MOVEMENT OF MATERIALS IN THE PLANT 147 



A simple experiment (Fig. 83) indicates the magnitude of the force that 

 draws water into the leaves to replace that lost by evaporation. If the cut end 

 of a leafy branch or stem is carefully sealed to the upper end of a glass tube filled 

 with water, and if the lower end of the tube dips into mercury, then mercury 

 is drawn up into the tube, replacing the water absorbed at the cut surface, 

 which in turn replaces that lost by evaporation from the leaves. In Bbhm's 1 

 experiments the mercury column rose 86 to even 90 cm. in the tube, thus 

 considerably exceeding the height of mercury column supported by atmospheric 

 pressure upon the free mercury surface below. AskenasyV experiments indi- 

 cate that the rise of the mercury column here shown has a simple physical cause. 

 In these experiments the upper, broad portion of a glass funnel, the neck of 

 which was fused to a long glass tube, was filled with a thick layer of plaster of 

 Paris; when the plaster hardened the apparatus was filled with water, the glass 

 tube dipping into mercury below. As water evaporated from the plaster 

 surface the mercury rose in the tube and attained a height of 82 cm., which is, 

 here also, noticeably greater than that attained under the action of atmospheric 

 pressure. The funnel may be covered with animal bladder instead of being 

 filled with plaster (Fig. 84)." 



[The bladder membrane has not been recorded as ever showing a rise of the 

 mercury column above the current height of the barometer column, and a 

 shorter rise demonstrates nothing but ordinary suction. The Askenasy experi- 

 ment often fails, even when porous procelain or plaster of Paris is used; without 

 special precautions the water column almost always breaks before the mercury 

 column has reached a height of 76 cm., although mere suctions of nearly a full 

 atmosphere are readily demonstrated. Because of the fundamental and far- 

 reaching importance of liquid tension in plants, and because this physical 

 phenomenon appears to be but vaguely appreciated, being rarely demonstrated 

 in university or college courses in either physics or plant physiology, the Editor 

 asked Dr. Grace Lubin, of the Laboratory of Plant Physiology of the Johns 

 Hopkins University, to prepare a brief account of the precautions that seem to be 

 necessary to secure the Askenasy demonstration. She has contributed the 

 following paragraphs and the accompanying diagram. 



Three limiting conditions present technical difficulties that oppose the de- 

 monstration of liquid tension by this method. In the first place the water- 

 impregnated porous material used must be so fine-pored that its air-water 

 menisci are capable of supporting for an indefinite period an excess air-pressure 

 of several atmospheres. In the second and third places, preliminary treatment 

 must insure adequate wetting of all surfaces in contact with the liquid that is 

 to be strained, and the removal from the, system, or the complete solution, of 

 all undissolved air. With these conditions in mind, a relatively simple proce- 



1 [Bohm, J., Capillaritat und Saftsteigen. Ber. Deutsch. Bot. Ges. n : 203-212, 1893.] 

 'Askenasy, 1896, 1897 [See note 1, p. 143.] Dixon, 1914. [See note r, p. 144.]. — Ed. 

 u In this connection see: Ursprung, A., Zur Demonstration der Flussigkeits-Kohasion. 

 Ber. Deutsch. Bot. Ges. 31 : 388-400. 1913 Idem, Ueber die Blasenbildung in Tono- 

 metern. Ibid. 33: 140 153. 1915. Idem, Ueber die Kohasion des Wassers im Farn- 

 annulus. Ibid. 33: 153-162. 1915. — Ed. 



