IV.

Before proceeding further it will be of advantage to describe another

tree-killing fungus, which has long been well known to mycologists as

one of the commonest of our toadstools growing from rotten stumps and

decaying wood-work such as old water pipes, bridges, etc. This is

_Agaricus melleus_ (Fig. 15), a tawny yellow toadstool with a ring

round its stem, and its gills running down on the stem and bearing

white spores, and which springs in tufts from the base of dead and

dying trees during September and October. It is very common in this

country, and I have often found it on beeches and other trees in

Surrey, but it has been regarded as simply springing from the dead

rotten wood, etc., at the base of the tree. As a matter of fact,

however, this toadstool is traced to a series of dark shining strings,

looking almost like the purple-black leaf stalks of the maidenhair

fern, and these strings branch and meander in the wood of the tree,

and in the soil, and may attain even great lengths--several feet, for

instance. The interest of all this is enhanced when we know that until

the last few years these long black cords were supposed to be a

peculiar form of fungus, and were known as _Rhizomorpha_. They are,

however, the subterranean vegetative parts (mycelium) of the agaric we

are concerned with, and they can be traced without break of continuity

from the base of the toadstool into the soil and tree (Fig. 16). I

have several times followed these dark mycelial cords into the timber

of old beeches and spruce fir stumps, but they are also to be found in

oaks, plums, various conifers, and probably may occur in most of our

timber trees if opportunity offers.

The most important point in this connection is that _Agaricus melleus_

becomes in these cases a true parasite, producing fatal disease in the

attacked timber trees, and, as Hartig has conclusively proved,

spreading from one tree to another by means of the rhizomorphs under

ground. Only the last summer I had an opportunity of witnessing, on a

large scale, the damage that can be done to timber by this fungus.

Hundreds of spruce firs with fine tall stems, growing on the hillsides

of a valley in the Bavarian Alps, were shown to me as "victims to a

kind of rot." In most cases the trees (which at first sight appeared

only slightly unhealthy) gave a hollow sound when struck, and the

foresters told me that nearly every tree was rotten at the core. I had

found the mycelium of _Agaricus melleus_ in the rotting stumps of

previously felled trees all up and down the same valley, but it was

not satisfactory to simply assume that the "rot" was the same in both

cases, though the foresters assured me it was so.

[Illustration: FIG. 15.--A small group of _Agaricus (Armillaria)

melleus_. The toadstool is tawny yellow, and produces white spores;

the gills are decurrent, and the stem bears a ring. The fine hair-like

appendages on the pileus should be bolder.]

By the kindness of the forest manager I was allowed to fell one of

these trees. It was chosen at hazard, after the men had struck a large

number, to show me how easily the hollow trees could be detected by

the sound. The tree was felled by sawing close to the roots; the

interior was hollow for several feet up the stem, and two of the main

roots were hollow as far as we could poke canes, and no doubt further.

The dark-colored rotting mass around the hollow was wet and spongy,

and consisted of disintegrated wood held together by a mesh work of

the rhizomorphs. Further outward the wood was yellow, with white

patches scattered in the yellow matrix, and, again, the rhizomorph

strands were seen running in all directions through the mass.

[Illustration: FIG. 16.--Sketch of the base of a young tree (s) killed

by _Agaricus melleus_, which has attacked the roots, and developed

rhizomorphs at r, and fructifications. To the right the

fructifications have been traced by dissection to the rhizomorph

strands which produced them.]

Not to follow this particular case further--since we are concerned

with the general features of the diseases of timber--I may pass to the

consideration of the diagnosis of this disease caused by _Agaricus

melleus_, as contrasted with that due to _Trametes radiciperda_.

Of course no botanist would confound the fructification of the

_Trametes_ with that of the _Agaricus_; but the fructifications of

such fungi only appear at certain seasons, and that of _Trametes

radiciperda_ may be underground, and it is important to be able to

distinguish such forms in the absence of the fructifications.

The external symptoms of the disease, where young trees are concerned,

are similar in both cases. In a plantation at Freising, in Bavaria,

Prof. Hartig showed me young Weymouth pines (_P. Strobus_) attacked

and killed by _Agaricus melleus_. The leaves turn pale and yellow, and

the lower part of the stem--the so-called "collar"--begins to die and

rot, the cortex above still looking healthy. So far the symptoms might

be those due to the destructive action of other forms of tree-killing

fungi.

