INTRODUCTION 9 



The second factor, however, has its inception in a peculiar 

 condition of the organization of the plant, which is either present 

 only at certain times, or is only peculiar to and innate in certain 

 individuals, or, finally, has been acquired under the influence 

 of definite external conditions. All these peculiarities in the 

 organization of the plant may be quite normal in their nature 

 that is to say, the organism as such appears to be perfectly 

 healthy in which case the predisposition is said to be " normal." 

 On the other hand, however, the predisposition to disease may 

 be " abnormal," as is the case when the plant is only predisposed 

 to one disease because it is already suffering from another. 

 Abnormal or disease-inducing predisposition may arise, for 

 example, in the neighbourhood of a wound through which alone 

 some particular parasite could gain entrance to the plant. The 

 entire group of infectious wound-diseases may be placed in this 

 category. 



Under normal predisposition, therefore, we are to understand 

 every condition, even if only temporary, in the anatomical struc- 

 ture, in the chemical constitution, or in the vital functions of an 

 organism, which, though not in itself prejudicial to the individual, 

 induces a disease when a second, and that an external, factor co- 

 operates in addition, even though the latter is in itself innocuous 

 to the plant. 



In addition to these cases of normal and abnormal disposition 

 residing in the organism itself, we may also speak of a pre- 

 disposition to disease which is due to the locality. 



There are a great number of fungi which can only attack a 

 certain species of tree when plants of another species occur in 

 the vicinity on which the particular fungus, at certain seasons 

 of the year, may complete its development. Localities in 

 which many aspens grow impart to the pines a predisposition 

 for the disease known as "Pine-twist" (caused by Melampsora 

 Tremulce pinitorqumit}. Rhododendrons abounding in a district 

 make the spruces liable to "Leaf-blister" (caused by Chry- 

 somyxa Rhododendri], while barberry bushes are associated 

 with the "rust" of wheat. The mere existence of uninter- 

 rupted woods, composed of a single species of tree, may give 

 rise to dangers leading to extensive epidemics. Pure larch 

 woods away from mountainous regions almost always succumb 



