146 PHYSIOLOGY OF NUTRITION 



is then moving through the piece at a uniform rate throughout. Similar 

 experiments with tubes filled with sand containing air and impregnated with 

 water gave concordant results with those obtained with the pieces of stem. 

 Votchal conceives that air bubbles in the wood act simply as resilient springs 

 that transmit and distribute the thrust imparted to them more slowly and 

 evenly than would a continuous, homogeneous water column. The effective 

 forces applied at the ends of the conducting channels — i.e., the force of foliar 

 transpiration and that of root pressure — furnish energy to account for the 

 ascending water current in plants. Root pressure, produced by osmotic forces, 

 exerts a pressure upon one end of the water column in the wood, while evapo- 

 ration of water from the leaves establishes traction at the opposite end. 8 



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 Böhm's 1 

 experiments the mercury column rose 86 and even go 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)." 



These experiments indicate the great magnitude of the force of cohesion 

 existing between the molecules of water; the water column is not broken 

 even when it is subjected to a considerable stress. These experiments 

 also give some idea of the magnitude of the imbibition force resident in 



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



8 Root pressure is not to be considered as generally important in the ascent of water through 

 plant stems. The mere existence of "negative gas pressure" in the vessels shows that the 

 liquid above the gas bubbles is not being forced upward by a pressure applied below. Perhaps 

 the simplest argument in favor of dismissing root pressure from consideration in the general 

 problem of rise of sap lies in the fact that this pressure is found to be highest when water 

 movement is slowest and lowest when movement is most rapid. — Ed. 



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



u But the bladder membrane has not been recorded as ever showing a rise of the mercury 

 column above the height of the barometer. The experiment usually fails to demonstrate this 

 important point, even with porous porcelain or plaster of Paris; the water column almost 

 always breaks before a stress of one atmosphere is developed. In this connection see: Ur- 

 sprung, A., Zur Demonstration der Flüssigkeits-Kohäsion. Ber. Deutsch. Bot. Ges. 31 : 388- 

 400. 1913. Idem, Ueber die Blasenbildung in Tonometern. Ibid. 33: 140-153. 1915- 

 Idem, Ueber die Kollusion des Wassers im Farnannulus. Ibid. 33: 153-162. 1915. — Ed. 



