304 HOW CROPS FEED. 
ishing solution, by the amount of free acid or of iron 
present, nor by the illumination. Hampe observed that 
from these trials it seemed that the plants, while young, 
were unable to assimilate ammonia or did so with diffi- 
culty, but acquired the power with a certain age. 
In 1868, Wagner (Vs. S¢., XI, 288) obtained exactly 
the same results as Hampe. He found also that a maize- 
seedling, allowed to vegetate for two weeks in an artificial 
soil, and then placed in the nutritive solution, with phos- 
phate of ammonia as a source of nitrogen, grew nor- 
mally, without any symptoms of disease. Wagner ob- 
tained one plant weighing, dry, 26'|, grams, and carrying 
48 ripe seeds. In experiments with carbonate of ammonia, 
Wagner obtained the same negative result as Beyer had 
experienced in 1866. | 
Beyer reports (Vs. S¢., XI., 267) that his attempts to 
nourish the oat-plant in solutions containing ammonia- 
salts as the single source of nitrogen invariably failed, 
although repeated through three summers, and varied in 
several ways. Even with solutions identical to those in 
which maize grew successfully for Hampe, the oat seed- 
lings refused to increase notably in weight, every precau- 
tion that could be thought of being taken to provide 
favorable conditions. It is not impossible that all these 
failures to supply plants with nitrogen by the use of am- 
monia-salts depend not upon the incapacity of vegetation 
to assimilate ammonia, but upon other conditions, unfa- 
vorable to growth, which are inseparable from the meth- 
ods of experiment. A plant growing in a solution or in 
pure quartz sand is in abnormal circumstances, in so far 
that neither of these media can exert absorbent power 
sufficient to remove from solution and make innocuous any 
substance which may be set free by the selective agency 
of the plant. 
Further investigations must be awaited before this 
point can be definitely settled. It is, however, a matter 
