KNOP'S NUTRIENT SOLUTION 
457 
Discussion of Results 
The yield in dry weight of tops is in close agreement with the general 
appearance of the crops. It will be noted that the yield of tops of the 
barley cultures with a high application of ferric hydrate exceeded the 
control cultures by over fifty percent. The depression with the low con- 
centration of silicic acid, as. compared with the ineffectiveness of the 
high concentration, with both pea and barley plants, is interesting; 
however, these results are based upon the observation of too few 
plants to warrant any conclusion on this point. 
The very decided depression in the yield of tops of barley when 
carbon black was applied would at first suggest the presence of toxins, 
in spite of the repeated washings of this material. In view of the fact, 
however, that the barley seedlings in distilled water, to which carbon 
black had been added, grew for three weeks with a good development 
of root system, while seedlings in distilled water alone were stunted in 
growth after a few days, it would seem that the carbon black was not 
inherently toxic. The young pea seedlings with the carbon black had 
an appearance of increased growth as long as they were supplied with 
nourishment from the cotyledons, but they began to look yellow as 
soon as this reserve was exhausted. This late development of the 
unfavorable effect would seem, therefore, to indicate some complex 
interaction between the plant and the solution, rather than a direct 
toxic effect of the carbon black. 
The comparative uniformity, in all cultures, of both dry weight and 
length of root is in contrast with the results of Breazeale (3), who 
found a distinct stimulus to root growth on the addition of similar 
solids to cultures in toxic soil extracts. It is probable that the in- 
crease in dry weight of roots from the high application of silicic acid 
was due largely to small particles of the gelatinous material that could 
not be washed off. It will be noted that the high concentration of 
carbon black was accompanied by a depression of both length and dry 
weight of barley roots. Apparently, the increased growth of tops 
with ferric hydrate added to the solution was not caused by an in- 
creased surface of roots available for absorption but was due to some 
internal change in the plant, such as a change in the usual course of 
metabolism of the plant cells. 
There is a consistent relation of the dry weight of tops in the 
various cultures to the hydrogen-ion concentration of the solution, 
