ROOTS AND THEIR WORK 



89 



so delicate that water and mineral substances from the soil can 

 pass through it into the interior of the root. 



How THE Root absorbs Water. — This process can best be under- 

 stood by means of the following experiment: ^ Crack the shell of a fresh egg 

 at one end and pick it, bit by bit, from the delicate membrane that lies 

 underneath it until about one square inch of the 

 membrane is exposed. Now break a small hole in the 

 opposite end of the egg just large enough to admit a 

 small glass tube. After putting the tube in place, 

 cement it in with sealing wax or paraffin. Place the 

 egg with the large end in a glass of water. Examine 

 it after a few hours, and the contents of the egg will be 

 found to have risen in the glass tube to a considerable 

 distance. The membrane through which the water 

 has passed has no holes in it. It allows the passage 

 of certain fluids through it, and is hence called a 

 permeable membrane. In the- experiment just per- 

 formed, a little of the contents of the egg passes into 

 the glass, as can be proved by the proteid test applied 

 to the contents of the glass. On the other hand, a 

 considerable amount of water from the glass has 

 passed into the egg through the membrane. 



Osmosis. — The 'process hy which two fluids, 

 separated by a membrane, pass through the 

 membrane and mingle with each other is called osmosis. In this 

 process the greater flow is always toward the more dense medium. 

 The method by which- the root hairs take up soil water is exactly 

 the same process as we see in the egg. It is by osmosis. The 

 white of the egg is the best possible substitute for living matter; 

 it has, indeed, almost the same chemical formula as protoplasm. 

 The animal membrane separating the egg from the water is much 

 like the delicate membrane which separates the protoplasm of 

 the root hair from the water in the soil surrounding it. The fluid 

 in the root hair is more dense than the soil water; hence the 

 greater flow is toward the interior of the root hair. 



Passage of Soil Water within the Root. — We have already seen 

 that in an exchange of fluids by osmosis the greater flow is 

 always toward the denser fluid. Thus it is that the root hairs 



* This experiment, although not illustrating osmotic action in the strict sense, 

 appeals to the pupil as does no other. 



Experiment to show 

 osmosis in an egg; 

 L, level of the fluid 

 in the tube. 



