ABSORPTION AND OSMOSIS 65 



This response of roots to moisture is called hydrotropism, and 

 will cause roots to turn toward a water supply if the surroundings 

 be dry, even though they turn partly away from the direct down- 

 ward line. 



To Demonstrate Osmosis. Fill an artificial diffusion shell (such 

 as can be purchased from dealers in laboratory supplies) with 

 molasses and fasten it tightly to a long glass tube by wiring it 

 to a rubber stopper. Insert the shell in a jar of water. Here 

 are the three essentials for osmosis. The shell is the osmotic 

 membrane, the molasses, the dense liquid, and the water, the 

 less dense liquid. 



The rise in the tube will be rapid and usually reaches a height 

 of several feet. This illustrates in a way the action of a root hair 

 in causing root pressure, though the root hair, because of its 

 protoplasmic lining, selects what will be absorbed, while the 

 apparatus does not. 



With the same apparatus, starch or proteid or fat can be placed 

 in the shell, and it will be found that no osmosis goes on, and that 

 they cannot be found in the water outside the diffusion shell. 

 On the other hand, the sugar, peptone, or other soluble food stuff, 

 will pass through the membrane, and can be found by test in the 

 water outside. 



Not only does plant absorption depend upon osmosis but nearly 

 all the life processes of plants and animals utilize this process in 

 some degree, as will be seen as we proceed. 



COLLATERAL READING 



ABSORPTION 



Elementary Botany, Atkinson, pp. 22-27; Lessons in Botany, Atkinson, 

 pp. 36-44; Physiological Botany, Gray and Goodale, pp. 230-232; Text- 

 book of Botany, Bessey, pp. 175-176; The Story of Plants, Allen, pp. 53-73; 

 Introduction to Biology, Bigelow, pp. 41-45. 



GEOTROPISM 



Lessons with Plants, Bailey, p. 330; How Plants Grow, Bailey, p. 

 350; Plant Relations, Coulter, pp. 69, 89-91, 138-141; Textbook of Botany, 

 Stevens, pp. 24, 43, 61, 114; Plant Structures, Coulter, pp. 303-309; Ele- 

 mentary Botany, Atkinson, pp. 82-84; First Studies in Plant Life, Atkinson, 



