Page:Popular Science Monthly Volume 25.djvu/401

Rh the controlling treatment; but in other cases, where the disorder is strictly of a local character, it may be a simple matter to remedy the trouble. In the case of those peculiar developments of the oak which give us the gall-nuts of commerce, or of similar abnormal developments in the tissues, we have instances of well-defined disease, but it is of a strictly local nature; the disturbance of functional activity does not extend beyond very narrow limits. It becomes, then, a simple matter to treat the case, because the part may be removed without inflicting injury upon other organs of the plant, and thus the knife is the sure remedy. Or, again, certain diseases may originate in the breaking of a limb or the fracture of a surface tissue. In such cases the disease will follow the injury and progress slowly, but it is often a simple and easy matter to prevent its introduction into the general system by properly caring for the wounded part in the first instance. Nature herself provides the means of warding off disease in just this way, and within certain limits her provisions are most effective. If a structure such as a vigorously-growing plant be injured, there at once appears a clear fluid, which gradually thickens into a mucilaginous substance, and finally becomes dry and hard. Under its early protection, a tissue of cork is formed over the wound as a healing and protective structure, impervious to air or water. Under it, the injured parts, now excluded from the air, are able to perfect the healing process by the formation of new tissue.

In plants, as in animals, diseases may be developed through a great variety of causes, but it is possible to bring them into a rude system of classification by means of which their consideration is greatly facilitated. The best arrangement of the kind which we have at present, one which answers very well, is that of Hartig, according to which diseases are developed through the action of—

 1. Phenogamic plants. 2. Cryptogamic plants. 3. Injuries. 4. Soil influences. 5. Atmospheric influences.

Under the first head we have to deal with those plants, like the mistletoe and dodder, which grow upon others and draw their nourishment directly from them—hence are truly parasitic. Plants of this kind may contain a certain amount of chlorophyl, but usually possess no true roots; hence they are not only incapable of drawing nourishment directly from the soil, but they are also incapable of performing the assimilative functions by which materials for the formation of cellular structure are developed, in more than a limited manner if at all. Such plants, therefore, must depend entirely upon the already elaborated sap contained in their hosts, and, feeding exclusively upon this, the latter must suffer in a degree which is proportional to the