Page:Popular Science Monthly Volume 25.djvu/407

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It has further been shown by myself that in some cases of disease, notably the one just referred to, there are important modifications of cellular structure and cell-contents as typical of the pathological condition. The growth becomes depauperate as a whole, and also in the various anatomical elements of the structure. At the same time the foliage assumes an abnormal color, and fails to perform its functions in the assimilative process. Yet, again, with reference to the storage of assimilated material, there is often an abnormal accumulation of such elaborated food in parts where it should not appear, except in limited quantity.

All these conditions in the case of peach-yellows are coincident with the development of the peculiar chemical conditions as noted above; and it is further a most interesting fact that, while the excess of lime and want of potash occur together with depauperate structure, loss of color in the foliage, and excessive storage of starch, an increase of the potash and decrease of lime occur simultaneously with a disappearance of these various abnormal conditions.

We are now led to inquire as to the proper course to pursue in making a diagnosis.

When the disease is strictly localized, as when produced by injuries, or by the action of parasites proper; when the local disturbance is of sufficiently recent origin to render it improbable that the general system has become involved—then the diagnosis is in most cases a simple matter, and chiefly involves the correct recognition of the cause of the disturbance—i.e., the name and character of the insect or parasite, or the particular means by which the injury was first inflicted.

When the disease involves the entire system, and the conditions become more complicated, then the difficulty increases. A correct and complete diagnosis can then be made only when we consider—

 1. The chemical composition in health and disease. 2. The internal features, including the— (a) Cellular structure. (b) Cell-contents. (c) Presence of fungi in the cells. 3. The external features, embracing the— (a) Color and size of the foliage.