In the months of April and May, the younger needle-like leaves of the
Scotch pine are occasionally seen to have assumed a yellow tinge, and
on closer examination this change in color, from green to yellow, is
seen to be due to the development of what look like small orange
colored vesicles standing off from the surface of the epidermis, and
which have in fact burst through from the interior of the leaf (Fig.
31). Between these larger orange yellow vesicles the lens shows
certain smaller brownish or almost black specks. Each of the vesicular
swellings is a form of fungus fructification known as an _Æcidium_,
and each of the smaller specks is a fungus structure called a
_Spermogonium_, and both of these bodies are developed from a mycelium
in the tissues of the leaf. I must employ these technical terms, but
will explain them more in detail shortly: the point to be attended to
for the moment is that this fungus in the leaf has long been known
under the name of _Peridermium Pini_ (var. _acicola_, i.e., the
variety which lives upon the needle-like leaves).
[Illustration: FIG. 31.--To the right is a pair of leaves of the
Scotch pine, with the blister-like Æcidia a. of Peridermium Pini
(var. acicola) projecting from their tissues: these blisters are
orange yellow in color, and contain spores, as shown in Fig. 33.
Between the blisters are the minute spermogonia, b. To the left
is a small branch, killed at a a a by Peridermium Pini (var.
corticola), the blister-like yellow Æcidia of the fungus being
very conspicuous. (Reduced, after Hartig.)]
On the younger branches of the Scotch pine, the Weymouth pine, the
Austrian pine, and some others, there may also be seen in May and June
similar but larger bladder-like orange vesicles (_Æcidia_) bursting
through the cortex (Fig. 31); and here, again, careful examination
shows the darker smaller _Spermogonia_ in patches between the
_Æcidia_. These also arise from a fungus mycelium in the tissues of
the cortex, whence the fungus was named _Peridermium Pini_ (var.
_corticola_). It is thus seen that the fungus _Peridermium Pini_ was
regarded as a parasite of pines, and that it possessed two varieties,
one inhabiting the leaves and the other the cortex: the "varieties"
were so considered, because certain trivial differences were found in
the minute structure of the _Æcidia_ and _Spermogonia_.
[Illustration: FIG. 32.--Blisters (Æcidia) of Peridermium Pini (var.
corticola) on a branch of the Scotch pine: some of the Æcidia
have already burst at the apex and scattered their spores, b, b;
the others are still intact. (Natural size, after Hess).]
If we cut thin vertical sections through a leaf and one of the
smallest _Æcidia_, and examine the latter with the microscope, it will
be found to consist of a mass of spores arranged in vertical rows,
each row springing from a branch of the mycelium: the outermost of
these spores--i.e., those which form a compact layer close beneath the
epidermis--remain barren, and serve as a kind of membrane covering the
rest (Fig. 33, p). It is this membrane which protrudes like a blister
from the tissues. The hyphæ of the fungus are seen running in all
directions between the cells of the leaf tissue, and as they rise up
and form the vertical chains of spores, the pressure gradually forces
up the epidermis of the leaf, bursts it, and the mass of orange yellow
powdery spores protrude to the exterior enveloped in the aforesaid
membrane of contiguous barren spores. If we examine older _Æcidia_, it
will be found that this membrane bursts also at length, and the spores
escape.
Similar sections across a _Spermogonium_ exhibit a structure which
differs slightly from the above. Here also the hyphæ in the leaf turn
upward, and send delicate branches in a converging crowd beneath the
epidermis; the latter gives way beneath the pressure, and the free
tips of the hyphæ constrict off very minute spore-like bodies. These
minute bodies are termed _Spermatia_, and I shall say no more about
them after remarking that they are quite barren, and that similar
sterile bodies are known to occur in very many of the fungi belonging
to this and other groups.
Sections through the _Æcidia_ and _Spermogonia_ on the cortex present
structures so similar, except in minute details which could only be
explained by lengthy descriptions and many illustrations, that I shall
not dwell upon them; simply reminding the reader that the resemblances
are so striking that systematic mycologists have long referred them to
a mere variety of the same fungus.
Now as to the kind and amount of damage caused by the ravages of these
two forms of fungus.
In the leaves, the mycelium is found running between the cells (Fig.
33, h), and absorbing or destroying their contents: since the leaves
do not fail the first season, and the mycelium remains living in their
tissues well into the second year, it is generally accepted that it
does very little harm. At the same time, it is evident that, if very
many leaves are being thus taxed by the fungus, they cannot be
supplying the tree with food materials in such quantities as if the
leaves were intact. However, the fungus is remarkable in this
respect--that it lives and grows for a year or two in the leaves, and
does not (as so many of its allies do) kill them after a few weeks. It
is also stated that only young pines are badly attacked by this form:
it is rare to find _Æcidia_ on trees more than twenty years or so old.
