CHAPTER II. THE ASCENT OF THE SAP. 23 1 



account of its acid properties, in bringing mineral matters into 

 solution, which are afterward absorbed by the plant. (5) The 

 process of imbibition is also greatly aided by the avidity for 

 water which is possessed by the protoplasm of the young and 

 growing root cells. 



The Ascent of the Sap. The laws of diffusion and osmose 

 also help us to understand the ascent of the sap. For the same 

 reason that the exterior root-cells absorb water from the soil, 

 those adjacent to them absorb from these, which, in turn, yield 

 up a portion of their contents to others still farther interior, and 

 so the sap is passed on from cell to cell until finally it reaches 

 the most remote parts of the- plant. The process is, however, 

 greatly aided by what is called root-pressure. This is due to 

 the fact that the root-cells continue to absorb, even after they 

 are no longer able to hold any additional liquid, and the pres- 

 sure exerted by their turgid walls forces a part of the sap-con- 

 tents through the inner walls into adjacent cells ; that is, it filters 

 under pressure from cell to cell. The fact that the liquid is 

 forced inward, rather than outward, into the soil, may be 

 accounted for by supposing that the interior wall, or face of the 

 cell, has a structure somewhat different from that of the exterior 

 one. This pressure, in the growing season, makes itself felt in 

 the remotest part of the plant. The exudation of water in drops 

 from the water-pores of certain leaves, is due to this cause. For 

 this reason, also, plants often bleed copiously when wounded. 

 The amount of pressure of this kind, exerted by a plant, may be 

 measured, and has been found in some instances to considerably 

 exceed one atmosphere, or fifteen pounds to the square inch ; 

 but it varies greatly in different plants, and at different times in 

 the same plant. 



The ascent of the sap has been found to take place very 

 largely, though not exclusively, along the fibro-vascular bundles. 

 In the stems of trees it is chiefly along the sap-wood. It must 

 not be understood, however, that these tissues are in any proper 

 sense circulatory organs, such as we find in the higher animals. 

 No such organs exist in plants. While there is a distinctly trace- 

 able upward movement of the crude sap along certain tissues, 

 no such distinct downward movement of the elaborated sap can 

 be observed, but there occur chiefly the slow movements of diffu- 



