48 TRANSPIRATION IN A DESERT PERENNIAL. 



and a new curve was made by connecting the points of intersection. Figure 

 23 gives these resulting curves one space below their actual position in order 

 to avoid numerous intersections with the original curves. This method of 

 obtaining curves comparable in time is based on the assumption that both 

 the relative transpiration and the water content change at a constant rate 

 between the successive readings. 



DISCUSSION. 



The curves resulting from the plotting of water content determinations 

 of the two trees, I and II, are fairly consistent in general shape. In the 

 case of the end twigs there is a minimum water content about noon followed 

 by a subsequent rise. The decrease begins in one case about 8 h 30 m a. m. 

 and in the other at 10 h 30 m a. m. ; in the former case recovery is slower than 

 in the latter. In the case of branches a meter from the ends it may be said 

 that in general the slopes of the curves are in opposite directions to the 

 slopes of the twig curves. The branches 2 meters from the ends behave 

 differently in the two trees; in I 3 the curves are, in general, parallel to thel2 

 curve, while curve III 3 is nearly flat or only slightly lower in the middle of 

 the day than at the two ends of the day. 



The curve of relative transpiration from experiment III (see p. 20) is 

 plotted at the top of fig. 22. Although experiment III was performed in 

 March and experiment XVIII in August, the conditions of soil moisture and 

 evaporating power of the air at the two dates make it seem fair to compare 

 the two sets of readings. The soil moisture at a depth of 30 cm., as given by 

 weekly determinations made by Dr. Forrest Shreve, was 15 per cent on 

 March 12 and 15.7 per cent on August 6. The summer rains which had 

 intervened allowed for recovery from the extreme conditions of drought 

 obtaining during the arid fore-summer. It may be, of course, that trans- 

 piration conditions were different in the two cases, but the probability is that 

 they were much the same. 



The following theory for the behavior shown is suggested. The relative 

 transpiration rate increases until about 9 h 30 m a. m., when the demand for 

 water made upon the plant by the evaporating power of the air exceeds the 

 supply of water furnished by the transfer from the lower portions of the plant, 

 and there results a slight drying out of the walls of the cells which form the 

 true transpiring surface, both internal and external. This operates in two 

 directions : the one, a lessening of the relative transpiration rate because the 

 evaporating surface of the intercellular spaces has receded into the pores of 

 the cell walls; the second, an assumption of the tensile state of the water in 

 the vessels and cell walls, which results in a "pull" of water from the lower 

 portions of the same stem and, as an immediate result of this, a transfer 

 of water from lower to higher portions faster than the supply still farther 

 down can stand, and therefore an increase in the size of the air bubbles in 

 the vessels caused by the vacuum formed and hence a lower water content 



