480 PHYSIOLOGY 
8. THE DEATH OF PLANTS 
The cycle ends. From the foregoing it has become evident that the 
growth and development of plants does not proceed uniformly, but that it 
is profoundly influenced one may even say controlled by external 
conditions; and since many of these external conditions evince a de- 
cided periodicity, growth and development exhibit a corresponding 
periodicity. But it has also become apparent that growth and develop- 
ment are likewise affected, and in many particulars as profoundly 
affected or controlled, by factors that are wholly internal, so far as is 
known at present. It is found, further, that these factors may give rise to 
periodicity in growth and development; for, however uniform the exter- 
nal conditions may be, neither proceeds uniformly. In nothing is this 
more impressively shown than in the fact that the cycle of development, 
in spite of all that can be done, sooner or later comes to an end, and the 
plant perishes, leaving behind comparatively few living cells, if indeed it 
leaves any, out of the unnumbered millions that may have constituted 
its body. 
No inherent reason for death. There does not seem to be any in- 
herent reason why a plant should die. The material of which it is com- 
posed is all the while undergoing decomposition and repair. In a per- 
ennial plant, like a tree, the tissues in great part are renewed annually, 
so that though the living and the dead stand together as a sort of unity, 
which may have occupied the place for centuries, the oldest of the living 
parts is only a minute fraction of these centuries old. In such a plant, 
however, it becomes increasingly difficult to supply the extremities with 
the needful materials, because they are steadily becoming separated by 
greater and greater distances. The leaves are yearly further from the 
ports of entry for water, and the roots are yearly further from the source 
of food. With expanse of branching, mechanical overthrow threatens 
more and more. Thus the physical conditions are steadily becoming 
more severe, and it is easy to imagine why the plant must finally suc- 
cumb. Yet the long persistence, even after it has become evident that 
a tree has reached the practical limit of growth, shows that there is 
nothing in the living parts themselves which determines the end; and 
still more is this shown by the fact that cuttings may be taken from an 
old tree and successfully started upon a new cycle which may be as long 
as the parent's. Thus, the Washington elm at Cambridge has been 
struggling against adversity for more than a quarter of a century, slowly 
