3 



5- A salt solution separated by a rnembrane from water absorbs 

 some of the water and increases it own volume. — To understand 

 better how the root-hairs absorb their water, we will study that 

 physical process already mentioned. First dissolve one ounce of 

 saltpetre, which we may use as a fertilizer solution, in one pint 

 of water, calling this solution I. For use in some experiments 

 later on, also dissolve a piece of saltpetre not larger than a peach 

 pit (about \ oz.) in about one gallon of water, calling the latter 

 solution II. Now fill the tube A in figure 3 almost full of the 

 strong solution I., and tie a piece of animal 

 membrane (hog's bladder is excellent for 

 this purpose) over the large mouth. A 

 small funnel, with a long stem, may be 

 used if one cannot obtain a tube like A. 

 Then sink the tube, bladder part down- 

 ward, into a large bottle of water until the 

 level of liquid in the tube stands at the 

 same height as that in the bottle C. The 

 tube may be readily secured in this posi- 

 tion by passing it through a hole in the 

 cork of the bottle. In a short time, we 

 notice that the liquid in A' begins to rise, 

 and in an hour or so it stands as at b, say. 

 This is an important result : see that we 

 do not forget it, for it explains many things. 

 The facts are that the liquids tend to diffuse, 

 but the strong solution in A' cannot pass 

 through the bladder B as rapidly as the 

 water outside can pass into A'. Then 

 there is evidently absorption of water and 



pressure in A' which forces the liquid up higher than in C. The 

 liquid in A' would continue 10 stand higher than in C while this 

 absorption goes on. Thus we know that a strong fertilizer solu- 

 tion, or any solution denser than water, separated from water by 

 membrane, will absorb water. This is an instance of that which 

 physicists have named osmosis. It is osmotic action. 



6. The cell sap of the root-hair absorbs water from the soil by 

 osmotic actio7i. — The experiment above detailed enables us to 



Ho7v to illustrate 

 osmosis. 



