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,