NATURE OF PLANTS 53 



and more or less cutinized ring of cells. This tissue is supposed 

 to function in preventing an undue loss of water from the con- 

 ducting strands of xylem to the cortex. It should be stated here 

 that the root hairs not only absorb fluids but also force out the 

 absorbed substances into the adjoining cells of the cortex. All 

 living cells have this power of absorbing and expressing fluids. 



FIG. 37. Cross-section of a root of lupine showing the origin of the lateral 

 rootlets. Lettering as in Fig. 36. H. O. Hanssn. 



So it comes about that the absorbed fluids are forced by the root 

 hairs into the cortical cells and by them they are passed on to the 

 cells of the xylem when by some force not known they are drawn 

 up the stem into the leaves. The force exerted by these myriad 

 number of cells in the root exerts a pressure that often amounts 

 to considerably more than one atmosphere, i. e. } 15 Ibs. to the 

 square inch. It is the steady expressing of fluids by these cells 

 that causes the familiar phenomena of the " bleeding," or flow 

 of water from stumps in the spring ; likewise the " bleeding " of 

 injured branches, or the flow of sap, for in the stem the living 

 cells are constantly absorbing and giving off water as in the root. 

 In the summer, stumps and stems do not " bleed " as a rule be- 

 cause the water lost by transpiration nearly empties the cells, 



