HOW PLANTS FEED AND GROW 



27 



back from their tips. These projections wind about in every 

 direction among the soil particles, to which they closely adhere. 

 The root hairs may increase the absorbing surface of corn roots 

 five and one-half times, and of barley roots twelve times. 



29. How root hairs secure water from the soil. The soil 

 is made up of a multitude of fine particles. Each particle is 

 surrounded by a thin film of water held there by the force of 

 gravity. The water nearest the soil par- 

 ticle is held more firmly than that farther 

 away. The way in which the root hairs 

 are able to pull the water from the soil 

 particles may be shown by a simple ex- 

 periment. If a bladder or a bag made 

 from parchment paper is filled with 

 water containing 10 per cent of sugar, 

 and if one end of a glass tube is fast- 

 ened in the bladder and the bladder 

 placed in a pan of pure water, interest- 

 ing results follow. The sugar solution 

 will rise in the tube, and the distance to 

 which it rises, hour by hour, can be 

 watched. Imagine the thin wall of the 

 root hair to correspond to the bladder, 

 and the sap inside the root-hair cell to 

 correspond to the sugar solution. The 

 sap is a denser or more concentrated 

 solution than the water in the soil out- 

 side of the root hair. The water in the 



soil moves through the wall of the root hair, just as the water 

 in the basin moves through the bladder toward the inside, so 

 that the quantity of water on the inside increases ; that is, the 

 less dense solution always moves toward the more dense solu- 

 tion. This gives rise to a pressure (osmotic pressure) which is 

 sometimes considerable, and is called root pressure. It is one 

 of the factors which cause the sap to rise in plants. It is one of 

 the causes, perhaps the chief cause, of the bleeding of injured 



FIG. 1 5. Root hairs grow 



from the surface of the 



smallest rootlets 



Root hairs are produced in 

 very great numbers and ex- 

 tend into the soil, where they 

 take up water and substances 

 in solution in water 



