Plants hi their Rtlation to Disease. 173 



borer of the peach tree, however, we find a somewhat interesting relation of 

 this insect to disease, since, so far as known, it oviposits in the bark of trunk 

 and branches only when the tree has become diseased from other causes ; 

 and when other symptoms fail us it affords a clue to the pathological condi- 

 tion of the subject. 



The influence of injuries extends much beyond the action of insects. In 

 fact, it is through the infliction of injury that meteorological conditions 

 sometimes exert their most lasting influence. The action of frost, especially 

 upon those plants which have failed to fully ripen their structure, is a well- 

 known and fruitful source of disorder. Man, likewise, frequently develops 

 conditions which, if not properly controlled, serve as the open door through 

 which disease may enter. The laceration of roots, improper pruning, the 

 breaking of limbs, and a multitude of other injuries, either at the hand of 

 man or arising from accidental causes incident to natural conditions of 

 growth, all produce their well-deflned influence. Nature, herself, invariably 

 makes a strong recuperative efibrt in such cases, and oftentimes with success, 

 but most generally some assistance at the hand of man is needed for complete 

 results. 



Parasitic plants, such as the mistletoe, act as pathogenic agents in a two- 

 fold manner ; flrst, by absorbing from and appropriating to themselves those 

 assimilated materials which the host has provided for its own use; second, 

 by the mechanical action which its growth within the tissues of the host 

 necessarily exerts. The effect in this class of parasites is doubtless to be 

 ■considered as strictly local, and the treatment becomes simple, since it 

 involves the removal of the parasite with the affected part alone. It is thus 

 not from this class that we may expect the most serious results, but rather 

 from that much larger group of fungoid parasites, which, from their very 

 diminutive size, our want of exact knowledge concerning the life history and 

 mode of development of many of them, and the difficulty of obtaining precise 

 knowledge concerning their mode of action, are not only exceedingly difficult 

 to deal with, but must be numbered among the most far-reaching and destruc- 

 tive of all the influences which continue to promote disease. Indeed, it often 

 seems as if the destructive energy of these organisms was developed inversely 

 to their size and our knowledge concerning them. This class of organisms, 

 as a whole, will be found to meet the best conditions for growth under the 

 combined influence of excessive warmth and moisture, conditions which, as 

 we have seen, directly promote debility and thus enable these parasites to 

 secure a firmer hold. We have, further, to consider that these organisms 

 occur everywhere, in the sick and in the well, on the living and the dead, 

 but that they seem to particularly predominate whenever the vitality of the 

 host is largely reduced, and especially when death occurs. The mere fact of 

 association by no means establishes their pathogenic character to determine 

 which it is essential that they be capable of inducing disease in a fully normal 

 organism, rather than in one which has already become debilitated and 

 diseased from other causes. If, also, we can restore a diseased structure to 



