INTRODUCTION 15 



All the disease-inducing conditions which have been dis- 

 cussed may be designated as normal, because the peculiarities 

 noted are in themselves quite in accordance with the nature of 

 the plant-organism, and only become prejudicial when some 

 other external circumstance co-operates, and which is termed 

 the cause of the disease. 



There still remain to be noticed numerous abnormal disease- 

 inducing conditions which depend on an unsound state of the 

 plant. To these belong all those wounds in whose train some 

 disease or other of the interior of the plant may follow. 



When a tree is pruned it thereby incurs an abnormal predis- 

 position for a series of wound-diseases, infectious or otherwise, 

 which can be got rid of by the application of timely and appro- 

 priate that is to say, antiseptic dressings. Injury to a root, 

 e.g. the severance of a rootlet, is in itself damage, but when this 

 leads to decay spreading from it into the stem, we designate 

 such an injury as an abnormal disposition to disease. 



Insects of various kinds live in the cortex of sound trees, 

 which they injure, and thus open doors, as it were, to the entrance 

 of parasitic fungi into the interior, so that the trees are ultimately 

 killed. 



A hailstone strikes the cortex of a tree and injures it. This 

 creates an abnormal condition, which may lead to an infectious 

 disease should certain fungi settle on the cortex. 



When trees or shrubs are transplanted in any year, and their 

 development is so much retarded by the operation that the new 

 shoots have not completed their development when frost appears 

 that is to say, when lignification has not been completed 

 they possess an abnormal disposition to injury from frost. 

 Such plants may survive in mild winters, but if intense cold sets 

 in they may die off completely. 



From what has been said it will be clear how endless are 

 the phenomena which dispose to disease, and also how only one 

 group of these, "the inherent tendencies," possess the character 

 of inheritability. The phases of natural development, which 

 were first discussed, and which are passed through by every 

 plant, may be left out of account in connection with the ques- 

 tion of inheritability. Neither acquired predisposing causes nor 

 those due to an unhealthy state can, however, be transmitted 



