8 DISEASES OF TREES 



hand, one or other of the nutritive materials may be partially 

 exhausted. 



With the reduction in the transference of nutrient matters to 

 the crown of the tree, the latter becomes stunted and partly dies, 

 and this is followed by diseases which finally kill it altogether. 



There are, however, always demonstrable external influences 

 at work in the matter, so that the question whether the debility 

 of old age is in itself a natural condition manifesting itself, for 

 instance, in the enervation of the organization of a cambium 

 cell, or in the separation of a bud from a tree, must in the mean- 

 time be answered in the negative. Thus, when we speak of the 

 natural duration of life of a plant-species, we are to understand 

 the period of time during which a plant is able to live without 

 succumbing to the unfavourable external agencies in the soil 

 and the climate, or to the varied attacks of parasitic and 

 saprophytic organisms. 



The above considerations lead us to the natural classification 

 of the different kinds of disease which we shall examine in the 

 following pages, according to the external influences which 

 induce them. 



1. Diseases induced by Phanerogams. 



2. Diseases induced by Cryptogams. 



3. Wounds. 



4. Diseases due to unfavourable conditions of the soil. 



5. Diseases due to unfavourable atmospheric conditions. 



In the case of most diseases it is demonstrable that the 

 individuals of a given species of plant, which are subjected to 

 certain prejudicial influences, do not all succumb to these 

 influences to an equal extent, but that certain individuals or 

 varieties prove perfectly or almost perfectly resistant, while 

 others soon become diseased or die. These observations show 

 that it is not the environment alone which determines the origin 

 of a disease, but that, on the contrary, a plant contracts disease 

 only when subjected to definite pre-existing conditions ; that a 

 predisposition or tendency to disease must exist, and that there- 

 fore, to a certain extent, the origin of a disease is determined by 

 the co-operation of two factors. The first factor is the external 

 cause of the disease, and this is, as a rule, easy of demonstration. 