On uprooting a young pine, killed or badly attacked by the agaric, the

roots are found to be matted together with a ball of earth permeated

by the resin which has flowed out; this is very pronounced in the case

of some pines, less so in others. On lifting up the scales of the

bark, there will be found, not the silky white, delicate mycelium of

the _Trametes_, but probably the dark cord-like rhizomorphs; there may

also be flat white rhizomorphs in the young stages, but they are

easily distinguished. These dark rhizomorphs may also be found

spreading around into the soil from the roots, and they look so much

like thin roots indeed that we can at once understand their

name--rhizomorph. The presence of the rhizomorphs and (in the case of

the resinous pines) the outflow of resin and sticking together of soil

and roots are good distinctive features. No less evident are the

differences to be found on examining the diseased timber, as

exemplified by Prof. Hartig's magnificent specimens. The wood attacked

assumes brown and bright yellow colors, and is marked by sharp brown

or nearly black lines, bounding areas of one color and separating them

from areas of another color. In some cases the yellow color is quite

bright--canary yellow, or nearly so. The white areas scattered in this

yellow matrix have no black specks in them, and can thus be

distinguished from those due to the _Trametes_. In advanced stages the

purple-black rhizomorphs will be found in the soft, spongy wood.

The great danger of _Agaricus melleus_ is its power of extending

itself beneath the soil by means of the spreading rhizomorphs; these

are known to reach lengths of several feet, and to pass from root to

root, keeping a more or less horizontal course at a depth of six or

eight inches or so in the ground. On reaching the root of another

tree, the tips of the branched rhizomorph penetrate the living cortex,

and grow forward in the plane of the cambium, sending off smaller

ramifications into the medullary rays and (in the case of the pines,

etc.) into the resin passages. The hyphæ of the ultimate twigs enter

the tracheides, vessels, etc., of the wood, and delignify them, with

changes of color and substance as described. Reference must be made to

Prof. Hartig's publications for the details which serve to distinguish

histologically between timber attacked by _Agaricus melleus_ and by

_Trametes_ or other fungi. Enough has been said to show that diagnosis

is possible, and indeed to an expert not difficult.

It is at least clear from the above sketch that we can distinguish

these two kinds of diseases of timber, and it will be seen on

reflection that this depends on knowledge of the structure and

functions of the timber and cambium on the one hand and proper

acquaintance with the biology of the fungi on the other. It is the

victory of the fungus over the timber in the struggle for existence

which brings about the disease; and one who is ignorant of these

points will be apt to go astray in any reasoning which concerns the

whole question. Any one knowing the facts and understanding their

bearings, on the contrary, possesses the key to a reasonable treatment

of the timber; and this is important, because the two diseases

referred to can be eradicated from young plantations and the areas of

their ravages limited in older forests.

Suppose, for example, a plantation presents the following case. A tree

is found to turn sickly and die, with the symptoms described, and

trees immediately surrounding it are turning yellow. The first tree is

at once cut down, and its roots and timber examined, and the diagnosis

shows the presence of _Agaricus melleus_ or of _Trametes radiciperda_,

as the case may be. Knowing this, the expert also knows more. If the

timber is being destroyed by the _Trametes_, he knows that the

ravaging agent can travel from tree to tree by means of roots in

contact, and he at once cuts a ditch around the diseased area, taking

care to include the recently infected and neighboring trees. Then the

diseased timber is cut, because it will get worse the longer it

stands, and the diseased parts burnt. If _Agaricus melleus_ is the

destroying agent, a similar procedure is necessary; but regard must be

had to the much more extensive wanderings of the rhizomorphs in the

soil, and it may be imperative to cut the moat round more of the

neighboring trees. Nevertheless, it has also to be remembered that the

rhizomorphs run not far below the surface. However, my purpose here is

not to treat this subject in detail, but to indicate the lines along

which practical application of the truths of botanical science may be

looked for. The reader who wishes to go further into the subject may

consult special works. Of course the spores are a source of danger,

but need be by no means so much so where knowledge is intelligently

applied in removing young fructifications.

I will now pass on to a few remarks on a class of disease-producing

timber fungi which present certain peculiarities in their biology. The

two fungi which have been described are true parasites, attacking the

roots of living trees, and causing disease in the timber by traveling

up the cambium, etc., into the stem; the fungi I am about to refer to

are termed wound parasites, because they attack the timber of trees at

the surfaces of wounds, such as cut branches, torn bark, frost cracks,

etc., and spread from thence into the sound timber. When we are

reminded how many sources of danger are here open in the shape of

wounds, there is no room for wonder that such fungi as these are so

widely spread. Squirrels, rats, cattle, etc., nibble or rub off bark;

snow and dew break branches; insects bore into stems; wind, hail,

etc., injure young parts of trees, and in fact small wounds are formed

in such quantities that if the fructifications of such fungi as those

referred to are permitted to ripen indiscriminately, the wonder is not

that access to the timber is gained, but rather that a tree of any

considerable age escapes at all.

