CHAPTER XXVIII 



SUCTIONAL RESPONSE AND ASCENT OF SAP 



Inadequacy of existing theories of ascent of sap— General considerations regarding 

 cellular activity and resultant propulsion of water — The Shoshungraph — 

 Balanced Shoshungraph for determining variations of suction — Hydrostatic 

 and Hydraulic Methods of Balance. 



THERE are few phenomena in plant- life which have attracted 

 keener interest and inquiry than that process of transport by 

 which water is carried, from below the surface of the earth 

 to the tops of the tallest trees. The obscurity of the subject 

 is so great, and the secondary co-operating agencies so 

 numerous, that the inquirer is apt to be led into the error 

 of confining his attention to some one of them alone, imagin- 

 ing it to be the principal element in the problem. In 

 studying this subject, then, our first effort must be to distin- 

 guish between the essential factor and others which are 

 merely subsidiary. 



For a statement of the inadequacy of these subsidiary 

 factors to the solution of the problem, it is only necessary to 

 refer to the summary of Strasburger and Pfeffer regarding 

 existing theories of the ascent of sap : l 



The theory of atmospheric pressure is discredited, inas- 

 much as water is known to be lifted, in certain cases, to many 

 times the height of the water-barometer. 



The theory of capillarity is inadequate, inasmuch as 

 continuous capillaries are absent, and the height to which 

 liquids could be raised by such means would not, moreover, 

 approach that of an ordinary tree. 



1 Pfeffer, Physiology of Plants, English translation, 1903, vol. i. p. 222, 

 el seq. ; Strasburger, Text-book of Botany, English translation, 1903, p. 188. 



