1919] CRIBBS—TILIA AMERICANA 



279 



tial single factor influencing the transpiration index (Briggs and 

 Shantz 2), and so far as considered in these studies it has been 

 found to be a factor of the first order. It is not always the con- 

 trolling one, however, for there are frequent variations in the 

 daily march which ow£ their appearance to other causes. . Tem- 

 perature, stomatal movement, and growth water, for instance, 

 at any given time may become the dominating factor, only to 

 be succeeded as the conditions vary by some other which for the 

 time being replaces it. 



The proportional increase in the soil temperature over that 

 of atmospheric temperature from A to E is of interest, and it may 

 safely be inferred that high soil temperatures have a prominent 

 part in the maintenance of the water supply, and hence indirectly 

 concern the transpiration rate. Until more accurate quantita- 

 tive measurements are made upon the effect of increase of 

 temperature of the soil upon the water absorbing power of roots, 

 however, the quantitative influence of this factor can only be 

 speculated. Such work as has been done by Rysselberghe (14) 

 on the effect of temperature changes on the rate of osmotic trans- 

 fusion through semipermeable membranes leads to the conclusion 

 that the much higher soil temperature, as of E over station A, 

 must be one of the factors which enable the exposed plants to 

 take over water from the soil with sufficient rapidity to withstand 

 the much higher transpiration loss. 



One of the noteworthy features of the dune studies appears 

 in the relation of the growth water to transpiring power. It may 

 be seen from the figure that there is a low average of growth 

 water at all stations, that for A being only about 2.46 per cent 

 and that for E 1 .25 per cent. While it is true that the highest 

 average is at A, the greater percentage here is due to the higher 

 content during the earlier summer months. During the latter 

 part of July and August, however, the water available for plant 

 absorption decreases more rapidly than on the open sand, and the 

 frequent drop in soil moisture to the wilting coefficient produces 

 a period of stress and leads to early abscission. Meanwhile 

 the growth water on the moving complex remains practically 

 ncostant, especially at a depth of 3 dm. or below, where it is about 



