HOW PLANTS GROW 



339 



Also showing air spaces. 

 Magnified. 



If we place some mustard, radish, or turnip seeds 

 between moist blotting paper and keep the paper moist 

 for two or three days, we shall find that the roots of the 

 germinating seeds are covered with tiny white threads, 

 standing straight out from the root, 

 and some of them about an eighth 

 of an inch long. These white threads 

 are called root hairs, and they take 

 up most of the moisture for the grow- 

 ing plant. Root hairs are composed 



of only one cell, and the soil water RoOT HAms IN THE SOIL 

 flows into them and thence into 

 the main root. All new roots are 

 covered with these root hairs. When large trees start to 

 grow in the spring, they first grow very small new roots at 

 the ends of the old ones and these new roots are covered 

 with root hairs which absorb the soil water for the trees. 

 These root hairs are composed of protoplasm and are 

 filled with sap which is more dense than ordinary soil 

 water; the soil water passes through the thin 

 wall of the root hair and then into the larger 

 root. This process by which a liquid flows 

 through an animal or plant membrane is 

 called osmosis, and the greater flow is toward 

 the denser liquid. The pressure caused by 

 osmosis in the roots is sufficient to lift the 

 sap many feet. 



If no osmosis apparatus is at hand, the 

 effect of osmosis can be shown in the follow- 

 ing way: Fasten a glass tube to the narrow end of an egg 

 with paraffin or sealing wax; then with a long wire care- 

 fully puncture the shell of the egg inside of the glass tube 

 so that the contents of the egg can flow up the tube. 



ROOT HAIRS 

 ON A YOUNG 

 ROOT 



