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. * * * * * THE SCIENTIFIC AMERICAN ARCHITECTS AND BUILDERS EDITION $2.50 a Year. Single Copies, 25 cts.