Nov. 17, 1923 
Absorption of Carbon by the Roots of Plants 
309 
tive charges. If, during the rapid absorption of potassium, there should 
be a tendency for the sap to become too basic, an absorption of C 0 3 would 
follow. On the other hand, a rapid absorption of an acid radical, such 
as N 0 3 , would necessitate an exudation of C 0 2 if equilibrium is to be 
maintained. This phenomenon of absorption and exudation of C0 2 for 
such a purpose holds in animal physiology, and the proof is not wanting 
that there is a striking analogy within the plant. 
The second column of Table I presents data upon this phase of absorp¬ 
tion. With an abundance of potassium present as potassium chlorid, 
as in number 2, the absorption of potassium, including that originally 
contained in the seed, reached 0.1435 gm. for a total of 100 plants. 
With sodium nitrate present, however, the absorption reached 0.2564 
gm. Subtracting the amount originally in the seed from both of these 
figures, we find an increase in the absorption of 105 per cent resulting 
from the presence of sodium nitrate. Ammonium nitrate, in number 
7, produced an absorption almost as great, followed in the order named 
by calcium carbonate, sodium carbonate, and sodium phosphate. Sodium 
phosphate is an alkaline salt, and, while it may prevent the nutrient 
solution from becoming acid during a rapid absorption of potassium, it 
carries no carbonate radical and produces no effervescence in the ash. 
At this stage of growth the normal rate of absorption of potassium by 
wheat seedlings is about equal to the absorption of N 0 3 , and therefore 
with both potassium chlorid and sodium nitrate present in the culture 
solution conditions are ideal for the maximum absorption of potassium. 
When potassium chlorid alone is present in the nutrient solution the 
absorption of potassium is limited. With the addition of a salt not pos¬ 
sessing the power of becoming alkaline under the conditions of the 
experiment, such as the sodium chlorid in number 6, no increase in 
the absorption of potassium was noted in the experiment. This regular 
order of the absorption of potassium will probably not be maintained in 
all experiments, but, with extreme care to eliminate all disturbing factors, 
it seems likely that the general order of these results will be maintained 
as is shown in Table II. 
When sodium nitrate is present in culture solutions, the opposite con¬ 
dition prevails at the beginning of absorption from that which prevails 
with only potassium chlorid present. There is a demand and conse¬ 
quently a rapid absorption of the nitrate ions, N 0 3 , which leaves the 
solution alkaline with an excess of base and the plant sap with an 
-excess of negative charges. Under these conditions the plant seems to 
be able to exude C 0 2 , which tends to bring the sap into equilibrium, and 
enters into combination with the basic radical sodium, in the culture 
-solution, to form sodium carbonate or bicarbonate. There may thus 
be an exchange of acid radicals between plants and the nutrient solu¬ 
tion. The solution may also absorb carbon dioxid from the air, and this 
may enter into the same compounds with the sodium. 
The alkalinity of a sap may be overcome in two ways, first by an exu¬ 
dation of some base not needed in the plant, or by the absorption of 
;some acid. The plant in its adaptation has elected to absorb an acid, 
C 0 3 , rather than to exude a base. Similarly, the acidity of the sap may 
be overcome by the absorption of a base or by the exudation of an 
acid radical, and the plant in its adaptation seems to have developed the 
power to do both. Carbon dioxid, under natural conditions of growth, 
Is practically always present in solution and available, and the C 0 2 
