1 86 OSMOSIS : ABSORPTION OF WATER 



Roots die or develop badly when plants are transplanted and 

 put into the soil too deeply. Although the root-hairs come 

 into very intimate contact with the small particles of earth, and 

 are specially adapted to use the thin films of water surrounding 

 the latter, they are not able to withdraw the whole of the water 

 which a soil is capable of holding. When soils are allowed . to 

 dry, plants growing in them begin to wither as soon as the water 

 present sinks below a certain amount, which varies with the 

 composition of the soil in question. Beans, tobacco and 

 cucumber plants have been found to wither and die in good 

 garden soils containing 12 to 15 per cent, of water and in loams 

 containing 8 per cent. 



Ex. 106. — Grow a dwarf-bean in a pot of sandy soil and one in a pot of good 

 garden soil. When the plants have three or four well-developed leaves allow 

 the soil to become dry and when the plants are dead shake out the soil from 

 each pot and determine what percentage of water remain.s in it. To do this, 

 weigh a. porcelain dish ; then place a small amount of the soil in the dish and 

 weigh again ; the difference gives the weight of the soil taken. Place the 

 dish with the soil in a ' water-oven ' to drive off all the water ; leave for five 

 or six hours and when cool weigh again ; the loss gives the amount of water 

 which has evaporated from the amount of the soil taken ; from these weights 

 calculate the percentage loss of water. 



Ex. 107. — Select three seedling cabbages as near the same size as possible ; 

 take one of them up carefully with a small amount of earth with it so as to 

 damage the roots as little as possible ; the second take up and shake off all 

 the soil ; take up the third and after shaking off all the earth from its roots 

 pull off all the finest rootlets. Then transplant all three and notice the 

 further growth of the three plants for ten days. 



3. Exudation-pressure. Root-pressure: 'bleeding' of plants. — 



After water has been absorbed from the soil by the root-hairs, it 

 passes by osmosis from the latter into the adjoining parenchyma- 

 tous cells of the cortex (r, 2, Fig. 7 2). The cortical cells then absorb 

 from each other until they all become highly turgid, and the same 

 turgid condition is soon reached by the parenchymatous cells 

 within the vascular cyhnder of the root. When a certain degree 

 of pressure is attained within the innermost parenchymatous cell.s 



