichobasis_ and not an _Uredo_, according to the present
limitation of the latter genus.
Footnote 7:
Two of these specimens were found at the bottom of a hedge-bank,
amongst grass, by the side of the road leading from Neatishead-street
to Irstead Rectory, and the third in a similar position by the Norwich
road, two hundred yards from the turning which leads to
Neatishead-street.
Although the evidence against the retention of the species of _Lecythea_
(as the genus is named) amongst Fungi as true species, on the ground of
di-morphism, is even stronger than against the group just illustrated,
we cannot pass them altogether in silence, especially in a popular
treatise. Those who are residents in town, and yet possess their little
plot of garden-ground, with only two or three pet roses, may have had
the misfortune of seeing them smothered with a yellow blight. This
golden visitation, unwelcome as it is, may afford a subject for the
microscope, and for a small space in this chapter. At first there will
not appear to be any important difference between the spores of the
yellow series of the last genus and those of the present; but a closer
examination will reveal one important distinction, viz., the presence of
colourless elongated, abortive spores. The species are not so numerous
by half as those of _Trichobasis_, even when three anomalous forms are
included, which species are included by some mycologists in two other
genera. One very common rust of this group has already been alluded to
(Plate II. fig. 37), and which is known botanically as _Lecythea Rosæ_.
A similar one is found on the bramble, and another on the burnet. All
these three species are produced at first on spots which are afterwards
more or less occupied by the long, many-celled spores of the dark brown
brands called _Aregma_ or _Phragmidium_, between which and the simple
yellow spores of the rust almost every intermediate form may often be
found in the same pustule. Thus, from the same mycelium as that of the
rose rust, the rose brand is afterwards developed; whilst from the nidus
of the bramble rust (Plate III. fig. 40) the bramble brand is also at
length produced; and the successor to the burnet rust (Plate III. fig.
31) is the burnet brand. Besides these, a rust belonging to the same
genus may be found on the leaves of the poplar, the spurge, and the
common valerian, and two or three species on willows. It can scarcely
have escaped notice, that the goat-willow is almost constantly afflicted
with a rust on the under surface of the leaves (Plate VIII. fig. 160).
This species will again come under notice as the summer spores of a
truly dimorphous species.
One of the rusts separated by some botanists from this genus is found
(possibly most commonly) on the leaves of the raspberry; but during the
past autumn we have met with it plentifully on the upper surface of the
leaves of one or two species of bramble, and have never seen it growing
on the raspberry, although in all descriptions of the species that is
stated to be its habitat. Even to the naked eye this is so distinct,
that no one could well confound it with any other. It appears very late
in the autumn, and the spots are scattered at some distance apart from
each other (Plate VIII. fig. 162); each spot or pustule forming a ring
(Plate VIII. fig. 163 enlarged) encircling a cluster of spermogones
which occupy the centre.
Another rejected species (_Lecythea Lini_, Lev.) occurs on the little
purging flax (_Linum catharticum_), forming small pustules on the leaves
(Plate VIII. fig. 165); these burst irregularly, and remain surrounded
by the remains of the ruptured epidermis (Plate VIII. fig. 166). The
yellowish spores are subglobose (Plate VIII. fig. 167), and in the first
instance concatenate, or chained together like a necklace, which
circumstance has been taken advantage of to place it, with one or two
other species, in a separate genus.
We cannot claim for the species brought into notice in the present
chapter any attractive features resulting from singularity of form,
complexity of structure, or delicate tracery, whereby they might commend
themselves to mere “searchers after curiosities,” or be recommended from
friend to friend as “sensation” objects for the microscope. They _do_
possess an interest and a value, but such as would not be appreciated by
those who seek to pass an idle half-hour by gazing at some new thing.
------------------------------------------------------------------------