34 TRANSPIRATION IN A DESERT TERENNIAL. 



OBSERVATIONS ON TRANSPIRATION EXPERIMENTS. 



1. The 18 relative transpiration curves taken in the sun all show a maxi- 

 mum earlier in the day than the time of maximum evaporation. 



2. All of the relative transpiration curves, except those from experiments 

 I, II, VI, and a part of IX, show an alirupt drop after the maximum, 

 followed b}-^ a rise which appears sometimes an hour earlier than the time 

 of maximum evaporation and sometimes coincident with it, but never later. 



1 A. An early maximum in actual transpiration corresponding to that 

 of the relative transpiration appears in all of the plants growing in the open, 

 except those in experiments I and II. In the case of potted plants this 

 maximum behaves somewhat irregularly. In experiment VI it is not pro- 

 nounced, in VII it is lower than a second maximum, in IX it fails to appear 

 in the case of three hot-house-grown plants and appears as a secondary 

 maximum in the case of the transplanted seedling. 



2 A. All curves of actual transpiration of plants growing in the open, 

 with the exception of I and II, show a drop and rise in the morning. As 

 in the case of the relative curves, the potted plants differ among themselves. 

 Experiment VI shows the drop and rise to a small extent; IX shows a 

 flattening of the slope in the morning followed l)y an increase in the angle 

 of slope in the three hot-house plants and a drop followed by a rise for the 

 transplanted seedling; X shows straight lines followed by a rise at the time 

 of the drop in the relative transpiration. 



3. It will be seen from table 14 that leafless seedlings and leafless branches 

 of adult trees give the same average for maximum relative transpiration 

 for the day, and that this average is lower than the average maximum for 

 branches in leaf, and this is in turn much lower than the average maximum 

 for potted j^lants. The one seedling which was transplanted from the oi^en 

 shows a maximum somewhat under the maximum average of branches in 

 leaf, but is higher than two of the individual maxima of which the average 

 is made up. 



4. The differences in anatomical features shown in the sections in figure 1 

 indicate that the same relation exists between the water-losing power 

 in leaves as compared to stems antl in green-house i)lants as compared to 

 those grown in the open, as is shown in the preceding paragraph. 



5. Experiment X shows that a change of soil moisture in one potted plant 

 from 15.1 per cent to 3.5 percent, i. e., a lowering of 76.8 per cent, was accom- 

 panied by a lowering of 98.4 per cent of the original amount of relative 

 transpiration; in a second plant a change of the soil moisture from 14.5 to 

 3.4 per cent, i. e., a loM-ering of 76.0 per cent, was accompanied by a lowering 

 of 97.2 per cent of the original relative transpiration. In only one case 

 were valid readings of soil moisture, at a depth of 30 cm., taken at times 

 corresponding to transpiration readings from trees, and this shows that a 

 change in soil moisture from 14.0 per cent on March 27 to 32.0 per cent 

 on August 22, i. e., an increase of 128 per cent, was accompanied by a 

 change in relative transpiration from 0.181 on March 27 to 0.302 on August 

 22, i. e., an increase of 72.3 per cent. 



