124 WATER-ABSORPTION AND TRANSPIRATION. 



The surface of the root is increased at least fifty or sixty 

 times by the root-hairs. The root-hairs come into contact 

 with the water around the soil-particles, and each hair 

 becomes surrounded by a film of water. Some roots contain 

 sugar (Carrot, Beet, Parsnip, etc.), and by chemical tests we 

 always find in roots sugar or other substances that are capable 

 of " attracting " water. We have here the conditions under 

 which osmosis occurs two solutions separated by a permeable 

 membrane (cell- wall of root-hair), and we may infer that 

 soluble substances in the soil-water diffuse into the root-hair, 

 while some of the substances dissolved in the cell- sap of the 

 root-hair diffuse into the soil. 



* (a) Root-hairs are especially well seen on the roots of seedlings (Figs. 

 12, 13, and 45). In seedlings of Wheat, Radish, Turnip, Cress, or 

 Mustard, grown on muslin stretched over a tumbler of water, the hairs 

 are very abundant. An even better method is to place the seeds in an 

 earthenware dish, or on pieces of brick or broken plant-pots, and keep 

 them moist, covering them with a sheet of glass or a bell-jar ; the root- 

 hairs are freely developed in the damp air, forming a white fleecy 

 covering on the roots. 



* (&) Make a rough model of a root-hair out of a long potato-tuber. Cut 

 off one end of the tuber so that it will stand upright, and with a knife 

 scoop out the middle part, leaving on the outside a layer about a 

 quarter of an inch thick. Half -fill the tuber with salt solution or sugar 

 solution (about 5 per cent, in each case), coloured with red ink, and 

 stand it in a dish of water, the level of which should not exceed that 

 of the salt or sugar solution inside the tuber. From day to day observe 

 the rise of the coloured solution, showing that water has been absorbed. 



158. How is Hoot-Pressure set up? The process of osmosis 

 through the thin cell-wall of a root-hair is largely influenced by the 

 layer of living substance (protoplasm) within the cell- wall, which allows 

 only weak solutions to pass into the hair, and which retains the watei 

 in the cell -sap at a high pressure. The tense condition of the root- hair 

 thus set up is called turgidity. It is really osmotic pressure, regulated 

 by the influence of the living protoplasm layer. If the "artificial root- 

 hair " is placed in a solution stronger than that which is inside it, water 

 is drawn out. Exactly the same occurs with the real root-hair. Its 

 stiffness or turgidity is, like the stiffness of an inflated bicycle-tyre, 

 due to the pressure inside it. This pressure in the root-hair is due 

 to absorption of water hence, when we draw this water out the root- 

 hair collapses or becomes flaccid. The presence of salts in large 

 amounts in soil-water hinders absorption by the root ; this is exactly 

 what the root has to endure in salt-marshes and peat-bogs. 



When the pressure thus set up in the root-hair reaches a certain 

 point the protoplasm of the root-hair undergoes a change, so that it 



