40 GROWTH AND WORK OF PLANTS 



There is a thin and continuous membrane of protoplasm 

 which lines the wall. Inside of this membrane the entire space 

 of the cell is not filled with protoplasm. There are large spaces 

 filled with a watery fluid called cell sap, which is a solution of 

 certain salts, sugars, etc., in water. If the root hairs are placed in 

 a 5 per cent salt solution, the membrane of protoplasm lies 

 between the cell sap inside of the cell and the salt solution. The 

 cell wall permits the water and solutions to filter through easily. 

 But the protoplasmic membrane is of a closer and different 

 texture, so that soluble substances do not pass through so readily. 

 The water of the solution, however, passes through the mem- 

 brane easily. The result is that the protoplasm contracts away 

 from the cell wall. This is because some of the water in the 

 cell sap flowed through the protoplasmic membrane into the 

 salt solution outside. According to well-known laws of physics, 

 the greater flow of water through such a membrane, which is 

 only half-way permeable (semi-permeable), is always in the 

 direction of the stronger solution. This shows then that the 5 

 per cent salt solution is stronger than the cell-sap solution. But 

 when the salt solution is replaced with water, the membrane of 

 protoplasm moves out again against the cell wall and is pressed 

 firmly against it, so that the elastic cell wall becomes slightly 

 stretched while the cell becomes firm and turgid, or is in a state 

 of turgescence, or tension, something like an inflated bladder. 

 This action of the protoplasmic membrane, the cell wall, and 

 the cell sap explains to us how it is that the delicate root hairs 

 can take up water and food solutions from the soil. 



64. The behavior of a root hair or other cell in the absorp- 

 tion of water is sometimes illustrated in the following way:* 

 Over the bulb of a thistle tube a piece of a bladder membrane is 

 tied after thoroughly soaking it. A saturated solution of sugar 

 in water with a small quantity of a red aniline dye to color it is 

 poured into the tube to fill the bulb and a short distance into the 

 tube. The bulb of the thistle tube is lowered into a bottle of 

 water so that the height of the water in the bottle and the solution 

 * Or by the well-known egg experiment. 



