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 various 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, we 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, climate, 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 roois by additions, the distance 
between the place of supply and of consumption 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 given for fungi to enter 
