Senile Changes in Leaves of Certain Plants 368 



of storage products that under normal conditions are unaffected, and in 

 such cases the nature of the stimulus may also be such as to increase the 

 permeability of the cells. 



In the form in which no sexual reproduction occurs, it is possible that 

 the accumulation of products resulting from the decrease in permeability, 

 when it reaches a certain stage, produces a stimulus of such powerful 

 nature that the original degree of permeability is restored. It is note- 

 worthy, in this particular, that passive stages or encystment are common 

 in such forms and aid in the process of waste elimination. 



Is there any reason to believe that sexual reproduction rejuvenates 

 through a restoration of the original degree of permeability? If senility 

 is clue in part to decreasing permeability, then rejuvenescence must be 

 due in the same degree to increased permeability. 



The effects of sexual reproduction can be most easily studied in the 

 eggs of marine forms, which are normally fertilized when free in the water. 

 A considerable number of researches on such material have been made 

 by animal physiologists, and among the functions studied was this one 

 of permeability. All these investigators have described a marked increase 

 in permeabiUty as the most characteristic effect of the entrance of the 

 sperm into the ovum. This union of the two sex cells, therefore, generates 

 a stimulus of such a powerful influence on permeability that the original 

 degree of permeability is restored. Lilhe (1909) has observed that pig- 

 ments present begin to pass out rapidly; Lyon and Shackell (1910) found 

 that dyes entered much more readily than before; while Loeb (1907), 

 Warburg (1910), and others have observed that oxygen entered much 

 more rapidly than before. Lillie (1909) thinks that this first remarkable 

 increase in permeability lessens to a certain degree soon after; but 

 this, if true, merely means that the first great increase in permeability 

 is even greater than that characteristic of youth — one that is valuable for 

 the sudden expulsion of accumulated waste, but too great to be suited for 

 the retention of food, for turgidity, and for protection in the living 

 embryo. 



The evidence seems strong, therefore, both from the standpoint of senility 

 and from that of rejuvenescence, that the duration of life is directly linked 

 with the degree of permeability in that part of the living cell which places 

 it in contact with the universe about it, and that as the activities of hfe 



