Senile Changes in Leaves of Certain Plants 337 



TABLE 52. Summary of Results on Rate of Eltmtnation of Carbon Dioxide from 

 Picked Leaves op Vitis vulpina of Different Ages 



Average age of young vines used 



Average age of older vines used 



Number of determinations made 



Average length of each determination 



Rate of elimination of carbon dioxide in young vines. 

 Rate of elimination of carbon dioxide in older vines . . 



7 . 3 years 

 25 . 4 years 

 92 

 23.86 hours 



. 0349 per cent per hour 

 . 0297 per cent per hour 



The general average for the plants tested indicates that the rate of 

 elimination of carbon dioxide from the picked leaves was about 17 per 

 cent more rapid in the young plants than in the older ones. Any function 

 so complex as respiration is affected by many factors. While conditions 

 were identical in these tests as regards external conditions, and while the 

 results doubtless indicate a true difference in the metabolic power of the 

 respective tissues, nevertheless irregularities are present which point to 

 differences in some factor other than age. The amount of water present 

 is certainly one of these differences. 



The respiratory values here shown do not necessarily indicate a slower 

 katabohc activity on the part of each individual cell of the leaf. This 

 may or may not be the case. The total decrease for the leaves of the older 

 plants is doubtless due in part to the lesser total volume of parenchym- 

 atous tissue in them as compared with that in the leaves of the younger 

 plants. Whatever the cause of the slower respiration in the older leaves, 

 its presence may indicate that the leaves of the younger vines are more 

 efficient organs. 



The evidence presented seems to show that the encroachment with age 

 of vascular tissue on the space occupied by photosynthetic cells in the leaf 

 of a young plant results in decreasing assimilation and respiration. The 

 decrease in size of the vein islets is therefore a real degeneration in the 

 structure of the leaf, just as the invasion of connective tissue into the 

 higher tissues in senile animals is a degeneration. As the replacement 

 of photosynthetic tissue by vascular cells progresses with age, each new 

 crop of leaves will be less efficient in the production of carbohydrates than 

 the preceding crop; and ultimately the plant will begin to suffer from lack 

 of food, and if no other cause of death intervenes it will in time starve to 

 death. The rapidity of the progress of senile degeneration in plants varies 

 with the species, as it does in animals, but ultimately senile death will occur. 



