74 BIOLOGY OF DEATH 



ance of senescence in the cells. But it may be objected 

 that an element of uncertainty is injected into the case, 

 by the fact that, as Carrel and Ebeling have lately dis- 

 cussed in some detail, it has been necessary in carrying 

 along this long-continued culture to add regularly to the 

 culture medium a small amount of "embryonic juice." 

 One might urge that, but for the ' ' embryonic juice, ' ' cellu- 

 lar senescence and death would have appeared. But 

 suppose this to be granted fully. It does not mean 

 that senescence is a necessary and inevitable consequence 

 of life, but only that to realize a potential immortality 

 the cells must have an appropriate environment, one 

 element of which is presumably some chemical combination 

 which, so far, one has supplied only through "embry- 

 onic juice. ' ' 



An entirely different sort of e^'idence and one of 

 great significance is found in the facts of clonal propaga- 

 tion of plants, well known to horticulturists. An individ- 

 ual apple tree grows old, and eventually dies, as a tree. 

 But at all periods of its life, including all stages of 

 senescence up to the terminal one, death, it produces 

 shoots each spring. If one of these shoots is grafted to 

 another root, it will, in the passage of time, make first 

 a young tree, then a middle aged tree, and finally an old, 

 senescent tree; which, in turn, will make new shoots, 

 which may, in turn, be grafted to new roots, and so on 

 ad infinitum. It is not even absolutely necessary that 

 the shoot be grafted to a new root; though, of course, 

 this is the manner in which the great majority of our 

 orchards are, in fact, propagated, and have been since 

 the beginning of horticultural history. Anyone who is 

 familiar with the woods of New England, not too far from 

 settlements, has seen apple trees in the woods where a 



