Page:Popular Science Monthly Volume 25.djvu/399

Rh of the nature and extent of these disorders in plants, as well as of their influence, that it seems desirable to present a general outline of the subject according to our present knowledge. We will, therefore, pass over a special consideration of the various scientific investigations of the last few years, and deal with the conditions under which the diseases develop, as well as some of the most important results of their action. First, let us briefly consider what constitutes a disease.

In the animal the system is considered diseased when the functions of the body cease to be performed in a normal manner, and the disease is more or less serious, according to whether one or more functions are involved, as well as the degree to which the impairment of a particular function is carried. In instituting pathological comparisons between animals and plants, we have to keep in mind that there are important structural differences and physical peculiarities which may favor the development of disease more in one case than the other. In animals the mass of their structure is composed of highly vitalized and actively growing cells and tissues; while in the higher plants, where the differentiation of structure is carried to a high degree, there is a very considerable portion of the body which has become incapable of further growth, and is virtually dead. In many cases, as in trees, the permanent structure predominates, and the vitalized tissues are relatively few. Furthermore, through a delicate nervous system which penetrates the most remote parts of the body, the whole animal organism is brought into more or less active sympathy with the diseased portion, even though the disorder be one of a strictly local nature, while the blood as a general medium of circulation tends to distribute the affection and thus bring the entire system into a diseased condition. In the vegetable kingdom we find no fluid which would be strictly equivalent to the blood of animals and capable of disseminating disease through the organism in a similar manner. Recent researches by Hillhouse, however, seem to strongly confirm the previous observations of Gardiner, Strasburger, Frommann, and others, that there is a well-defined continuity of the protoplasmic substance between adjoining cells through their walls, thus rendering it highly probable that, in plants, the protoplasm may act in a manner similar to the nerves of animals to bring somewhat remote parts into more or less active sympathy, and this consideration must have weight in the future, as giving us a more correct insight into the operation of disease and the possibilities of its complication.

If we clearly recognize that the physical basis of life is the same in both plant and animal, and that it is through disturbance, primarily, of the protoplasmic functions that the functions of the organism as a whole are disordered, then from this and what has already been stated it becomes evident that the pathology of plants and animals is the