OIBOUlATIOlf OS SAP. 85 



CHAPTER Vir. 

 CIRCULATION OF SAP. 



Plants obtain the principal part of their nourishment 

 from the liquids and gases absorbed by their roots. The 

 fluids and gases thus absorbed is called crude sap, and this, 

 meeting previously assimilated matter in the cells, min- 

 gles with it, and going forward or upward untU it reaches 

 the buds, twigs, or expanded leaves, is there exposed to 

 or meets both air and light, producing chemical changes 

 resulting in what is termed organizable matter. 



The movement of fluids in endogenous plants is not so 

 readily determined as in the exogenous, owing to the in- 

 termingling of the woody and vascular bundles. It is, 

 however, quite probable that both take part in the move- 

 ment, and as we find cambium near the vascular bundles, 

 it may serve the same purpose as this material in the 

 exogens. But experiments are wanting to show how the 

 transmission of sap takes place in the various and com- 

 plex structure of endogenous stems ; still it is known 

 that there is both an upward and downward flow, but its 

 movement has not been so accurately determined as in 

 the exogenous stems. 



The crude sap, or liquid taken in by the roots by the 

 process of imbibition, does not pass upward through open 

 tube-like vessels, but from cell to cell by an endosmose 

 and exosmose action, as explained in Chapter I. ; conse- 

 quently, the crude liquid does not remain separate from 

 the old or previously assimilated sap in the cells, but the 

 new and thinner liquid lessens the density of the older, 

 and both, thus mingled, flow on upward or outward, as 

 the case may be, to the ends of the branches, the result 

 of some force not fully understood. Physiologists do not 

 agree in regard to the cause of motion in the liquids of 

 plants. Some attribute it to what they term capillary 



