THE ASCENDING SAP 



121 



[E. Step. 



FIG. 155. TOOTHWORT (Lathrcea squamaria). 



A parasite upon the roots of Elm and Hazel. The leaves are represented by hollow scales into which minute creatures 

 enter ; and it is believed they are digested and absorbed. 



of which the inward flow of water is the chief, have been set up. With 

 these results in view, substitute in imagination a root-hair for your 

 glass tube, and for your bladder the exterior cell-walls of the hair, and 

 the experiment will have been made to some purpose. 



That there is a slight outward flow of sap from the plant, in addition 

 to the more important inward passage of water and its concomitants, may 

 be shown in another way. If a plant be grown with its roots in water, 

 the surrounding fluid is soon found to contain some of the peculiar sub- 

 stances contained in the descending sap. Thus a Pea or Bean will dis- 

 engage a gummy matter, a Poppy will communicate to the water an 

 opiate impregnation, and a Spurge will give it an acrid taste. This 

 passage of the sap through the cell membranes, from within outwards 

 is called exosmose (Greek exo, outside ; osmos, impulsion). 



Once the fluid from without has entered the root-hairs, it diffuses from 

 cell to cell till it reaches the fibro-vascular system that wonderful 

 arrangement of vessels and woody cells which forms the framework or 

 skeleton of the plant ; and so it mounts and mounts, chiefly by way of the 

 wood elements, from root to stem, from stem to branch, from branch to 

 slender twig, till it reaches the leaves as little changed during its whole 



