Senile Changes in Leaves of Certain Plants 283 



first month than the second, indicating that a decrease in the rapidity 

 of growth sets in even before birth. 



Data are abundant regarding the yearly percentage of gain in weight 

 made by children. The average increase in weight during the first year, 

 relative to the weight at birth, shows a percentage of increase of 200; 

 that of the second year only from 22 to 30; that of the third year from 

 14 to 20. From the fourth year to the twenty-fifth the rate of increase 

 is subject to irregular variations, mostly downward. From the twenty- 

 fifth year to the fiftieth there is httle or no change in weight, on the average. 

 After fifty years, as a rule, there is a loss in weight. Detailed tables, 

 with graphs showing the steady decrease in rate of growth with increasing 

 age, can be found in Donaldson's papers (1895, 1906, and 1908); while 

 Minot gives carefully selected data from various sources, to which he has 

 added graphic illustrations. 



It is evident that the rate of growth in animals thus far investigated 

 declines from the very beginning of mdividual life, and that this is a 

 manifestation of inherent forces which tend to slow vital activities. 



Decrease in rate of respiration.— The rate of respiration in mammahan 

 animals can be investigated only in the years after birth. There is then 

 evident a gradual decrease of the same general type as that found for 

 growth. In the last stage of fife a shght increase occurs, similar to that 

 shown in the heartbeat and explained under that topic. 



Decrease in rapidity of heartbeat. — The following figures are taken 



from a compilation by W. T. Porter, quoted in Minot's book, and refer 



to man: ,^ . 



Mean frequency 



of heartbeat 



Age per minute 



Fetus 140 



1st month 134 



5th year 103 



10th year 91 



15th year 82 



20th year. 74 



25th year 72 



55th year 72 



65th year 73 



80th year 79 



