Nov. 17,1923 
Absorption of Carbon by the Roots of Plants 
305 
When sodium carbonate goes into solution the salt is partially ionized 
with the formation of sodium ions which bear a positive charge and C0 3 ions 
which bear a negative charge. A C0 3 ion will also be formed when calcium, 
magnesium, potassium or any other carbonate goes into solution. The 
plant seems to feed upon ions only. The results of these experiments 
suggest that the plant, when grown in a solution containing a dissolved 
carbonate, absorbs the C0 3 ion by means of its roots. 
Under normal conditions most plants feed more heavily upon nitrates, 
or nitrate radicals (N0 3 ), than they do upon such basic radicals as 
sodium or calcium. When sodium nitrate occurs in a solution the normal 
rate of absorption of N0 3 by plants of this age, in comparison with that 
of sodium, is about in the ratio of 10 to 1 . The plant first takes up 
the N0 3 ion, leaving the solution basic with sodium; the sodium then 
by combining with water becomes a hydroxid and this in turn combines 
with the carbon dioxid dissolved in the nutrient solution from the air 
or from that given off by the roots, and forms sodium carbonate (Na 2 C0 3 ) 
or bicarbonate (NaHC0 3 ). The plant then seems to absorb the ionized 
C0 3 or HCO s radical of the carbonate thus formed. As fast as the C0 3 
or HC0 3 radical is taken up by the plant, the sodium again through the 
agency of water unites with the carbon dioxid of the solution, and more 
sodium carbonate or bicarbonate is formed. 
There is some reason for believing that a demand for any element of 
plant food must exist within the plant before much of it can be absorbed. 
A certain amount of almost any salt available in solution will be 
taken up by the plant, but the amount will be small if the salt be useless. 
The demand generated in the tissues is the one which dominates absorp¬ 
tion, and a plant can not be forced to take up any considerable amount 
of material not needed. 
There are strong indications that the plant absorbs the C0 3 ion from 
solution by means of its roots, and that the carbonate may be derived 
from mineral carbonates, the bases of which may take up carbon dioxid 
from the air or from the carbon dioxid dissolved in the solution. The 
absorption of C0 3 does not seem to take place unless some soluble 
base is present. Upon more than one occasion sets of wheat seedlings 
have been grown for two weeks or more in such solutions as, (a) distilled 
water (control), (b) distilled water, saturated with carbon dioxid gas 
every day, (c) distilled water, with excess of ferric hydroxid, saturated 
with carbon dioxid every day, (d) distilled water, with excess of alu¬ 
minum hydroxid, saturated with carbon dioxid every day, (e) distilled 
water, with excess of calcium carbonate, saturated with carbon dioxid 
every day. With ferric hydroxid and aluminum hydroxid the plant 
had to deal with relatively insoluble bases, while in the case of calcium 
carbonate it had a slightly soluble salt. Only when calcium carbonate 
was present did an ash result that effervesced with acids. Distilled 
water saturated with carbon dioxid would in a few hours come practi¬ 
cally into equilibrium with the air. Nevertheless, for a considerable 
time, especially in cold weather, a large amount of gas would be held 
in the water as carbonic acid, (H 2 C0 3 ), and this would be ionized slightly, 
with the same ions, HC0 3 , and probably a very small amount of C0 3 , 
as were in solution in the case of sodium bicarbonate. With sodium 
bicarbonate the ions appear to be readily absorbed, but in the case of 
carbonic acid it seems impossible for the plant to absorb them. This 
phenomenon of nutrition as shown by the plant may either be indicative 
