594 The » Philéon’ ine Journal of Science 1920 
For concentrations above 1.60 atmospheres the T7R1C1 graph 
lies above the other three for all concentrations. The water 
requirement for these cultures, which are very high in chloride, 
is thus higher than for the other three sets for all concentrations 
between these limits. The graphs for the other three sets of 
salt proportions lie very close together, and cross one another 
frequently, so that there appear to be no significant differences 
between them. 
If equal dry weights in different cultures corresponded to 
equal leaf areas, then the water requirement would of course 
give an approximate measure of the transpiring power per 
unit area of leaf surface; that is, of the ability of a unit area 
of leaf surface to give off water by transpiration. *? It is of 
course not to be expected that equal dry weights do corre- 
spond at all rigidly to equal leaf areas, and hence the use of 
water requirement as a measure of transpiring power would 
be only a rough approximation at best. But the relations that 
have been observed in these studies between concentration and 
water requirement are likely to be approximately the same as | 
those between concentration and transpiring power of unit 
leaf area. Transpiring power might be expected to decrease, 
in the way represented by the graphs, with increasing osmotic 
concentration of the medium surrounding the roots. Trans- 
- piring power, for the whole plant, is taken to be practically 
identical with water-absorbing power, measured by total water 
absorption. 
The graphs for the water requirement of roots (fig. 12) 
show the same general features as those already described for 
tops. There is a general decrease in the water requirement 
of roots with an increase in concentration of the culturé medium. 
The graphs frequently cross, and all have about the same slope, 
though above 3.50 atmospheres the T1R1C1 graph falls less 
rapidly than do the others. 
The water requirements of roots may be taken as an approxi- 
mation of the water-absorbing power of the roots per unit root 
weight and also of their absorbing power per unit area of root 
surface. This is seen to decrease with an increase in the osmotic 
concentration of the medium surrounding the roots, as might 
be expected on physico-chemical grounds. 
“ Livingston, B. E., The resistance offered by leaves to transpirational 
water lost, Plant World 16 (1918) 1-85. Livingston, B. E., and Hawkins, 
Lon A., The water-relation between plant and soil, Publication Carnegie 
Inst. Washiigton 204 (1915) 5-48. 
