1 86 



EXCHANGES AND MOVEMENTS OF FLUIDS 



spaces may be measured however, only by connecting the body of 



the plant with some form of a 

 manometer by which this pres- 

 sure is transmitted to the in- 

 strument directly. The fric- 

 tion encountered by fluids in 

 their expansion and contrac- 

 tion in the narrow spaces in 

 the plant, and the difficulty of 

 making perfect fittings with 

 the apparatus are such, that all 

 results are only approximate, 

 and generally do not indicate 

 the full extent of the com- 

 pression or expansion under 

 which the free fluids of the 

 body are sometimes found. 

 Very rarely does the tension 

 of the gases and the liquids 

 in the trachea and intercellular 

 spaces coincide with that of 

 the external atmosphere. 

 Tests of this character are 

 necessarily confined to plants 

 with firm woody stems and 

 branches, and may be made as 

 follows : Provide a manometer 

 of the closed arm type, which 

 may be made by sealing one 

 end of a glass tube and then 

 bending it into the form of a 

 U, the arms of which are at 



Fig. 90. Measurement of tensions of 

 fluids in stems. A, stump of stem. B, 

 section of rubber tubing. C, level of 

 mercury in manometer. Z>, stopcock. 



<Zxih*=a 



