xi ABSORPTION AND MOVEMENT OF WATER 139 



Osmosis. Since nutrient substances must pass through the 

 closed walls of cells in order to reach their interior, it follows 

 that they must be in a soluble condition. 



How is the interchange between the fluid in the plant and 

 that in the soil brought about ? The cell-sap in the plant is 

 separated from the water in the soil by the permeable cell-walls. 



The absorption of the solution from the soil is nothing more 

 than a mixing of two fluids of different densities. The mixing 

 of fluids through a permeable membrane is called osmosis, and 

 for this to take place it is necessary for the fluids to be of 

 different densities. There are two currents set up, one from the 

 exterior of the plant to the interior called the endosmotic 

 current, and one from the interior of the plant to the exterior, 

 called the exosmotic current. Since the cell-sap is much 

 richer in substances which set up osmotic currents than the 

 water in the soil, or in other words is heavier bulk for bulk, a 

 considerable endosmotic current of \\-ater from the soil is set up 

 while very little of the cell-sap passes into the soil. The giving 

 out of the acid cell-sap by the plant in exchange for the solution 

 in the soil plays a very important part in absorption. In the soil 

 is a variety of materials insoluble in pure water but which are 

 dissolved in a weak acid. If a plant is grown over a slab of 

 polished marble so that the roots come in contact with it, the 

 acid sap in the cells of the younger portions of the roots leave 

 their impression on the slab of marble. These impressions 

 are produced by the acid cell-sap dissolving some of the 

 marble. 



EXPT. 126. (i) Dip a piece of bhie litmus paper in a weak solution 

 of sulphuric acid. Note 



(ii) It turns red. 



(iii) Dip a piece of red litmus paper in a little caustic soda solution ; it 

 changes its colour and becomes blue. 



(iv) These tests are used to see if a substance is acid or alkaline. 



EXPT. 127. Pull up a grass plant b/ the roots and place a piece of 

 blue litmus paper against the tip of a young root. Note 

 (i) The paper gradually becomes red. 

 (ii) The roots are therefore acid. 



EXPT. 128. Obtain a piece of limestone or marble and polish it by 

 rubbing one side on a piece of flagstone. Place the polished limestone 

 in a pot along with some soil, and plant a young seedling above the 

 limestone. Keep the plant moist and place the pot where there is 



