Ch. V, 4] ABSORPTION BY ROOTS 227 



the cortical cells under considerable pressure, as manifest to 

 the eye when a suitable pressure-gauge is attached to the 

 cut stump of an active plant (Fig. 167). Thus tested, potted 

 plants will often show a root pressure, i.e. a pressure of water 

 in the ducts, sufficient to raise water over thirty feet, while 

 some trees show two or three times as much. This pressure 

 is not enough to raise water to the tops of the tallest trees, 

 but it does give the sap a good start up the stem, after 

 which it is lifted to the leaves by the forces we have earlier 

 considered (page 147). This root pressure, however, is the 

 source of the "bleeding" of broken or pruned stems in the 

 spring, and also of guttation. 



What then is the nature of the power by which the root 

 hairs absorb water and give it so forcible a push up the 

 stem ? Evidently the water absorbed by the hairs and passed 

 through the cortical cells must pass through walls and proto- 

 plasm, which are membranes, and through the cell solutions, 

 which, for simplicity, we can consider as solutions of sugar, 

 their most prominent constituent. Such absorption is 

 known in physics under the name osmosis, and so important 

 a part does osmosis play, not only in absorption of water, 

 but also in other physiological phenomena, that the student 

 should not fail to make its acquaintance through experiment. 

 Any simple device in which a membrane, e.g. a piece of 

 parchment, separates a sugar solution from water, will 

 serve the purpose ; but a specially convenient arrangement 

 is represented in the osmoscope shown in Figure 168. When 

 a solution (molasses is a very convenient solution of sugar) 

 is placed in the parchment tube, which then is immersed 

 in water, the solution will rise in the vertical tube at a 

 distinctly visible rate. If instead of water a solution 

 weaker than that in the parchment tube be used, the result 

 is the same, though the rise is slower. If the water be 

 placed inside and the solution outside, there is no rise, but the 

 tube soon empties, shrinks, and collapses. These phenom- 

 ena are typical, and the osmotic process may be generalized 



