30 Disease and Death of Trees — Generalities 



Often it is difficult even for the expert to determine the 

 exact cause of final death. The cause frequently lies hid- 

 den under ground, or is only to be discovered in a compli- 

 cation of \arious conditions, operative at different times. 



Finally, however, it is mostly interference with normal 

 nutrition and especially deficiency in water-supply that is 

 the ultimate cause of death. 



Old Age. Since the truly living portion of the tree is 

 annually renewed, it follows that when we speak of the 

 length of the natural life of trees, w^e can mean only that 

 period of time during which the tree without human aid 

 may, as a rule, successfully resist unfavorable external influ- 

 ences of soil, cUmate, or attacks of parasites, and may repair 

 mechanical injuries. 



Although we do not know in trees the decline which 

 characterizes the old age of animals, yet we find that, as in 

 animals, extreme youth as well as extreme old age predis- 

 poses the tree to diseases, and that serious consequences 

 and fatal results are more apt to follow the attacks of 

 enemies in the younger and in the older stages than in middle 

 life. 



One reason for the greater danger to old trees seems to 

 be in their mode of growth. Since trees lengthen their 

 shoots as- well as their roots by additions, the distance 

 bet^veen the place of supply and of consumj^tion of water 

 constantly increases. Finally the forces at work can not 

 readily pump the water against gravity and friction from 

 the ever farther removed root tips to the utmost twigs in 

 sufficient quantity to satisfy the demands of a drouthy 

 season. The twigs die back again and again, and thus a 

 limit to height growth is set. At least this is the theory of 

 the writer, and no better one has as yet been presented. 



But there is also opportunity gi\-en for fungi to enter 



