PLANT FOOD 



65 



from the soil without destroying them. The roots take 

 some part in absorbing the soil solution, but the root- 

 hairs do most of this work. 



63. Osmosis. Tie a piece of parchment or a piece of 

 bladder over the end of a thistle tube. Fill this with a 

 strong solution of salt. Invert in water 

 so that the height of the water and the 

 solution are the same. Allow this to 

 stand for some time and observe the 

 result. The height of the water in the 

 tube rises above that outside the tube. 

 This shows that the water has passed 

 through the membrane more rapidly than 

 the salt solution. The water passes 

 through the membrane more readily 

 than the denser solution. 



Pare a potato and cut slices from it. 

 Place some of these in water and some 

 in a strong solution of salt. Examine 

 in about an hour. The pieces in water 

 will be found very plump and rigid. 

 This shows that water passes into the 

 potato faster than the sap passes out of it. The pieces 

 in the salt solution will be flexible or wilted. This shows 

 that the concentrated salt solution did not pass into the 

 potato cells so fast as the cell sap was lost. The potato 

 "wilts" when immersed in salt solution. 



The process of the interchange of fluids, either liquids 

 or gases, through a membrane is called osmosis. Whenever 

 a plant or animal membrane separates two solutions, there 

 is an interchange of the two. The less dense the solution, 



Fig. 39. 



Apparatus ready for 

 osmosis experiment to 

 show how root - hairs 

 take in soil- water. 



