MOVEMENT OF WATER THROUGH THE STEM i6l 



227. Osmose. — The rise of sap is partly due to the pres- 

 sure caused by the constant absorption of soil water through 

 the absorbent hairs of the root. The passage of liquids 

 through the walls of cells and tissues is known as osmose 

 and takes place when liquids of different densities are 

 separated by a thin membrane, the principle governing 

 the direction of the flow being that the thinner, lighter 

 liquid passes toward the denser. The nature of the sub- 

 stances, also, must be considered ; those that are crystalline 

 and easily soluble, like sugar and salt, pass readily through 

 membranes, while gelatinous ones pass with difficulty or not 

 at all. 



Chip away a bit of the shell from the big end of an egg, 

 taking care not to injure the thin membrane underneath. 

 Make a small puncture through both shell and membrane 

 in the small end and place the egg in a cup with its big 

 end in salt water. In a few hours the contents will be found 

 running out of the puncture at the other end, having been 

 forced out by the water that made its way in below. And 

 there are no pores visible, even with the most powerful 

 microscope, in the membrane that lines the eggshell. 



The same principle is well illustrated by the experiment 

 described in Section 204, the water passing by osmose 

 through the walls of the cells that make up the substance 

 of the stem. Take one of the stem sections after it has 

 lain in fresh water, and transfer it to a five per cent solution 

 of salt water (about a tablespoonful of salt to a tumbler 

 of liquid). Allow it to remain as before, and then exam- 

 ine. It will be found to have become straight again, or 

 perhaps even to have coiled over in the opposite direction. 

 This is because the thinner liquid of the cells has passed 

 out by osmose into the thicker salt solution, so that the 

 interior cells have become flabby, while the exterior ones, 

 protected by the epidermis, remain distended and thus 

 cause the section to curve inward. 



The passage of liquids into a sac or cell is called endos- 

 iMose, out of it, exosmose. Which is it that takes place 

 between the soil water and the root? 

 Andrews's box. — 11 



