Transpiration of the Ascent of Sap. 65 



4. Pfeffer strangely enough urges as an objection "that the 

 walls of the tracheae can absorb or give off water with equal 

 readiness, and that the negative pressure exerted by a continuous 

 water-column would tend to cause an inward and downward stream 

 of water in the upper parts at least". ^) This objection is endorsed 

 by Steinbrinck. ■-) As is well known this surmise has been 

 frequently shown to be exactly in accordance with fact. "When the 

 upper or any tracheae are exposed and supplied with water, the 

 lower transpiring organs determine a flow towards themselves from 

 the point of supply downwards through the upper tracheae. This 

 experience constitutes one of the many arguments against polarity and 

 pumping actions in the stem and at the same time is confirmatory, 

 as this objection strikingly shows, of the tension theory. — When 

 there is no supply of liquid water afforded to the exposed tracheae, 

 menisci are developed at the water-surfaces in their walls. These 

 menisci support the tension in the water and if the tension is 

 sufficiently great and the external state of saturation suitable, con- 

 densation must take place upon them and a flow of condensed water 

 from the exposed tracheae into the plant be determined. 



5. Steinbrinck^) express another doubt, viz.. that the motion 

 of the water in the conducting tubes might render it tensilely weak. 

 He thinks that possibly in experiments, in which considerable tensions 

 have been developed in, and successfully withstood by water, the water 

 particles have been at rest, and that movement of an}- kind would 

 have destroyed their cohesion. In the author's experiments even at 

 the highest tensions the relative motion of the parts of the tensile 

 water was frequently demonstrated by the motion of the enclosed 

 pieces of wood downwards in the water. This motion never determined 

 the rupture of the tensile fluid. Nor would such a result be expected. 



Shortly after the publication of the tensile theory of the ascent 

 of sap the late Dr. E. A s k e n a s y ^) published a paper assigning great 

 importance to the cohesion of water in the transpiration current. Coming 

 at a time when Botanists were beginning to recognise the cohesive 

 properties of water this paper called attention to a point of view 



M AV. Pfeffer, The Physiology of Plants. Eng. Trans, by A. J. Ewart, 

 Oxford 1900, Vol. 1 p. 224. 



'^) C.Steinbrinck, Ueber dynamische Wirkungen innerer Spannungsdiffereuzen 

 von Flüssigkeiten und ihre Beziehung zum Saftsteigeproblem der Bäume. Flora, 

 Bd. 93, 1904, p. 127. 



') C. Steinbrinck, loc. cit. 



*) E. Askenasy, Heber das Saftsteigen. Verb. d. Naturhist.-Med. Vereins zu 

 Heidelberg, N. F., Bd. V. 1895. Idem , Beiträge zur Erklärung des Saf tsteigeus. 

 Loc. cit. 1896. 



Progressus rei botanicae III. "^ 



