often find the breakage of a limb to be but the open door by which rot is introduced into the interior tissues. We may consider, however, that with most of the mycelial forms of fungi their action is more or less localized, as in the smut of corn (Ustilago maydis), or the disease called cedar-apples (Gymnosporangium Sabinæ), or the curl of the peach-leaf (Exoascus deformans). So far as they are localized, therefore, their treatment is a simple matter, since it only involves cautery or removal of the affected part. Owing, however, to their peculiar habits of growth, and the insidious rapidity with which the spores may be disseminated, they may cause a disease of the general system when the conditions of the latter are favorable. But it is not such an easy matter to dispose of all these organisms. In the animal, it is now well demonstrated that disease may be directly produced by the action of certain schizomycetes, such as the micrococci and allied germs; and it is even claimed by some that they have a corresponding pathogenic function in the vegetable organism. These latter views, however, rest upon insufficient evidence at present; but, in considering certain diseases of plants at least, analogy would dictate measures of caution in formulating an opinion which wholly disregards the importance of these minute structures as pathogenic agents. Whether actually the cause of disease, or only of secondary features, in either case they are most difficult elements to deal with.
The third class embraces a variety of causes which may be directly controllable by man or not. Injuries may be inflicted by insects, as so generally occurs in the formation of galls upon leaves; in the punctures which various boring insects, as the scolytus and ægeria make for the deposition of their eggs; and, more especially, as in the subsequent action of the larvæ. There are, also, injuries which may be inflicted by animals and man, either by accident or design, and which permit the operation of fungoid growths with the development of secondary features. All these are of a strictly local nature, and the question whether or not the entire system will be involved in disorder must largely depend upon the extent and nature of the injury in the first instance.
The treatment may or may not be difficult. Where insect action is strictly local, as in galls, the amputation of the parts is sufficient; but, where the injury is inflicted by boring larvæ, the grub must first be destroyed, and this requires certain knowledge of the habits of these insects in the different stages of development. In the case of the scolytid borers the treatment is especially difficult, as the beetles are very small, and hard to destroy; but it is an interesting fact that the ovipositing of these insects is in itself indicative of an already diseased condition,[1] so that the surest and best remedy is a
- ↑ Professor Riley tells me that, so far as he knows, these borers oviposit only in diseased trees, though they may feed on healthy trees; and, in my observations of the last two years, I have been unable to collect a single fact opposed to this view.
