NOTES AND COMMENT 



In discussing before elementary botany classes the role of the various 

 physiological processes that occur in leaves it is customary to introduce 

 some experiment which demonstrates the suction force resulting from 

 the transpiration of water from a cut-off leafy branch. While it may 

 be self-evident to many students that such a phenomenon is the nat- 

 ural effect of the physical process of evaporation, just as "root pres- 

 sure" is attributable to osmosis, nevertheless empirical proof that a 

 similar reaction may be secm-ed by purely artificial means is instructive. 

 For this purpose the writer for several years has employed to advantage 

 the diffusion shells manufactured by Schleicher and Schiill. The scheme 

 may not be a new one, but in view of its extreme simplicity, the ready 

 availability of the materials needed, and the fact that the ■\\Titer has 

 never seen this particular application suggested elsewhere, it seems 

 worth while to call attention briefly to the method followed. 



The shell is soaked in water for a few hours, then filled wth water, 

 and tightly connected (wired) by means of a rubber stopper to a glass 

 tube, which is also filled with water. The lower end of the tube is 

 then dipped into a phial of mercury, this part of the manipulation 

 being performed under water to avoid the entrance of air into the shell, 

 and finally tube and shell are clamped in an upright position. One 

 essential part of the apparatus has not been mentioned. In order to 

 prevent its collapse, a roll of wire gauze is closely fitted inside the shell 

 before filling with water. Fifteen or twentj^ minutes may elapse before 

 the mercury commences to rise in the tube, due to the preliminary 

 drying off of the surplus water from the surface of the shell, together 

 with the imperfect fit of the gauze framework, but once started it 

 ascends with a fair degree of rapidity. Using the ordinary 100 by 116 

 mm. shell, attached to 3 mm. (inside diameter) tubing, the column of 

 mercury rises at a rate of 2 to 3 mm. per minute — the rate would of 

 course vary with external conditions — and ordinaril}^ reaches a maxi- 

 mum height of about 17.5 cm. The porous clay cup atmometer would 

 be equally adaptable to such an experiment, but the striking super- 

 ficial resemblance of the shell to a plant cell makes the experiment as 

 described doubly impressive. — G. E. Nichols. 



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