One of the commonest of these is _Polyporus sulphureus_, which does

great injury to all kinds of standing timber, especially the oak,

poplar, willow, hazel, pear, larch, and others. It is probably well

known to all foresters, as its fructification projects horizontally

from the diseased trunks as tiers of bracket-shaped bodies of a

cheese-like consistency; bright yellow below, where the numerous

minute pores are, and orange or somewhat vermilion above, giving the

substance a coral-like appearance. I have often seen it in the

neighborhood of Englefield Green and Windsor, and it is very common in

England generally.

If the spore of this _Polyporus_ lodges on a wound which exposes the

cambium and young wood, the filaments grow into the medullary rays and

the vessels and soon spread in all directions in the timber,

especially longitudinally, causing the latter to assume a warm brown

color and to undergo decay. In the infested timber are to observed

radial and other crevices filled with the dense felt-like mycelium

formed by the common growth of the innumerable branched filaments. In

bad cases it is possible to strip sheets of this yellowish white felt

work out of the cracks, and on looking at the timber more closely (of

the oak, for instance), the vessels are found to be filled with the

fungus filaments, and look like long white streaks in longitudinal

sections of the wood--showing as white dots in transverse sections.

It is not necessary to dwell on the details of the histology of the

diseased timber; the ultimate filaments of the fungus penetrate the

walls of all the cells and vessels, dissolve and destroy the starch in

the medullary rays, and convert the lignified walls of the wood

elements back again into cellulose. This evidently occurs by some

solvent action, and is due to a ferment excreted from the fungus

filaments, and the destroyed timber becomes reduced to a brown mass of

powder.

I cannot leave this subject without referring to a remarkably

interesting museum specimen which Prof. Hartig showed and explained to

me last summer. This is a block of wood containing an enormous

irregularly spheroidal mass of the white felted mycelium of this

fungus, _Polyporus sulphureus_. The mass had been cut clean across,

and the section exposed a number of thin brown ovoid bodies embedded

in the closely woven felt; these bodies were of the size and shape of

acorns, but were simply hollow shells filled with the same felt-like

mycelium as that in which they were embedded. They were cut in all

directions, and so appeared as circles in some cases. These bodies

are, in fact, the outer shells of so many acorns, embedded in and

hollowed out by the mycelium of _Polyporus sulphureus_. Hartig's

ingenious explanation of their presence speaks for itself. A squirrel

had stored up the acorns in a hollow in the timber, and had not

returned to them--what tragedy intervenes must be left to the

imagination. The _Polyporus_ had then invaded the hollow, and the

acorns, and had dissolved and destroyed the cellular and starchy

contents of the latter, leaving only the cuticularized and corky

shells, looking exactly like fossil eggs in the matrix. I hardly think

geology can beat this for a true story.

The three diseases so far described serve very well as types of a

number of others known to be due to the invasion of timber and the

dissolution of the walls of its cells, fibers, and vessels by

hymenomycetous fungi, i.e., by fungi allied to the toadstools and

polypores. They all "rot" the timber by destroying its structure and

substance, starting from the cambium and medullary rays.

To mention one or two additional forms, _Trametes Pini_ is common on

pines, but, unlike its truly parasitic ally, _Tr. radiciperda_, which

attacks sound roots, it is a wound parasite, and seems able to gain

access to the timber only if the spores germinate on exposed surfaces.

The disease it produces is very like that caused by its ally; probably

none but an expert could distinguish between them, though the

differences are clear when the histology is understood.

_Polyporus fulvus_ is remarkable because its hyphæ destroy the middle

lamella, and thus isolate the tracheides in the timber of firs;

_Polyporus borealis_ also produces disease in the timber of standing

conifers; _Polyporus igniarius_ is one of the commonest parasites on

trees such as the oak, etc., and produces in them a disease not unlike

that due to the last form mentioned; _Polyporus dryadeus_ also

destroys oaks, and is again remarkable because its hyphæ destroy the

middle lamella.

With reference to the two fungi last mentioned I cannot avoid

describing a specimen in the Museum of Forest Botany in Munich, since

it seems to have a possible bearing on a very important question of

biology, viz., the action of soluble ferments.

