Senile Changes in Leaves of Certain Plants 355 



equivalent to an increase in the distance from the veinlets. If one effect 

 of age on the plant protoplasm is to decrease its permeabiUty to SiUj or all 

 of the substances that pass in and out of the cell, the effect on the central 

 cells of the vein islets wiU be essentially the same as removing them 

 further from the veinlets. 



On the assumption that such a decrease in permeability is one of the 

 marks of senile degeneration occurring in plant cells, the differences 

 between the sizes of vein islets, the number of stomata, the size of the 

 cells, and the physiological activities, found between the leaves of young 

 and of old plants, can be rather plausibly explained. The smaller size 

 of the cells with age causes an additional interference with the freedom 

 of osmosis; but this, it is believed, is secondary to the change in permea- 

 bihty, and probably is a result of it. The evidence given here of a possible 

 decrease in permeability as a senile change in protoplasm, suggests its 

 appUcation to the theories of senility. 



THEORIES OF SENILITY 



The theories of senility may be grouped in two general classes: (1) those 

 considering the cause to be failure of some specific organ in the body, and 

 (2) those considering the cause to be specialization of the cells or change 

 in the physiological activities of the cells. The theories in the first class 

 cannot be considered as fundamental. 



Localization of senile changes 



Demange (1886) and Osier (1892) considered arterial sclerosis as the 

 cause of senility, while Lorand (1912) ascribed senility to a degeneration 

 of the thyroid and other ductless glands. Metchnikoff (1903 and 1908) 

 explained senile degeneration in man as due to the action of intestinal 

 bacterial toxins, and in plants to the formation of poison in the flowering 

 shoot. 



Metchnikoff's theory of the poisoning of the body by toxins produced 

 in one part of it represents merely a different phase of the " guilty-organ " 

 class of theories. He thinks that it is the large intestine in man, and the 

 flowering shoots in the higher plants, that serve as centers of poison 

 production which ultimately poisons the whole organism. In animals 

 he considers the poison to be produced by bacteria in the intestine. The 

 toxins thus formed are carried to all parts of the body by the blood, and 



