48 HOW CROPS FEED 
fice to provide a rapidly growing vegetation with carbon 
is demonstrated by numerous facts. Here we 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 anormal 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 H. 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, whether 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, whose 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. (H. C. G., p. 200.) Finally, experiment 
shows that oxygen is given off (and carbonic acid decom- — 
posed with fixation of carbon) only from those parts of 
