50 THE RELATION OF DESERT PLANTS TO 



by the weighing method, the ratio of water loss from three vessels, 

 one containing free water, the second containing moist soil, and the 

 third similar to the second but with growing plants of Xeranthemum 

 bracteatum. The amount of transpiration was obtained by subtracting 

 the decrease in weight of the second dish from that of the third, the 

 assumption being made that the soil would lose water at the same rate 

 whether with or without plants. This writer's periods of observation 

 were so long, being about a week, that his results failed to bring out 

 the variations in the rate of relative transpiration with which we are 

 chiefly interested here. 



A study of the relation of external factors to this physiological 

 periodicity will be instructive. In the first place, it was noticed at 

 once that these periods do not coincide at all with the periods of nycti- 

 tropic movement. The leaves were observed to take their nocturnal 

 position between 3"30 m and 5 h 30 m in the afternoon and to return to their 

 diurnal position at about the same time in the morning, while relative 

 transpiration began to decrease, and even to decrease rapidly, several 

 hours earlier in the day than there was any evidence of leaf folding. 

 Thus the leaf movements are shown to be of comparatively little im- 

 portance in determining the rate of relative transpiration, and stomatal 

 or internal adjustments appear to be the probable controlling factor. 

 This plant transpires mainly from the lower surface of the leaves, and 

 the closing of these organs does not affect evaporation from this surface. 



To facilitate the study of the relations existing between external 

 conditions and these variations in relative transpiration, the minimum 

 and maximum points on the curve of the latter have been designated in 

 figure 8 (p. 47) by heavy vertical lines. The lighter vertical lines merely 

 divide the day from the night periods. 



A study of the points where the heavy lines intersect the other 

 curves and the axis of abscissas brings out certain interesting facts. 

 Obviously the maximum for the first day of the experiment is not 

 shown. The first minimum is at 8 p.m. Following down the line from 

 this point to the other curves, we find that increase in relative trans- 

 piration began at a time when the air temperature was 75 F., and 

 when the evaporating power of the air was such as to produce evapora- 

 tion from unit water surface at the rate of 0.029 gram per hour. De- 

 termining these data for each maximum and for each minimum point 

 throughout the curve of relative transpiration, we arrive at the facts 

 presented in Table IX. This table gives the hour, temperature, and 

 hourly rate of evaporation from unit water surface, together with the 

 maxima and minima of relative transpiration with which they are coin- 

 cident in time. 



