206 



NUTRITION 



dies (fig. 242). It dies 



Fig. 242. — Sunflower- 

 plants. The two left-hand plants 

 have been cultivated in a soil 

 containing combined nitrogen 

 in the form of potassic nitrate. 

 The two right-hand plants have 

 been cultivated in a soil similar, 

 excepting that there is no com- 

 bined nitrogen in it. (After 

 A. Mayer.) 



for want of nitrogen, though it has 

 an inexhaustible supply of free 

 nitrogen in the air around it and in 

 the air dissolved in the culture- 

 solution. This proves tha,t the plant 

 cannot obtain from the air the nitrogen 

 it requires. The plant must have com- 

 bined nitrogen — preferably nitrogen 

 containing salts — supplied to its roots. 

 Hence the roots absorb all the 

 elements required by the plant with 

 the exception of carbon.* The 

 members of the Bean-family (Legum- 

 inosce) form an apparent exception to 

 this rule. They have peculiar swell- 

 ings on their roots — the so-called 

 tubercles or nodules — which are caused 

 by microscopic fungi or bacteria. 

 These Leguminosm can live and grow 

 vigorously when the nitrogen is 

 supplied to them only in the form 

 of free nitrogen gas. But if the roots 

 of a leguminous plant are not infected 

 with the tubercle-bacterium, the plant 

 remains stunted and soon dies when 

 not supplied with nitrates or other 

 compounds of nitrogen. In some 

 way the bacteria enable leguminous 

 plants to employ free nitrogen as 

 food. 



CURRENT OF WATER AND SALTS UP THE STEM 

 TO THE LEAVES. 



If we observe the amount of water absorbed by the roots 

 dipping in a culture-solution, we see that it is many times as 

 great as the volume of the roots. The roots are not large enough 

 to have retained all the water they absorbed ; it is therefore 

 evident that some of the liquid must have passed up into the 

 stem. Salts also are carried up into the stem and to the 

 leaves, as is shown by the fact that large quantities of the 



* The roots can also absorb organic carbon-compounds, though they can- 

 not take in carbonic acid. 



