RESISTANCE BY LEAVES TO WATER LOSS 



31 



that the lower leaf surface of this same plant possessed a much 

 greater transpiring power at the time of observation than did the 

 lower leaf surface of the xerophytic form. The lower foliar sur- 

 face of the latter plant exhibits transpiring power about five times 

 as great as that of the upper surface of the same leaves. 



Experiment IX. On September 8, 1912, tests of the foliar 

 transpiring power of Sphaeralcea pedata Torr. were made above 

 Oracle, and on the following da}" the same tests were applied to 

 sensibly similar plants at the Desert Laboratory. The plants 

 were vigorous and in bloom in both cases and were of about the 

 same size. The leaves were apparently alike at both localities, 

 large, thin and practicalh' without hairs, these tests being per- 



TABLE 6 

 Sphaeralcea pedata 



LEAF SURFACE 



TIME 



IXDEX OP TBANSPIKING POWER 



RELATIVE ORACLE, 

 ^ INDEX AS UNITY 



Oracle 



Des. Lab. 



Oracle 



Des. Lab. 



Des. Lab. 





seconds 



seconds 









Upper 



36 



44 



0.378 



0.289 



0.76 



Lower 



29 



42 



0.469 



0.302 



0.64 



Entire 







0.424 



0.296 



0.70 



Water test (saturated paper 





13 









formed at the end of the summer rainy season, i- The results of 

 the two tests are presented in table 6, which explains itself. In 

 both cases, the data apply to hour 13. 



It is clear that the indices of transpiring power are lower for 

 the Oracle plants than for those at the Desert Laboratory. The 

 average index for the two leaf surfaces is 30 per cent lower for * 

 the plants at the higher station than for those at the lower. This 

 is obviously due to a greater difference (36 per cent) between 

 the indices for the lower surface and a smaller one (24 per cent) 



'2 In the dry season the leaves of this species are markedly different from those 

 produced with low evaporation rates; in Maj' and June the plants are characterized 

 hy small, thick, denseh- hairj- leaves. In this connection see the following paper: 

 Livingston, B. E., Evaporation as a climatic factor influencing vegetation. Horti- 

 cultural Society of New York. ]\Iemoirs 2 : 43-54, 1910. 



