Senile Changes in Leaves of Certain Plants 361 



excretion may not be of equal harm to the cell. The loss in efficiency of 

 excretion will affect the cell first. This is the argument which supports 

 the idea that defective excretion may be the cause of senility, rather than 

 the argument based on relative surface and mass. 



But here again, if the efficiency of excretion becomes lessened, it must 

 be due to some chemical or physical change occurring in the cell, and this 

 change must then be the primary senile deterioration of which the decrease 

 in excretion is a result. 



A change of this nature seems to be indicated in the cells of the leaves 

 of Vitis vulpina and certain other plants. A change in permeability, 

 which would explain the decrease in size of the vein islets if present in all 

 living cells, both plant and animal, would directly and inevitably decrease 

 the efficiency of excretion of all cells both internal and external. 



decreasing permeability as an explanation of some observed 

 senile degenerations 



It should be emphasized at this point that a decrease in permeability 

 with age must be the result of some deeper-lying physical or chemical 

 senile degeneration in protoplasm. It is probable that this fundamental 

 cause produces other changes besides that of decreasing permeability. 

 This will be touched upon shortly. With this in mind, it will be interest- 

 ing to see to what degree a senile decrease in permeability will produce the 

 effects which the other theories of senility assert are present and which 

 they explain in their own way. 



The presence of inert accumulations in cells would certainly be brought 

 about by an increasing difficulty of exit from the cells. Furthermore, it 

 is quite possible that decreasing permeability would favor the production 

 of changed metabolic products. It is equally evident that a certain degree 

 of impermeability is also essential to the cell, since on this depend the re- 

 tention of food, the protection against harmful solutes in the environment, 

 and the maintenance of turgidity. There must therefore be a constant 

 compromise in the living cell between permeability and complete impermea- 

 bility. It is possible that even in the youngest and most vigorous cell the 

 degree of permeability required is such that oxygen cannot enter as 

 rapidly as is needed at times of great activity. Under such partially 

 anaerobic conditions, it is well known that certain cells, bacteria for 

 example, may produce different katabolic products from those produced 



