KNOP'S NUTRIENT SOLUTION 
455 
precipitation with ammonium oxalate and titration of the precipitate 
with potassium permanganate (12). 
When the plants were removed from the culture solutions, the tops 
were severed from the roots, cut into small pieces, dried in watch 
crystals at 104° C, and weighed. The roots were measured, washed 
free from adhering solids, and treated in the same way as the tops. 
Results 
At the end of one week, the barley receiving the higher concen- 
tration of ferric hydrate showed a decidedly darker color and a greater 
growth than any of the other cultures. At this time the foliage of the 
peas receiving carbon black appeared somewhat darker, but the barley 
similarly treated was yellow, as compared with the respective control 
cultures. 
After twenty-one days of growth the peas were just beginning to 
show the influence of the nutrient solutions, despite the previous 
removal of a large part of the reserve material of the seed. At this 
time the plants receiving carbon black were beginning to turn yellow 
and those with ferric hydrate had assumed a very dark green color. 
In the barley cultures, at the end of the experiment, there was a 
striking contrast in the appearance of tops between the plants treated 
with ferric hydrate and those that had received carbon black. In the 
former cultures, the leaves were noticeably broader and the stems 
thicker than those of the control cultures, and the plants were of a very 
dark green color. The carbon-treated culture had produced slender 
plants of a yellowish green color whose general appearance was much 
below that of the control plants. There was a very close similarity in 
appearance between the plants which had received silicic acid and those 
of the control cultures. Those plants to which ferric hydrate had been 
applied appeared to have more freely branching root systems than the 
others, but on the whole there was a considerable ur^iformity in the 
roots of all cultures. An extra control culture of barley in distilled 
water plus carbon black was healthy at the end of twenty-one days, 
whereas the barley in distilled water alone had died some time before ; 
hence, this added solid could hardly have been very toxic to this plant. 
As more important criteria of growth than the preceding observa- 
tions, the dry weights of roots and tops were measured. These results, 
as well as those of the study of the changes in the solution, are pre- 
sented in table i. 
