CHAPTER VIII . 



THE RELATION OF PLANTS TO WATER 

 THE MECHANISM OF ABSORPTION 



OSMOSIS 



The particles of the soil from which roots absorb water and soil 

 salts are surrounded by delicate films of water (Fig. 69) in which 

 the dissolved portions of the soil necessary to plant life are held 

 in very dilute solution ; namely, from .0001 to .03 per cent. 

 In order to understand the method by which roots absorb this 

 soil water and its dilute salt solutions, the student must first 

 understand something of the laws of osmosis, upon which all 

 absorption and much of the movement of fluids in the plant 

 depend. A simple experiment illustrated in Fig. 67, a, will serve 

 to give the necessary data for understanding the application of 

 osmosis to the movement of water and soil salts into and through 

 the plant. Fig. 67 shows a parchment tube which is not unlike 

 a root hair in its form and in its osmotic properties. If now 

 the parchment tube has been filled with a strong solution of 

 common salt before being placed in the distilled water, the results 

 of osmosis will shortly begin to be manifest to the observer. The 

 water will be seen to rise slowly in the glass tube, until a column 

 several feet in height is attained. At the same time it will 

 be found, by chemical analysis or by the taste of the water in 

 the jar, that minute quantities of salt have flowed out of the 

 parchment tube into the pure water. 



In the above experiment we have illustrated the essential facts 

 regarding osmosis, or the diffusion of substances in solution 

 through an osmotic membrane which separates two solutions of 

 different composition. In such cases the substance (for example, 

 salt) dissolved in a liquid (for example, water) is called a solute, 



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