52 THE VEGETATIVE ORCxANS 



cell-contents and the selective power of the plasmatic 

 membranes. 



Each of the fine particles of which the soil is composed 

 is coated with a thin film of water containing mineral salts 

 in solution. The root-hairs press against and become 

 moulded to the particles, and water (together with a selec- 

 tion of the dissolved mineral salts) is drawn into the root- 

 hair. This continues as long as the osmotic attraction of 

 the cell-contents can overcome the surface tension of the 

 film of water surrounding the soil-particle. If water is 

 renewed from adjacent particles absorption will continue: 

 if not, the plant will be unable to obtain a sufficient supply, 

 and the effect on the plant will be seen by the drooping 

 of the leaves. 



Turgidity. When a plant is dug up and transplanted it 

 wilts, but after a time recovers. Why does it wilt ? What 

 changes take place that enable it to recover ? We have 

 seen how very small and tender the root-hairs are and how 

 easily injured, and we have noticed also their great impor- 

 tance as absorbing organs. The movement of the soil in 

 digging up and transplanting will obviously break off a large 

 number of these hairs and so reduce the absorbing power 

 of the root, hence wilting will occur. New root-hairs, 

 however, are gradually formed, which absorb water and 

 soon bring the plant back again to its fresh condition. 

 The condition of living cells, whereby their elastic walls 

 are stretched by the pressure of internal fluids, is called 

 turgidity. The freshness of a shoot depends upon its 

 turgidity ; loss of turgidity is the cause of wilting. 



Potato osmometer. Another experiment will enable us 

 to realize how the water taken up by the root-hairs may 

 travel through the outer to the inner tissues of the root. 

 Obtain two long and similar potatoes with uninjured skins. 

 Boil one of these for fifteen minutes and allow it to cool. 

 Then prepare both the boiled and the unboiled potatoes 



