66 



THE RELATION OF DESERT PLANTS TO 



from the last 24 hours of the experiment, or for as nearly that period 

 as was possible from the data at hand. The rates marked with an 

 asterisk (*) in the third column are for plants which had begun to 

 wilt at the end of the transpiration record; in the last column is given 

 the water content of the soil at the end of the experiment, in per cent 

 of its volume under water. These moisture determinations all cor- 

 respond to soils in which incipient wilting had just occurred. 



TABLE XV. Relation of Transpiration to Moisture Content of the Soil. 



It appears from the table that the wilting point for these plants, in 

 terms of moisture content of the soil, lies between 5.5 and 13.7 per 

 cent. It was lowest for Fouquieria, intermediate for Allionia and 

 Boerhavia, and highest for the squash plants of Experiment XV. 

 Judging from the other experiments with Boerhavia, the high moisture 

 content of the soil in Experiment XI is probably erratic. This general 

 arrangement of the different plants in regard to their power to with- 

 stand a dry soil is what should have been anticipated from their char- 

 acters. Fouquieria and Euphorbia are extreme xerophytes, while the 

 squash is a mesophyte. Allionia and Boerhavia, although they are 

 desert forms, are not active during the dry season, are not markedly 

 xerophytic in their structures. 



As has been noted, all of the determinations of moisture content 

 correspond to incipient wilting on the part of the plants involved. In 

 Experiment I this did not occur until 15 days later than the termination 

 of the transpiration record, so that the rate of transpiration given in the 

 table is probably somewhat too high to correspond to the last 24 hours 



