48 HOW CROPS FEED 



fice to provide a rapidly growing vegetation with carbon 

 is demonstrated by numerous facts. Here w^e need only 

 mention that in a soil totally destitute of all carbon, be- 

 sides that contained in the seeds sown in it, Boussingault 

 brought sunflowers to a normal development. The writer 

 has done the same with buckwheat ; and Sachs, Knop, 

 Stohmann, Nobbe and Siegert, and others, have produced 

 perfect plants of maize, oats, etc., whose roots, throughout 

 the whole period of growth, were immersed in a weak, 

 saline solution, destitute of carbon. (See 11. C. G., Water 

 Culture, p. 167 ) 



Hellriegel's recent experiments give the result that the 

 atmospheric supply of carbonic acid is probably sufficient 

 for the production of a maximum crop under all circum- 

 stances ; at least artificial supply, w^hether of the gas, of 

 its aqueous solution, or of a carbonate, to the soil, had no 

 effect to increase the crop. {Chem. Ackersmann, 1868, 

 p. 18.) 



Liebig considers carbonic acid to be, under all circum- 

 stances, the exclusive source of the carbon of agricultural 

 vegetation. To this point we shall recur in our study of 

 the soil. 



Carbon fixed by Chlorophyll.— The fixation of carbon 

 from the carbonic acid of the air is accomplished in, or has 

 an intimate relation with, the chlorophyll grains of the 

 leaf or green stem. This is not only evident from the 

 microscopic study of the development of the carbohy- 

 drates, especially starch, Avhose organization proceeds from 

 the chlorophyll, but is an inference from the experiments 

 of Gris on the effects of withholding iron from plants. In 

 absence of iron, the leaf may unfold and attain a certain 

 development ; but chlorophyll is not formed, and the plant 

 soon dies, without any real growth by assimilation of food 

 from without. (IT. C. G., p. fiOO.) Filially, experiment 

 shows that oxygen is given off (and carbonic acid decom- 

 posed with fixation of carbon) only from those parts of 



