i 7 8 



EXCHANGES AND MOVEMENTS OF FLUIDS 



The total duration of the experiment was 30 hours, and the 

 final pressure attained was sufficient to compress a column of air 

 from a length of 6.30 cm. to .78 cm. and the amount is indicated 

 as 630/78 = 8 atmospheres. This pressure was maintained for 

 two days and then began to decrease slowly, showing but 1.2 

 atmospheres a week later. 1 



247. Movements Caused by Imbibition. Secure a few awns of 

 Stipa avenacea, which are usually curved at right angles midway. 



Warm a cent piece and put a 

 drop of sealing wax in the 

 middle of one side. Thrust 

 the basal end of the awn into 

 the wax, and as soon as it is 

 cool set it in the center of a 

 glass dish about 2 cm. in di- 

 ameter, and about 3 cm. deep. 

 Mark the mouth of the dish 

 into fractions of a circle. Now 

 fill the dish with water and 

 note the movements of the 

 bent awn. The terminal por- 

 tion will sweep around the dish like the hands of a watch. After 

 a half hour pour the water from the dish and set in a warm place 

 and follow the reverse movement. The awn will not return to the 

 point from which it started for some time, perhaps days, since 

 the last of the water taken up is lost very slowly. The move- 

 ments are the result of forces set up by the imbibition of water 



1 MacDougal. Force exerted by swelling seeds. Jour. N. Y. Bot. Garden. 2 : 

 39. 1901. 



Fig. 85. S, awn of Stipa avenacea 

 fastened to metal disk A, and set in dish 

 of water to give hygroscopic movements. 



