128 



NATURE 



[June 7, 1888 



however, by the mycelium, which is continually extending 

 the area of injury : consequently the next zone of wood 

 (4 in the figure) extends even a shorter distance round 

 the stem, and so on with 5 and 6, the cambium being 

 now restricted to less than half round the stem — i.e. from 

 D to D, and the same with the living cortex. Of course 

 the injured area extends upwards and downwards also, 

 as shown by the lips of the healing tissue. As soon as 

 the injury extends all round, the stem dies — it is, in fact, 

 ringed. It is also interesting to note that the zones 4 and 

 5 (and the same would be true of 6 when completed) are 

 thicker than they would have been normally : this is 

 partly due to release from pressure, and partly to a 

 concentrated supply of nutritive materials. 



Much confusion still exists between the various cases: 

 some of them undoubtedly are due to frost or to the 

 intense heat of direct insolation ; these are, as a rule, 

 capable of treatment more or less simple, and can be 

 healed up. Others, again, can only be freed fro.n the 

 irritating agents (which, by the bye, may be insects as well 

 as fungi) by costly and troublesome methods. 



I shall only select one case for illustration, as it is 

 typical, and only too well known. As examples of others 

 belonging to the same broad category, I may mention the 

 "canker" of apple-trees, beeches, oaks, hazels, maples, 

 hornbeams, alders, and limes, and many others ; and 

 simply pass the remark that whatever the differences in 

 detail in the special cases, the general phenomena and 

 processes of reasoning are the same. 



Perhaps no timber disease has caused so much conster- 

 nation and difference of opinion as the " larch-disease," 

 and even now there is far too little agreement among 

 foresters either as to what they really mean by this term, 

 or as to what causes the malady. The larch, like other 

 timber-trees, is subject to the attacks of various kinds of 

 fungi and insects, in its timber, roots, and leaves ; but the 

 well-known larch-disease, which has been spreading itself 

 over Europe during the present century, and which has 

 caused such costly devastation in plantations, is one of 

 the group of cancerous diseases the outward and visible 

 signs of which are manifested in the bark and young 

 wood. 



The appearance presented by a diseased larch-stem is 

 shown in Fig. 29. In the earlier stages of the malady the 

 stem shows dead, slightly sunken patches, a, of various 

 sizes on the cortex, and the wood beneath is found to cease 

 growing : it is a fact to be noted that the dead base of a 

 dried-up branch is commonly found in the middle of the 

 patch. The diseased cortex is found to stick to the wood 

 below, instead of peeling off easily with a knife. At the 

 margins of the flattened patch, just where the dead cortex 

 joins the normal living parts, there may frequently be 

 seen a number of small cup-like fungus fructifications 

 (Fig. 29, b), each of which is white or gray on the outside, 

 and lined with orange-yellow. These are the fruit-bodies 

 of a discomycetous fungus called Peziza Willkommii 

 (Htg.), and which has at various times, and by various 

 observers, received at least four other names, which we 

 may neglect. 



In the spring or early summer, the leaves of the tree 

 are found to turn yellow and wither on several of the twigs 

 or branches, and a flow of resin is seen at the dead patch 

 of cortex. If the case is a bad one, the whole branch or 

 young tree above the diseased place may die and dry up. 

 At the margins of the patch, the edges of the sounder 

 cortex appear to be raised. 



As the disease progresses in succeeding years, the 

 merely flattened dead patch becomes a sunken blistered 

 hole from which resin flows : this sinking in of the de- 

 stroyed tissues is due to the up-growth of the margins of 

 the patch, and it is noticed that the up-growing margin 

 recedes further and further from the centre of the patch. 

 If this goes on, the patch at length extends all round the 

 stem or branch, and the death of all that lies above is 



then soon brought about, for, since the young wood and 

 cambium beneath the dead cortex are also destroyed, the 

 general effect is to " ring " the tree. 



To understand these symptoms better, it is necessary 

 to examine the diseased patch more closely in its various 

 stages. The microscope shows that the dead and dying 

 cortex, cambium, and young wood in a small patch, contain 

 the mycelium of the fungus which gives rise to the cup-like 

 fructifications — Peziza Willkommii — above referred to 

 (Fig. 30) ; and Hartig has proved that, if the spores of this 

 Peziza are introduced into the cortex of a healthy living 

 larch, the mycelium to which they give rise kills the cells 

 of the cortex and cambium, penetrates into the young 

 wood, and causes the development of a patch which 

 everyone would recognize as that of the larch-disease. 

 It is thus shown that the fungus is the immediate cause 

 of the patch in which it is found. 



The next fact which has been established is that the 

 fungus can only infect the cortex through some wound or 

 injury — such as a crack or puncture — and cannot pene- 

 trate the sound bark, &c. Once inside, however, the 

 mycelium extends upwards, downwards, sideways, and 

 inwards, killing and destroying all the tissues, and so 

 inducing the outflow of resin which is so characteristic of 

 the disease. The much-branched, septate, colourless 



Fig. 29. — Po-tion of stem ofa young larch affected with the larch-disease, as 

 indicated by the dead " cancerous " patch of cracked cortex, a : at and 

 near the margins of the patch are the small cup-like fructifications of 

 Peziza U> 'illkommii '(Htg.), which spring from mycelium in the dead and 

 dying cortex and cambium beneath. (After Hess.) 



hyphae can penetrate even as far as the pith, and the 

 destroyed tissues turn brown and dry up. 



After destroying a piece of the tissues in the spring, 

 the growth of the mycelium stops in the summer, the 

 dead cortex dries up and sticks to the wood, and the 

 living cortex at the margins of the patch commence to 

 form a thick layer of cork between its living cells and the 

 diseased area. 



It is this cork-formation which gives the appearance of 

 a raised rim around the dead patch. It has long been 

 known that the patches dry up and cease to spread in the 

 dry season. It should be pointed out that it is one of 

 the most general properties of living parenchymatous 

 tissue to form cork-cells at the boundaries of an injury : 

 if a slice is removed from a potato, for instance, the cut 

 surface will be found in a few days with several layers 

 of cork-cells beneath it, and the same occurs at the cut 

 surface of a slip, or a pruned branch, — the " callus " of 

 tissue formed is covered with a layer of cork. 



If it is remembered that the cambium and young wood 

 are destroyed beneath the patch, it will be at once clear 

 that in succeeding periods of growth the annual rings of 

 wood will be deficient beneath the patch. 



Next year, the cambium in the healthy parts of the 

 stem begins to form another ring ; but the fungus 



