WATER-RELATION BETWEEN PLANT AND SOIL. 43 



ENVIRONMENTAL ARIDITY (IXE). 



The February graph here exhibits its maximum (8.94) with hour 13, 

 the same hour as shows the maximum in evaporation. The February 

 period of low night values extends from hour 23 to hour 7 of the day 

 following, having an average value of 0.98. The ratio of the maximum 

 to this average is 8.6. The March graph shows a maximum (17.92) 

 an hour later than the first, its night period reaching from hour 23 to 

 hour 6. The night average is here 0.88 and the maximum is 20.4 

 times this average. The pronounced difference between the two 

 maxima, largely related to the corresponding difference between the two 

 graphs of soil-absorbing power, again illustrates the manner in which 

 internal conditions may indirectly affect external ones; the soil condi- 

 tion in our experiments depended very largely, at any one time, upon 

 the rate of root absorption during the period just past. 



DAILY MEAN CONDITIONS COMPARED. 



The daily averages at the base of table 3 show a general condition 

 of affairs resembling that shown for Coleus but differing from that for 

 Vicia. The evaporating power of the air (39 per cent greater in the 

 March series than in the February one), the resistance of the soil to 

 root absorption (7 per cent greater in the March series), and the 

 environmental product (71 per cent greater in March), agree in the 

 direction of their differences. On the other hand, the two remaining 

 powers here considered possessed higher magnitudes in the February 

 period. Absolute transpiration shows a daily average 16 per cent 

 greater in February, and transpiring power shows an average 65 per 

 cent greater in the earlier period. 



GENERAL RESULTS. 



From the foregoing presentation of some of the more outstanding 

 details for the three plants a few general points are fairly well brought 

 out, though the tentative nature of conclusions drawn from a first 

 study of such a complex inter-relation of factors as is here involved 

 must be clearly recognized, and anything like a reliable interpretation 

 of many of the features of the experimental data is quite impossible 

 at present. It appears, in the first place, that the study of the diurnal 

 march of the rate of loss from the irrigator, in connection with simul- 

 taneous study of the corresponding march of transpiration and of the 

 evaporating power of the air, promises to throw considerable light on 

 the general problem of the dynamics of the water-relations of plants. 

 The method here employed is susceptible of improvement in many 

 ways, but it appears to look in a promising direction, at least. 

 The value of this measure of the resistance offered by the soil to root 

 absorption, when employed in cultures where the soil-moisture content 

 is not maintained nearly constant, but is directly influenced by external 



