DISEASES OF PLANTS. 387 



tion 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, there- 

 fore, pass over a special consideration of the various scientific investi- 

 gations 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 dis- 

 ease is more or less serious, according to whether one or more func- 

 tions 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 Avhich 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 rela- 

 tively few. Furthermore, through a delicate nervous system which 

 penetrates the most remote parts of the body, the whole animal organ- 

 ism 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 dis- 

 tribute 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 dis- 

 seminating disease through the organism in a similar manner. Recent 

 researches by Hillhouse,* however, seem to strongly confirm the pre- 

 vious observations of Gardiner, Strasburger, Fromraann, 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 



* Bot. Centralbl., XIV, 1883, pp. 89-94 ; Journal Royal Mic. Soc, Ser. II, vol., 

 iii, p. 524. 



