40 WATER-RELATION BETWEEN PLANT AND SOIL. 



series (4.7). These ratios are greater than the corresponding ones for 

 evaporation (2.9, 4.6) and not markedly below those (5.5, 5.1) for 

 irrigator loss. 



Comparing these two graphs, it is apparent that transpiring power 

 was generally lower in the second series than in the first, a feature 

 probably to be related, as in the case of Coleus, to a maturing of the 

 leaves between the times of the two series. 



ENVIRONMENTAL ARIDITY (/x£). 



These two graphs agree fairly well. Both show maxima at the hour 

 of the greatest evaporation intensity, as should be expected from the 

 nature of the conditions, and both show about the same night values. 

 It is probably safe to infer that the low night value was not really 

 attained in either case until hour 23 and that the night period ended 

 with hour 6. On such a basis the average night values are 1.24and 1.20, 

 for the two series, respectively. The only apparent and considerable 

 differences between the two graphs lie in the facts that the earher series 

 shows a slightly higher maximum and a somewhat more rapid rise in 

 the early forenoon. The two maxima are 17.0 and 15.5 times the cor- 

 responding night averages. In this case, then, the ratio of maximum 

 to night average was greater in the February series, while the opposite 

 held for Coleus. 



DAILY MEAN CONDITIONS COMPARED. 



The daily averages at the base of table 2 show that, while the evapo- 

 rating power of the air was, in general, 39 per cent greater for the March 

 than for the February period, all the other features here considered 

 were greater for the earlier period, a fact almost surely due to lowered 

 transpiring power of the plant. The attraction of the soil for water 

 was 37 per cent and the environmental aridity was 2 per cent, the 

 mean transpiration rate was 32 per cent and the transpiring power was 

 70 per cent greater in March than in February. 



The disagreements between these features and the corresponding 

 ones ioT Coleus seem to illustrate how the internal conditions of the plant, 

 though appearing — at first thought — quite distinct from the external 

 surroundings, play an important role in determining the quantitative 

 conditions of the latter. The environmental aridity is conceived as a 

 function of the atmospheric water-extracting power and the power of 

 the soil to attract moisture. While the former of these two powers 

 may usually be regarded as independent of the latter, the latter is 

 seldom independent of the former — not at all so in our experiments. 

 The atmospheric evaporating power tends usually, in nature, to dry 

 the soil about the roots in two ways: (1) by direct evaporation from 

 the soil-air surfaces and (2) by increased root absorption. In our cul- 

 tures the plastiline seal prevented the former action ; the only way by 

 which the soil adjacent to the roots became less saturated with water 

 was through increased root absorption. But the latter is largely 



