4 So 



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 



