15^ PLANT LIFE. 



as osmosis, the protoplasm, braced by the cell-wall, acting as 

 the membrane, and the cell sap of the vacuole as the denser 

 fluid of the osmotic pair. (See Physics.) 



203. The development of the root system is related to 

 the character of the soil and to the amount and distribution 

 of water and organic matter within it. Branching of the 

 root system is much more profuse in the moister parts of the 

 soil, as well as in those which contain more organic matter. 



204. Movement of water within the plant. — Once the 

 water has gained entrance to the plant, it must move to those 

 parts where it is to be used — i.e., to all the organs of the 

 plant, but especially to the leaves, since from these there is 

 the largest loss of water by evaporation (% 209). From the 

 root hairs the water passes inward through the cells of the 

 cortex, and reaches the stele. The forces which determine 

 this movement and its direction are not fully understood, 

 though osmosis probably plays a chief part. They are com- 

 prehended under the general phrase root pressure. 



205. Root pressure. — The action of root pressure may be 

 demonstrated by severing a suitable stem close to the ground 

 and observing the water which flows out, after a short time, 

 from the cut end. Careful examination of the cut surface 

 will show that the water oozes out chiefly from the woody 

 parts of the stele. The force with which water is extruded 

 may be measured by attaching to the stump, by means of a 

 rubber tube, a manometer (fig. 172). In this way it may be 

 ascertained that in woody plants, such as the birch, the 

 .pressure sometimes becomes equal to that of five or six 

 atmospheres. 



206. Route to the leaves. — After entering the xylem 

 bundles of the roots, the water is thence transferred along the 

 stem in the same tissues, which are continuous with those of 

 the root. Since the xylem bundles form an unbroken line to 

 the most remote parte of the leaves, passing out in the ribs 