[Illustration: FIG. 33.--Vertical section through a very young Æcidium
of Peridermium Pini (var. acicola), with part of the subjacent
tissue of the leaf, h, the mycelium of the parasitic fungus
running between the cells of the leaf; immediately beneath the
epidermis of the leaf, the ends of the hyphæ give rise to the
vertical rows of spores (b), the outermost of which (p) remain
barren, and form the membrane of the blister-like body. The
epidermis is already ruptured at p by the pressure of the young
Æcidium. (After K. Hartig: highly magnified.)]
Much more disastrous results can be traced directly to the action of
the mycelium in the cortex. The hyphæ grow and branch between the
green cells of the true cortex, as well as in the vast tissues
beneath, and even make their way into the medullary rays and resin
canals in the wood, though not very deep. Short branches of the hyphæ
pierce the cells, and consume their starch and other contents, causing
a large outflow of resin, which soaks into the wood or exudes from the
bark. It is probable that this effusion of turpentine into the tissues
of the wood, cambium, and cortex has much to do with the drying up of
the parts above the attacked portion of the stem: the tissues shrivel
up and die, the turpentine in the canals slowly sinking down into the
injured region. The drying up would of course occur if the conducting
portions are steeped in turpentine, preventing the conduction of water
from below.
The mycelium lives for years in the cortex, and may be found killing
the young tissues just formed from the cambium during the early
summer: of course the annual ring of wood, etc., is here impoverished.
If the mycelium is confined to one side of the stem, a flat or
depressed spreading wound arises; if this extends all round, the parts
above must die.
When fairly thick stems or branches have the mycelium on one side
only, the cambium is injured locally, and the thickening is of course
partial. The annual rings are formed as usual on the opposite side of
the stem, where the cambium is still intact, or they are even thicker
than usual, because the cambium there diverts to itself more than the
usual share of food substances; where the mycelium exists, however,
the cambium is destroyed, and no thickening layer is formed. From this
cause arise cancerous malformations which are very common in pine
woods (Fig. 34).
[Illustration: FIG. 34.--Section across an old pine stem in the
cancerous region injured by Peridermium Pini (var. corticola). As
shown by the figures, the stem was fifteen years old when the
ravages of the fungus began to affect the cambium near a. The
mycelium, spreading in the cortex and cambium on all sides,
gradually restricted the action of the latter more and more; at
thirty years old, the still sound cambium only extended half way
round the stem--no wood being developed on the opposite side. By
the time the tree was eighty years old, only the small area of
cambium indicated by the thin line marked 80 was still alive; and
soon afterward the stem was completely "ringed," and dead, all
the tissues being suffused with resin. (After Hartig.)]
Putting everything together, it is not difficult to explain the
symptoms of the disease. The struggle between the mycelium on the one
hand, which tries to extend all round in the cortex, and the tree
itself, on the other, as it tries to repair the mischief, will end in
the triumph of the fungus as soon as its ravages extend so far as to
cut off the water supply to the parts above: this will occur as soon
as the mycelium extends all round the cortex, or even sooner if the
effusion of turpentine hastens the blocking up of the channels. This
may take many years to accomplish.
So far, and taking into account the enormous spread of this disastrous
disease, the obvious remedial measures seem to be, to cut down the
diseased trees--of course this should be done in the winter, or at
least before the spores come--and use the timber as best may be; but
we must first see whether such a suggestion needs modifying, after
learning more about the fungus and its habits. It appears clear, at
any rate, however, that every diseased tree removed means a source of
Æcidiospores the less. Probably every one knows the common groundsel,
which abounds all over Britain and the Continent, and no doubt many of
my readers are acquainted with other species of the same genus
(_Senecio_) to which the groundsel belongs, and especially with the
ragwort (_Senecio Jacobæa_). It has long been known that the leaves of
these plants, and of several allied species, are attacked by a fungus,
the mycelium of which spreads in the leaf passages, and gives rise to
powdery masses of orange yellow spores, arranged in vertical rows
beneath the stomata: these powdery masses of spores burst forth
through the epidermis, but are not clothed by any covering, such as
the _Æcidia_ of _Peridermium Pini_, for instance. These groups of
yellow spores burst forth in irregular powdery patches, scattered over
the under sides of the leaves in July and August: toward the end of
the summer a slightly different form of spore, but similarly arranged,
springs from the same mycelium on the same patches. From the
differences in their form, time of appearance, and (as we shall see)
functions, these two kinds of spores have received different names.
Those first produced have numerous papillæ on them, and were called
_Uredospores_, from their analogies with the uredospore of the rust of
wheat; the second kind of spore is smooth, and is called the
_Teleutospore_, also from analogies with the spores produced in the
late summer by the wheat rust. The fungus which produces these
uredospores and teleutospores was named and has been long
distinguished as _Coleosporium Senecionis_ (Pers.) We are not
immediately interested in the damage done by this parasite to the
weeds which it infests, and at any rate we might well be tempted to
rejoice in its destructive action on these garden pests. It is
sufficient to point out that the influence of the mycelium is to
shorten the lives of the leaves, and to rob the plant of food material
in the way referred to generally in my last article.
