. Elementary botany . Botany. 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
. Elementary botany . Botany. 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 Please note that
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