ELEMENTS 'OF GENERAL SCIENCE 



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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 membrane between a weaker 

 and a stronger solution, increasing the volume of the latter, is 

 called osmosis. The similarity of the results of this experi- 

 ment with osmosis to the results of root 

 absorption is striking. Osmosis appears 

 to be an explanation of absorption by the 

 root, but before we accept it as such we 

 must know whether the root is so con- 

 structed that it affords opportunity for 

 osmosis to take place. 



252. 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 the walls are permeable to 

 water, so that 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 is semi-permeable ; that is, 

 it permits water to pass 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 



FIG. 117. A root tip 



The tip of a clover root 

 covered with root hairs. 

 Very much magnified 



