vi TENSION REQUIRED TO RAISE THE SAP 123 



then, where the branch had only 500 leaves, while the 

 whole tree had 9,000,000 (1 : 18,000), it is probable that 

 the effectiveness of the former was much greater than 

 that of the remaining leaves of the tree. Such an over- 

 estimate in the amount transpired involves, according to 

 the method, an exaggeration in the velocity of the current 

 in the trunk. 



The control of transpiration exercised by the freedom 

 of supply may be easily observed by means of the weigh- 

 ing method. The amount transpired will be found to 

 fall off as the plant exhausts the water in the soil round 

 its roots, and to rise when the soil is again rendered moist. 

 The following numbers (Table 12) illustrate this fact 

 in the case of a small yew-tree which was exposed to 

 conditions favourable to transpiration on seven successive 

 days. The conditions were fairly uniform, as throughout 

 the experiment the sky was lightly over-cast and a light 

 east wind blew. 



Table 12. 



The dependence of transpiration on the supply is prettily 

 illustrated by Darwin's experiment with the horn hygro- 

 scope. Darwin records that when the hygroscope is 

 applied to the leaves of a branch severed from a plant, 

 it indicates a gradually diminishing rate of transpiration 

 as the store of water in the branch is gradually exhausted. 



