PLANT HABIT AND CONDITION. 73 



of the first part of July 1916, while leaves produced a month earlier 

 had only 173 stomata per square millimeter, averaging 7.9 M 

 long. The ratio of stomata to other epidermal cells was the same, 

 and hence the difference was merely one of expansion or the size 

 of the cells. This expansion does not affect guard-cells and 

 epidermal cells equally, since the average increase of area was 1.4 

 greater in the June leaves than those formed in July, while the area 

 of each pore when wide open was 1.7 greater. 



The diffusive capacity, n^/ab^ of the stomata when wide open 

 was very nearly the same, that of the July leaves being 1,103 and 

 the June leaves 966, a difference of less than 5 per cent. As this 

 was the greatest difference found, and the diffusion capacity of 

 the leaves usually checked within 1.6 per cent, there was no ad- 

 vantage in using this instead of the percentage of maximum opening, 

 except where it was desired to compare the diffusion capacity of 

 the different species other than stomatal movement. Hence, the 

 difference in expansion has little or no effect upon the diffusion 

 through the stomata, or upon the effect of stomatal movement upon 

 such diffusion. The stomata of these larger leaves, however, are more 

 sensitive to low humidity than those of the smaller leaves. It may be 

 pointed out in this connection that Eckerson's figures for the number 

 and size of stomata of various species apply only to greenhouse plants, 

 as field plants differ greatly from these in most cases examined. 



The effect of hairs, wax, and sunken stomata is to protect the 

 leaves from water-loss to a certain degree. In the case of waxy 

 leaves and leaves with sunken stomata, it obviously is impossible 

 to determine the effect of these by direct comparison, but in the 

 case of hairy leaves the hairs can be removed, at least in part. Two 

 experiments were performed upon Encelia farinosa in March 1918, 

 at Tucson, Arizona, comparing the movement in "shaved" and in 

 normal leaves. The woolly hairs of a number of leaves were re- 

 duced as much as possible with a safety razor. On the following 

 day a short series was made, beginning at 8 a. m. and ending at 

 noon. The stomata of the shaved leaves were wide open when the 

 series started and remained at maximum 1 hour. At 10 a. m. they 

 had closed to 60 per cent, at 11 a. m. to 20 per cent; at noon closure 

 was complete. The stomata of the normal leaves were only 70 

 per cent open at 8 a. m., opened to maximum at 9 a. m., and re- 

 mained in this condition to the end of the series. The second 

 experiment showed essentially similar results. The removal of 

 most of the hairs increased the amount of light reaching the stomata; 

 this accounts for the earlier opening in the shaved leaves. The 

 closure before noon may be ascribed to a greater water-loss from 

 the leaves when the hairs were removed. Inspection of the leaves 

 supported this view, but unfortunately no determination could be 

 made of the percentage of water in the shaved and normal leaves 

 at the time. Nevertheless, the stomatal closure and less turgid 