It has already been stated that some of these tree-killing fungi

excrete ferments which attack and dissolve starch grains, and it is

well known that starch grains are stored up in the cells of the

medullary rays found in timber. Now, _Polyporus dryadeus_ and _P.

igniarius_ are such fungi; their hyphæ excrete a ferment which

completely destroys the starch grains in the cells of the medullary

rays of the oak, a tree very apt to be attacked by these two

parasites, though _P. igniarius_, at any rate, attacks many other

dicotyledonous trees as well. It occasionally happens that an oak is

attacked by both of these polyporei, and their mycelia become

intermingled in the timber; when this is the case, the _starch grains

remain intact in those cells which are invaded simultaneously by the

hyphæ of both fungi_. Prof. Hartig lately showed me longitudinal

radial sections of oak timber thus attacked, and the medullary rays

showed up as glistening white plates. These plates consist of nearly

pure starch; the hyphæ have destroyed the cell walls, but left the

starch intact. It is easy to suggest that the two ferments acting

together exert (with respect to the starch) a sort of inhibitory

action one on the other; but it is also obvious that this is not the

ultimate explanation, and one feels that the matter deserves

investigation.

It now becomes a question--What other types of timber diseases shall

be described? Of course the limits of a popular article are too narrow

for anything approaching an exhaustive treatment of such a subject,

and nothing has as yet been said of several other diseases due to

crust-like fungi often found on decaying stems, or of others due to

certain minute fungi which attack healthy roots. Then there is a class

of diseases which commence in the bark or cortex of trees, and extend

thence into the cambium and timber: some of these "cankers," as they

are often called, are proved to be due to the ravages of fungi, though

there is another series of apparently similar "cankers" which are

caused by variations in the environment--the atmosphere and weather

generally.

It would need a long article to place the reader _au courant_ with the

chief results of what is known of these diseases, and I must be

content here with the bare statement that these "cankers" are in the

main due to local injury or destruction of the cambium. If the normal

cylindrical sheet of cambium is locally irritated or destroyed, no one

can wonder that the thickening layers of wood are not continued

normally at the locality in question; the uninjured cells are also

influenced, and abnormal cushions of tissue formed, which vary in

different cases. Now, in "cankers" this is--put shortly--what happens:

it may be, and often is, due to the local action of a parasitic

fungus; or it may be, and, again, often is, owing to injuries produced

by the weather, in the broad sense, and saprophytic organisms may

subsequently invade the wounds.

The details as to how the injury thus set up is propagated to other

parts--how the "canker" spreads into the bark and wood around--_are_

details, and would require considerable space for their description:

the chief point here is again the destructive action of mycelia of

various fungi, which by means of their powers of pervading the cells

and vessels of the wood, and of secreting soluble ferments which break

down the structure of the timber, render the latter diseased and unfit

for use. The only too well known larch disease is a case in point; but

since this is a subject which needs a chapter to itself, I may pass on

to more general remarks on what we have learned so far.

It will be noticed that, whereas such fungi as _Trametes radiciperda_

and _Agaricus melleus_ are true parasites which can attack the living

roots of trees, the other fungi referred to can only reach the

interior of the timber from the exposed surfaces of wounds. It has

been pointed out along what lines the special treatment of the former

diseases must be followed, and it only remains to say of the latter:

take care of the cortex and cambium of the tree, and the timber will

take care of itself. It is unquestionably true that the diseases due

to wound parasites can be avoided if no open wounds are allowed to

exist. Many a fine oak and beech perishes before its time, or its

timber becomes diseased and a high wind blows the tree down, because

the spores of one of these fungi alight on the cut or torn surface of

a pruned or broken branch. Of course it is not always possible to

carry out the surgical operations, so to speak, which are necessary to

protect a tree which has lost a limb, and in other cases no doubt

those responsible have to discuss whether it costs more to perform the

operations on a large scale than to risk the timber. With these

matters I have nothing to do here, but the fact remains that by

properly closing over open wounds, and allowing the surrounding

cambium to cover them up, as it will naturally do, the term of life of

many a valuable tree can be prolonged, and its timber not only

prevented from becoming diseased and deteriorating, but actually

increased in value.

There is no need probably for me to repeat that, although the present

essay deals with certain diseases of timber due to fungi, there are

other diseases brought about entirely by inorganic agencies. Some of

these were touched upon in the last article, and I have already put

before the readers of _Nature_ some remarks as to how trees and their

timber may suffer from the roots being in an unsuitable medium.

In the next paper it is proposed to deal with the so-called "dry rot"

in timber which has been felled and cut up--a disease which has

produced much distress at various times and in various countries.

* * * * *