AN INTRODUCTION TO PLANT BIOLOGY 91 



that some of the molasses osmosed. Evidently the molasses 

 did not osmose outward as rapidly as the water osmosed 

 into the molasses, otherwise the column of fluid would not 

 have been forced up the tube. 



This outward osmosis of molasses in the experiment simply 

 illustrates the fact that solutions will osmose through a mem- 

 brane ; but it must be understood that sugar solution in a 

 plant root would not osmose out into the soil, for the living 

 plant cell can prevent outward osmosis of its constituent 

 substances. However, the osmosis into roots of substances 

 dissolved in soil water might be illustrated by adding common 

 salt to the water outside the membrane in the above experi- 

 ment, and later it would be found that some of the salt 

 solution had osmosed into the molasses. A large number 

 of substances in solution will osmose through membranes, 

 but others (e.g., white-of-egg, and glue) will not osmose. 



89. Osmosis or Absorption by Roots. The above prin- 

 ciple of osmosis applies to the absorption of water by roots 

 as follows : The cells of the roots (especially the root-hairs, 

 Fig. 26) contain cell-substances which attract water very 

 much as the molasses did in our experiment. The walls 

 of the cells allow osmosis as the membrane of the diffusion- 

 shell did. In short, water from the soil, containing mineral 

 substances in solution, osmoses into the cells on the surface 

 of the root, especially the root-hairs. There is one dif- 

 ference between this osmosis in the root and that in our 

 experiment ; namely, that the cells of the root are filled 

 with substances which attract water but do not themselves 

 osmose out, as did the molasses. Hence water continually 

 osmoses into the root, while the chief cell-substances do not 

 osmose out into the soil. Some substances in root cells 

 do pass into the soil, but in very small quantities. 



Recalling the pressure made evident by the height of the 

 water column in the last two experiments ( 88) , we see that in 

 both cases pressure is the result of osmosis or absorption of 



