52 



THE RELATION OF DESERT PLANTS TO 



stood in the shade near the fixed evaporimeter. Withering of leaves 

 was beginning to be manifest at the time of the second weighing; 

 therefore no more weighings were made. The data for both experiments 

 are given in Table X. 



The symbols Rt, etc., of this table have the same significance as in 

 Experiment I. It will be noticed that the hourly rate of transpiration 

 from these plants was 0.839 or 0.83 mg. for unit leaf surface. No data 

 were obtained for the comparative rates of night and day periods.* 



TABLE X. Data for Experiment II and III. 



Experiment IV. The subject of this experiment was a thrifty plant 

 of Tribulus brachystylis standing in the shade on the north side of the 

 building. It was sealed and the record was begun at 9 h 30 ra a.m., August 

 13, and was continued until 8 h 30 m a.m., August 15. The total leaf area 

 was 39.69 sq. cm. 



This plant has a more marked nyctitropic movement than the 

 Euphorbia of Experiment I. The movement consists in the rising of 

 the leaflets of the pinnately compound leaves until their upper surfaces 

 approximate each other, after the manner of the similar movement in 

 Gleditschia triacanthos. 



The table of fundamental data will be omitted in this and the follow- 

 ing experiments, the essential points being brought out clearly by the 

 curves. The curve of relative transpiration for this experiment, 

 together with those of temperature and rate of evaporation, are given in 

 figure 9. These are constructed on the same plan as those of Experi- 

 ment I. 



An inspection of figure 9 shows that during the progress of the 

 experiment the rate of relative transpiration, while showing something 



*Prof. F. E. Lloyd was studying the transpiration of this species especially with 

 reference to night and day rates, while the present work was in progress. His paper 

 on this subject has not yet appeared. 



