Senile Changes in Leaves of Certain Plants 359 



the studies on Vitis vulpina supports the view of Hertwig and disapproves 

 Child's statement regarding certain plants at least. In forms that are 

 without sexual reproduction and that are capable of continuous existence, 

 the protoplasm must free itself from its accumulations more readily than 

 protoplasm that depends on sexual reproduction. 



It will be noticed that both Hertwig and Child picture the accumulation 

 of inert products as interfering primarily with metabolism as a whole, 

 not merely with katabolism. This accumulation theory must be dis- 

 cussed, as to its probability, in connection with the next theory of senility. 



Decreased elimination of waste products 

 The first thorough treatment of the idea that senility may be due to 

 the retention of poisonous waste in the tissues was that by Miihlmann 

 (1900). His argument is based on the fact that as the growth of a many- 

 celled organism goes on, the central cells are shut off from direct contact 

 with oxygen, and become dependent for food, oxygen, and elimination 

 of waste on the cells that still remain in contact with the surface. With 

 increased growth, moreover, the mass increases as the cube, while the 

 surface increases only as the square, of the dimensions. Therefore the 

 number of dependent cells is increasing much more rapidly than the 

 number of those cells that are able to excrete. The central cells are thus 

 forced to retain their poisonous waste products longer and longer, and 

 suffer in consequence an increasing degree of poisoning. Miihlmann 

 makes the interesting observation that the organs which represent the 

 surface continue to grow for the longest time — skin, lungs, intestines, 

 vessels, and heart, which grow for from forty to fifty years after birth; 

 while the musculature and the skeleton, which lie deeper in the body, 

 stop their growth earlier, and the nervous system, which is deepest of all, 

 grows but little after birth. 



Montgomery (1906) gives an interesting argument along the same Kne. 

 He says that death results from the insufficiency of the excretion process, 

 and that this becomes impaired because the excreting surfaces do not keep 

 pace with the increase in mass. He carries this idea so far as to postulate 

 that spore formation and gamete production are essentially excretion 

 processes, the spores or the gametes separating from the poisoned excre- 

 mentitious soma. He exactly reverses the old Hildebrand and Weismann 

 concept that the soma dies because the gametes have been produced and 



