THE PLANT COVERING OF THE EAKTH 323 



It is clear that the water passes through the partition and 

 dilutes the denser solution, thus increasing its volume. Later 

 the solutions have interchanged until they are very nearly if 

 not quite alike. The membrane is said to be semi-permeable, 

 because the water will pass through freely but the sugar will 

 not pass so readily. The same results may be secured with 

 other solutions and other membranes. This 

 process by which water or other liquids 

 may pass through a semi-permeable mem- 

 brane between a weaker and a stronger 

 solution, and by which the volume of the 

 latter is increased, is called osmosis. The 

 similarity of the results of this experiment 

 with osmosis to the results of root absorp- 

 tion is striking. 



325. Root structure. When the tip of 

 a root is examined, it is found to be com- 

 posed of cells, as is any other part of a 

 plant or animal. These cells resemble 

 those seen in the leaf. They are thin- 

 walled, and it is possible for water to pass 

 in through the cell walls. The interior of 

 the cell wall is lined with a layer of living 

 substance (protoplasm) with a space in 

 the center which contains cell sap. Cell 

 sap consists of water and some substances 

 (often including considerable sugar) in solution. This layer 

 of protoplasm permits water to pass inward but it does not 

 readily permit the sugar and most other substances which 

 are dissolved in the cell sap to pass outward. 



Each cell of the root is, in fact, an osmotic apparatus in 

 which the protoplasm acts as a semi-permeable membrane, 

 the cell sap being the more concentrated solution and the 

 soil water the weaker one. If in our experiment we had 

 used soil water and concentrated plant sap as the two liquids, 



FIG. 157. A root tip 



The tip of a clover root 

 covered with root hairs. 

 Very much magnified 