What we are here more directly interested in is the following. A few
years ago Wolff showed that if the spores from the _Æcidia Peridermium
Pini_ (var. _acicola_) are sown on the leaf of _Senecio_, the germinal
hyphæ which grow out from the spores _enter the stomata of the
Senecio leaf, and there develop into the fungus called Coleosporium
Senecionis_. In other words, the fungus growing in the cortex of the
pine, and that parasitic on the leaves of the groundsel and its
allies, are one and the same: it spends part of its life on the tree
and the other part on the herb.
[Illustration: FIG. 35. A spore of Peridermium Pini germinating. It
puts forth the long, branched germinal hyphæ on the damp surface
of a leaf of Senecio, and one of the branches enters a stoma, and
forms a mycelium in the leaf: after some time, the mycelium gives
rise to the uredospores and teleutospores of Coleosporium
Senecionis. (After Tulasne: highly magnified.)]
If I left the matter stated only in this bald manner, it is probable
that few of my readers would believe the wonder. But, as a matter of
fact, this phenomenon, on the one hand, is by no means a solitary
instance, for we know many of these fungi which require two host
plants in order to complete their life history; and, on the other
hand, several observers of the highest rank have repeated Wolff's
experiment and found his results correct. Hartig, for instance, to
whose indefatigable and ingenious researches we owe most that is known
of the disease caused by the _Peridermium_, has confirmed Wolff's
results.
It was to the brilliant researches of the late Prof. De Bary that we
owe the first recognition of this remarkable phenomenon of
_heteroecism_--i.e., the inhabiting more than one host--of the
fungi. De Bary proved that the old idea of the farmer, that the rust
is very apt to appear on wheat growing in the neighborhood of berberry
bushes, was no fable; but on the contrary, that the yellow _Æcidium_
on the berberry is a phase in the life history of the fungus causing
the wheat rust. Many other cases are now known, e.g.., the _Æcidium
abietinum_, on the spruce firs in the Alps, passes the other part of
its life on the rhododendrons of the same region. Another well known
example is that of the fungus _Gymnosporangium_, which injures the
wood of junipers. Oersted first proved that the other part of its life
is spent on the leaves of certain Rosaceæ, and his discovery has been
repeatedly confirmed. I have myself observed the following
confirmation of this. The stems of the junipers so common in the
neighborhood of Silverdale (near Morecambe Bay) used to be distorted
with _Gymnosporangium_, and covered with the _teleutospores_ of this
fungus every spring: in July all the hawthorn hedges in the
neighborhood had their leaves covered with the Æcidium form (formerly
called Roestelia), and it was quite easy to show that the fungus on
the hawthorn leaves was produced by sowing the _Gymnosporangium_
spores on them. Many other well established cases of similar
heteroecism could be quoted.
But we must return to the _Peridermium Pini_. It will be remembered
that I expressed myself somewhat cautiously regarding the
_Peridermium_ on the leaves (var. _acicola_). It appears that there is
need for further investigations into the life history of this form,
for it has been thought more than probable that it is not a mere
variety of the other, but a totally different species.
Only so lately as 1883, however, Wolff succeeded in infecting the
leaves of _Senecio_ with the spores of _Peridermium Pini_ (_acicola_),
and developing the _Coleosporium_, thus showing that both the
varieties belong to the same fungus.
It will be seen from the foregoing that in the study of the biological
relationships between any one plant which we happen to value because
it produces timber and any other which grows in the neighborhood there
may be (and there usually is) a series of problems fraught with
interest so deep scientifically, and so important economically, that
one would suppose no efforts would be spared to investigate them: no
doubt it will be seen as time progresses that what occasionally looks
like apathy with regard to these matters is in reality only apparent
indifference due to want of information.
Returning once more to the particular case in question, it is obvious
that our new knowledge points to the desirability of keeping the seed
beds and nurseries especially clean from groundsel and weeds of that
description: on the one hand, such weeds are noxious in themselves,
and on the other they harbor the _Coleosporium_ form of the fungus
_Peridermium_ under the best conditions for infection. It may be added
that it is known that the fungus can go on being reproduced by the
_uredospores_ on the groundsel plants which live through the winter.
* * * * *
In St. Genevieve and Cape Girardeau Counties, Mo., in the Niagara
limestone is found a handsome marble of a variegated liver color. Near
Sheppard Landing it is 80 feet thick, and at Janis Mill, in St.
Genevieve County, Dr. Shumard speaks of beds of fine texture and
various shades of flesh, yellow, green, pink, purple, and chocolate,
all handsomely blended.
* * * * *