Senile Changes in Leaves of Certain Plants 



343 



surface when growing in strong light than when growing in the shade, and 

 Brenner (1902) has stated that at first, when leaves are moved into the 

 shade, the increase in their size due to greater humidity causes the stomata 

 to become farther apart, but that later new Anlagen are formed and the 

 number is brought back ahnost to that found in sun-lit leaves. The dif- 

 ferences in number of stomata per square milhmeter on the leaves of Vitis 

 vulpina given above, however, cannot be due to any difference in hght 

 exposure or humidity or available water, since the leaves used were borne 

 on cuttings grown under identical conditions. It was noted also that the 

 average size of the apertures of the stomata decreases with age, as does 

 also the size of the guard ceUs. The figures given in table 57 were 

 obtained from accurate measurements made on the same material as was 

 used for counting stomata. Since the width of the apertures varies directly 

 with the degree to which the stomata are open, while the length of the 

 apertures is approximately the same at aU times, the measurement selected 

 as best representing the size of the stomata was the long diameter of the 

 stomatal opening between the guard cells. Measurements of the width of 

 the apertures of stomata when fully extended indicated that the relation 

 of the long to the short diameter was much the same in both young and 

 old vines. The long diameters of the stomata can therefore be used as a 

 measure of the size of the apertures. 



TABLE 57. Size of Stomata in Vitis Vulpina of Different Ages 



Long diameter of aperture in 





6 vines with 





6 vines with 





from 5 to 7 rings 





from 20 to 30 rings 





(microns) 





(microns) 





18.3 





9.2 





14.6 





9.2 





14.6 





11.0 





18.3 





12.8 





18.3 





9.2 





14.6 





9.2 





18.3 





11.0 





14.6 





12.8 





22.0 





9.2 





18.3 





11.0 





14.6 





12.8 





14.6 





9.2 





18.3 





9.2 





14.6 





11.0 





14.6 



Average . . 



14.6 



Average. . . 



16.6 



10.8 



