3io 
Journal of Agricultural Research 
Vol. XXVI, No. 
content of the soil air is usually much higher than that of the overlying 
atmosphere. If the plant, in its ages of adaptation, has felt the stimulus 
of absorbing or exuding an acid it is reasonable to suppose that it may 
have selected C 0 2 in preference to all others. 
Carbon dioxid seems to be present in combination with some soluble 
base in practically all soil solutions. The writer has never analyzed a 
soil solution that did not contain at least some bicarbonates. If, for 
example, only a few parts per million of calcium or sodium are present 
in the soil or culture solution, the carbonate radical necessary for combi¬ 
nation with such a small amount of base would not necessarily be 
sufficient to cause a perceptible effervescence with acid; but it must be 
remembered that the C 0 3 ion may be absorbed by the plant, while the 
calcium or sodium ion may remain in the solution to unite with the CO* 
contained, and form more CaC 0 3 or Na^COg, which in turn gives up its 
C0 3 to the plant. The amount of the carbonate ion which may be 
absorbed is therefore out of proportion to that originally combined with 
the calcium. In localities where waters high in calcium carbonate 
abound, or where the soil is calcareous, it is very difficult to keep control 
cultures, or cultures where no carbonate is supposed to be, entirely free 
from carbonate. The dust from the air in settling over a culture solution 
often may contain enough lime to cause the ash of the plants to effervesce 
with acid. In accurate work the plants must be shielded from all such 
sources of lime, and the culture pans and aluminum disks must be rinsed 
off with dilute acid before beginning the experiment. 
If practically all the phenomena of plant physiology are but phenomena 
of adaptation, as the writer firmly believes, the theory that the plant uses 
the carbonate radical in order to maintain equilibrium in its tissues 
is perfectly reasonable. In the culture solution containing only potassium 
chlorid the absorption of potassium was checked because of the acidity of 
the solution. Under field conditions this acidity is cared for naturally, 
as the great majority of our upland soils are basic, either with lime or 
magnesium carbonates, or with the oxides of iron or aluminum. The 
plant, therefore, in its adaptation, has never felt the stimulus of exuding 
a base in order to prevent the soil solution from becoming acid. The 
writer has never been able, by the use of many methods, to force a plant 
to exude a base in appreciable amounts. On the other hand, the plant 
has felt the stimulus of exuding an acid radical, as the hydroxids of the 
alkalies are much more toxic than the carbonates or bicarbonates. 
With plants that thrive in bog soils it is possible to conceive of a 
scarcity or even an absence of aerobic bacteria, and the consequent 
scarcity of carbon dioxid. This might cause such plants to develop 
a different system. Soluble silica or some other acid radical might be 
used instead of C0 3 to maintain equilibrium in the tissues. The high 
silica content of many such plants may perhaps be ascribed to this 
process. 
The absorption of any element of plant food is probably dependent 
upon the state of equilibrium of both the plant sap and the nutrient 
solution. This equilibrium may not necessarily be neutral, but may be 
either acid or akaline, and it will probably be different with each plant. 
Further, the plant in absorbing food may have certain powers of accom¬ 
modation enabling it to absorb upon both sides of the equilibrium line at 
almost the same time. This absorbing zone will probably be wide with 
some plants and narrow with others. 
