vii OSMOTIC PRESSURES OF LEAF CELLS 153 



weighed vessel containing water was supplied to its 

 protruding end, and then the pressure was lowered 

 to that of the atmosphere. After ten minutes, the 

 vessel was re- weighed and was found to have lost 0T08 gr. 

 due to the elastic recovery of the conduits. When this 

 allowance is made in experiment C, Table 18, the amount 

 transpired becomes 0*219 gr. instead of 0T11 gr. 



In order to determine whether this elastic contraction 

 of the conduits occurred chiefly in the conduits of the 

 stem or leaf, experiments were made in which a branch 

 was first exposed to a pressure of 6 atmospheres for 

 ten minutes, and while this was still maintained, a 

 weighed quantity of water was supplied to its lower end 

 which protruded from the high-pressure apparatus. The 

 pressure was then immediately lowered, and the branch 

 was left to draw up water from below for ten minutes by 

 means of its elasticity, the amount drawn up being 

 measured by a second weighing. When this amount is 

 compared with the amount drawn up in a similar experi- 

 ment with the same branch when all the blades of the 

 leaves are removed, it is found that the former is very 

 much greater than the latter quantity. Thus, with a 

 branch of Tilia americana bearing 11 leaves, the first 

 amount was 0T08 gr., while the latter was only 0*02 gr., 

 a quantity which approaches the limits of error of the 

 experiment. From this we may conclude that the elastic 

 contraction takes place chiefly in the conduits of the 

 leaves. 



Determinations with compressed air. At a sub- 

 sequent date, when it was possible for me to generate 

 higher air pressures, it was found that the leaves were able 

 to withstand considerably higher pressures when not 

 exposed to the harmful effects of carbon dioxide. Thus 

 the leaves of Helianthus multiflorus in air do not collapse 

 until a pressure of 20 atmospheres is applied to them, 

 while those of Cytisus laburnum and Tilia americana did 



