STOMATA AND RELATIVE HUMIDITY. 



8 9 



tribute to the brevity of the life span of the primary leaf, though I do not 

 for my own part believe so. 



In table 46 measurements are given of the stomata of both primary and 

 secondary leaves. 



Table 46. — Measurements of stomata oj primary and secondary leaves. 



I was unable to take the relative humidity in these days, but judging from 

 other experiences a difference of 30 per cent between July 25 and 26 would 

 be a very conservative guess, and it is not improbable that it was as great 

 as 60 per cent. By the morning of the 28th the humidity would have 

 dropped back to 20 per cent or thereabout. Assuming these figures to be in 

 the main correct, it would appear that, after a showery night, the high relative 

 humidity made possible, on July 26, the opening of the stomata of both 

 primary and secondary leaves to an average transverse measurement twice 

 that on the day previous at the same hour. It is, of course, possible that this 

 was the result of an increased supply of soil-water or of water in the stem 

 absorbed through the areas immediately contiguous to the basis of the rosettes 

 of secondary leaves, or, again, of a conservation of the supply of water in the 

 stem by the wetting and relative humidity. Facts are already at hand to 

 show that the ocotillo is especially sensitive to increased soil-water (Cannon, 

 1905), to meteoric water (Lloyd, 1905), and probably also to a high relative 

 humidity independently of these (Cannon, 1906). It will be seen, however, 

 that on the following two days, July 27 and 28, the stomata of the secondary 

 leaves gradually increased their dimensions at the same hour (from 3 to 6 micra) 

 while those of the primary leaves decreased (from 9 to 7 micra) , and this under 

 a much reduced relative humidity. Even allowing for individual differences 

 in leaves and errors of measurement, it appears conclusive that the two sets 

 of stomata behaved differently; and this is difficult to explain. The behavior 

 of the stomata of the secondary leaves suggests the effect of soil-water, but 

 why this factor did not influence the primary-leaf stomata similarly is quite 

 unaccountable. It is possible that the formation of scission tissue had set in, 

 which results in the separation of the leaf-blade from the endogenously formed 

 spine (Robinson, 1904), but I endeavored to guard against this possibility 



