2 l6 
Journal of Agricultural Research 
Vol. XXVII, No. 4 
The results (Table V) show again and more clearly that the preference 
of the seedlings at an early stage of growth is for nitric acid and at a 
later stage for phosphoric acid. The age factor in this experiment also 
includes the factor of previous mineral nutrition. As a rule, however, 
the two are inseparable, as age in the sense of advanced stage of growth 
can not be measured merely by the number of days which elapse from 
the time of germination. 
DISCUSSION OF EXPERIMENTAL RESULTS 
The fact that the activity of seedlings in decreasing the initial acidity 
of the medium is preferential, resulting in the greatest decrease in acidity 
in one acid at one set of conditions and in another acid at another set 
of conditions, indicates that the cause of these changes is preferential 
absorption by the plants. 
The term “preferential absorption” seems more appropriate here than 
the commonly used term ‘‘selective absorption.” “Selective absorp¬ 
tion” might imply that some substances are excluded, while “preferential 
absorption” indicates merely a higher rate of absorption of one substance 
as compared with that of another. 
If the changes in the initial reaction of the acid solutions were a result 
of neutralization bysecreta from the roots, they would be expected to 
be controlled by the active acidity factor only, and different acids with 
the same initial hydrogen-ion concentration and the same dissociation 
constants would behave alike. If the preferential action of the plants 
were in favor of nitric acid throughout, reduction by microorganisms 
might have suggested itself. It has been shown elsewhere (4), however, 
that the possibility of nitrate reduction is eliminated under the condi¬ 
tions of these experiments. But the fact that the preference of the 
older seedlings is for phosphoric acid lends additional strength to the 
absorption theory, which is further supported by the fact that in plant 
life nitrogen and phosphorus are, at least quantitatively, the most essen¬ 
tial of the acid-forming elements dealt with in this investigation. 
SUMMARY 
Wheat seedlings were grown in solutions of hydrochloric, nitric, sul¬ 
phuric, phosphoric, formic, acetic, oxalic, succinic, benzoic, and phthalic 
acids. The changes in reaction produced by their growth, recorded at 
certain intervals, show that: 
Of the inorganic acids, the greatest changes were produced in nitric 
acid at early stages of growth of the seedlings and in phosphoric acid at 
later stages. Phosphorus and nitrogen being the most essential elements 
of plant growth contained in the acids used, it may be concluded that 
the changes in initial reaction produced by plants in the medium in 
which they grow are due to absorption rather than to neutralization. 
The previous growth of the experimental seedlings in nutrient solu¬ 
tions deficient in acid-forming elements diminished their ability to 
decrease the acidity of the acid solutions. Apparently the deficiency 
of the previous nutrient solutions produced functional disturbances in 
the seedlings. 
The greatest changes from the initial reactions were produced in the 
solutions of the organic acids. This, however, may have been due partly 
or wholly to microbiological activity and needs to be studied further. 
