12 DISEASES OF TREES 



is checked with the beginning of vegetative activity in the 

 host-plant. A similar condition of things is met with in those 

 fungi which at all seasons luxuriate in the wood of trees, and 

 even kill its living cells, but which are incapable of penetrating 

 the living tissues of the cortex till these have succumbed to 

 drought due to the death of the wood, when they may be 

 easily occupied. The tissues of the wood and those of the 

 cortex appear to differ in their power of resisting parasites. 



The amount of water in plants, determined by the weather, 

 also influences the development of endo-parasites. During 

 periods of much rain, when the plant-tissues contain more 

 water than during periods of drought, many perennial fungi 

 flourish with special vigour inside the plant. This is particularly 

 evident in the case of Melampsora Tremula pinitorquum and 

 Rosellinia quercina. 



In contradistinction to those phenomena of predisposition 

 which have been already discussed, and which to a certain 

 extent only appear periodically, there is a second category of 

 peculiarities which are innate, as it were, only in certain 

 individuals or varieties, which are thereby specially predisposed 

 to certain diseases. Variation in the vegetable kingdom may 

 find expression in morphological, chemical, and physiological 

 peculiarities, and in each of these directions forms may tend to 

 occur which are more or less susceptible to one disease or other. 

 As regards the morphological aspect, it need only be called to 

 mind that there are varieties of potato which possess a very 

 delicate skin ; others, a thick periderm ; and it is easy to 

 explain why the former are far less secure against the attack 

 of the fungus that causes potato disease than those with a 

 thick skin. 



Amongst the Douglas firs there is a bluish glaucous variety 

 whose leaves, owing to an abundant waxy covering, are much 

 better protected against atmospheric drought than the pure 

 green form. That the latter possesses a predisposition to perish 

 from drought in a continental climate is to be expected from 

 the fact that it is naturally confined to the west-coast region of 

 North America. 



That individual differences with respect to the chemical com- 

 position, and especially to the amount of water, occur in plants 



