72 



ROOTS 



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From the text on. pages 71, 72 explain why 

 the water rises in one tube. 



and the sugar molecules do not diffuse easily, it will be seen that 

 the inner side of the membrane does not present so much space 



for the diffusion of water par- 

 ticles as does the other side. 

 Hence the flow of water into the 

 tube is more rapid than the flow 

 out of the tube, and the water 

 gradually rises in the thistle 

 tube. This diffusion of water 

 through a semipermeable mem- 

 brane is known as osmo'sis. 

 It will be seen that the greater 

 flow of water particles or mole- 

 cules is from the point of greater 

 concentration of water to the 

 point of lesser concentration of 

 water; hence it is a true diffusion. And since the solution Tvithin 

 the thistle tube is inclosed, the process causes a pressure by the 

 solution within these closed walls. This is knowTi as osmot'ic pres- 

 sure. This pressure, if continued, would burst the egg membrane 

 in the experiment first noted and is a very important force in cir- 

 culating the water in the root hair. 



Why the Root Hair absorbs Water and Soil Salts. — The wall of 

 the root hair readily takes in water and dissolved soil salts by im- 

 bibition. The outer edge of the protoplasm forms a semi-per- 

 meable membrane, which, while allowing water and mineral salts 

 in solution to diffuse toward the inside, will not allow the diffusion 

 outward of the sugar and other soluble materials mthin the cell. 

 Hence an inward flow of soil water is started. As soon as the outer 

 cells have increased their holdings of soil water, an osmosis inward 

 is started because the water tends to flow from the place of its 

 greater concentration to the place of lesser concentration. Mineral 

 salts in solution are carried along with the water so that the needed 

 soil substances are carried along from cell to cell, until they reach 

 the small tubes of the central cjdinder. Here other factors help 

 the water up in the root ; of these, capillarity and the pull exerted 

 by evaporation from the upper parts of the plant are believed to be 

 the most important. We shall learn more about this later. 



