THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 253 



they appropriated? What substances are absolutely uecessarj, and 

 why ? 



C, H, O, N, also K, Ca, Mg, P, and S, are the elements of which 

 the food-substances are composed. Na, CI, and Si seem to belong 

 to the group of useful rather than necessary elements. Among 

 fungi^ rubidium and csesium may be substituted for K ; Mg, Sr, or 

 Ba for Ca. Fungi may subsist without Fe, since they contain no 

 chlorophyll. Marine-plants contain iodine and bromine in addition 

 to the elements mentioned above. 



Among plants and K are the only elements which occur in 

 the free state— N as a gas, O as a gas and in solution in water. The 

 remaining elements occur almost exclusively as binary, ternary, or 

 even higher compounds. 



Since plasm is chemically closely related to albuminous com- 

 pounds, and since the cell-wall and starch consist of carbohydrates^ 

 it becomes evident that C, H, O, N, S are the necessary elements,, 

 eventually also P. 



Oxygen alone enters into the plant-metabolism as an element. 

 Iron enters the plant in the form of an oxide in solution. It occurs 

 only in small quantities, though it is absolutely necessary in chloro- 

 phyll-formation and therefore also in assimilation. Sulphur and 

 sometimes ^A(9S^Ao/*ws are necessary in the formation of albuminous 

 substances. Potassium and calcium are also necessary, though their 

 true significance is not understood. (See below.) 



According to Boussingault, the free nitrogen of the air cannot 

 be utilized as food by the plant. It is usually introduced into the 

 plant by way of the roots ; not in the free state, but in the form of 

 compounds, such as nitrates, nitric acid, and amnionia in solution in 

 water. In the years 1851-1854 Boussingault apparently demon- 

 strated the fact that when all nitrogen-bearing compounds were ex- 

 cluded from the soil and atmosphere, the elementary K did not in- 

 crease the nitrogenous compounds of the plant; the plant would die 

 after all the reserve nitrogen in the form of compounds had been util- 

 ized. This belief prevailed until recent years, when Ville, Joulie, 

 Atwatek, Frank, Hellbiegel, and others carried on experiments 

 which tend to prove that the free nitrogen of the air may be utilized 

 by the plants. Frank based his conclusions upon experiments with 

 algaa, fungi, and several phanerogams. He has demonstrated that 

 not only are leguminous plants which bear root-tubercles containing 



