Senile Changes in Leaves of Certain Plants 357 



the adult tissues. The cytoplasm not only increases in amount relative 

 to the nucleus, but becomes structurally differentiated as well, both changes 

 being of such a nature as to fit the cell for the efficient performance of the 

 special work that it is to perform for the whole organism. 



Minot, in 1891, was the first to advance the theory that this specializa- 

 tion of the cell was the cause of senility. He maintained at that time 

 and later (1908) that the advance in efficiency in the function utilized 

 by the organism was at the expense of the functions necessary to the con- 

 tinuous vigor of the cell. According to his view, therefore, each advance 

 in efficiency of a special function was at the cost of deterioration in the 

 self-sustaining power of the cell, and this deterioration he considered to be 

 the true senile degeneration, which progressed to a final natural death. 



Hertwig (1906) agrees with Minot's theory to this extent, expressing 

 his idea in his own terms. He says that a free cell is completely free to 

 carry on the activities required in order to maintain the vigor of the cell, 

 and these activities he includes in the term cytotypical. The cells of the 

 embryo, while largely under the cytotypical laws, are to some extent 

 modified in their actions by the organotypical laws, which govern the 

 interactions of the cells in each many-celled organism. As development 

 proceeds, the cytotypical laws are gradually dethroned by the organo- 

 typical, with the same result as that postulated by Minot. 



In another respect, however, Hertwig differs from Minot. Minot main- 

 tains that cytoplasmic differentiation is irreversible, that having once 

 started it must continue to senility and death. Hertwig, on the other 

 hand, believes that under certain conditions the specialized cells can 

 throw off the organotypical yoke and become again cytotypically active. 

 His theory of cancer, for example, is that its cells are of this type, being 

 successful revolutionists against the organotypical control. 



There seems little doubt that Minot's position is untenable in this 

 respect, except possibly for some of the most highly differentiated cells 

 in the highest animals. The results of regeneration experiments in the 

 lower annuals (Child, 1913), and the recent successes in growing animal 

 tissues in plasma (Carrel and Burroughs, 1910, Carrel, 1912, and others), 

 seem to show conclusively that specialized tissue has not irretrievably 

 lost its earlier powers. 



For forty years the evidence from the plant side has been amply sufficient 

 to show that Minot's views as to the irreversibility of differentiation would 



