ABSORPTION AND MOVEMENT OF WATER 135 



In spite of the inadequacy of the means for measuring 

 sap-pressure, figures of very considerable magnitude are to 

 be found in the published studies on this subject. For ex- 

 ample — 



in Riciuus communis 

 Urtica dioica 

 Vitis vinifera 

 Betula alba 



Strangely enough, since the researches of Clark* in 1873, 

 little attention has been paid in this country to the phe- 

 nomena of bleeding, in spite of the facts that the important 

 maple-sugar industry depends upon it, and that there are 

 botanists at the Agricultural Experiment Stations of the 

 sugar-making States. 



For the very natural but also very poor reason that sap- 

 pressure has often been measured on the stumps of small 

 plants cut off near the ground — that is, where the sap-pres- 

 sure must develop almost wholty within the root — it has 

 been commonly called 7'oof-pressure. That this is a mis- 

 nomer follows not only from the foregoing consideration of 

 the physics of sap-pressure, but also from the introductory 

 experiments of Pitra,t repeated and extended by others, 

 upon the sap-pressures which may be developed in parts 

 above ground, for example, branches cut off from the main 

 stem and thus wholly separate from the root. The sap- 

 pressure of such amputated parts may even be higher than 

 of those left attached to the root. It is true that the sap- 

 pressure develops first in the lowest parts and gradually 

 ascends the stem or branch, but this is owing to the absorb- 

 ing part being below. It is the roots, or the lower ends of 

 amputated branches, which absorb the water, and it is 

 necessarily the cells nearest the absorbing parts that first 

 develop pressure and from which, after a time and after the 

 pressure goes beyond a certain height, sap is pressed out 



* Clark, W. S. Circulation of sap in plants. Lecture before Mass. State 

 Board of Agriculture, 1874. 



t Pitra, A. Versuche iiber die Druckkraft der Stammorgane bei den 

 Erscheinungen des Blutens und Thranens der Pflanzen. Jahrb. f. w. Bo- 

 tanik, Bd. XI., 1877. 



