Senile Changes in Leaves of Certain Plants 331 



may occur. There are two principal causes for such an improvement, 

 both resulting from repeated exercising of the special activity involved. 

 The first is the improvement in concerted action between the different 

 speciahzed tissues involved in even the simplest activity. The second 

 is the increase in size and power of the specialized cells concerned, which 

 not only continues as long as the original impetus of vital activity persists, 

 but is stimulated by the mutual interaction of related tissues. The benefit 

 of practice in athletics, for example, is due to improved coordination in 

 action of muscles, nerves, heart, lungs, and kidneys, as well as to the 

 increased specialized power that exercise stimulates in the cells of the 

 tissues of these organs. In animals, therefore, the increase in efficiency 

 is due to a longer period of interaction and development of specialized 

 tissues. 



The specialized tissues in the leaf of the old vine have neither worked 

 longer together, nor had a longer period of development, than the 

 specialized tissues in the leaf of the young vine, and therefore there is no 

 possibility of any such increase in efficiency of any of their tissues as 

 that which may occur in animals. Decrease in assimilating tissue means, 

 therefore, a decrease in the amount of carbohydrate production by the 

 leaf. 



In order to get actual figures on this point a number of determinations 

 were made of the increase in weight during daylight by the leaves of old 

 and of young plants. As the leaves of the cuttings were reserved for another 

 purpose, the leaves of the paired vines had to be used. Since in only a 

 relatively few cases a sufficient number of leaves with identical exposure 

 to light could be obtained, the determinations are too few to give more 

 than a general corroboration of the reasoned effect of the change in 

 venation. The impossibility of using gasometric methods on a wide 

 range of wild vines justifies the use of a method that is not strictly accurate, 

 but that is capable of giving at least relative results. The method used 

 was to punch disks, with a Ganong leaf cutter, from both sides, the base, 

 and the apex of each of a number of selected leaves on the paired vines 

 in the morning, and again at night, carefully drying the disks for three 

 days at, 80° C, and determining the gain in weight as a percentage of 

 the weight of the morning disks. Only relative results were sought for, 

 and therefore no system for measuring light intensity or recording tem- 

 perature was used. The disks were killed by steam immediately after 



