206 
AMERICAN JOURNAL OF BOTANY 
[Vo1. io 
seedlings were about io cm. high, specimens of uniform height were selected 
for use. These were mounted in wide-mouthed jars of about 960-cc. capacity 
(quart Mason jars), covered with heavy paper so as to exclude light and 
restrict the growth of algae. The cultures were supported on rotating tables 
for the purpose of equalizing the exposure of all to variations of light and 
temperature. This procedure is explained in the manual for collaborators 
( 3 )- 
The experiment continued from April 14 to May 2, 1922. No records 
were taken of climatic conditions within the greenhouse. However, the 
thermostat of the house was adjusted for 15.5 0 C. night temperature. The 
conditions of solar radiation by day may be considered from the following 
data derived from records of the local weather bureau station: 
Daily percentage of sunshine: 
Max. 100.0, Min. 0.0, Mean 58.3. 
Daily calories per sq. cm. per hour: 
Max. 674.0, Min. 49.0, Mean 472.6. 
The pH values of the solutions with various amounts of iron added were 
determined in the manner described by Clark (2). These values appear 
in table 2. 
Table 2. Values of Solution pH Value 4.2, with Various Forms 
and Amounts of Iron added 
Ferric Citrate 
Fe 2 (S 0 4 ) 3 
FeS 0 4 
FeP 0 4 
Mg. Fe per 1 . 
50 
IO 
2 
50 
10 
2 
10 
10 
pH. 
5-0 
4.8 
4.4 
3-0 
3-6 
4.0 
4.2 
4.4 
From these data it appears that Fe 2 (S0 4 )3 increased the hydrogen-ion 
concentration of the nutrient solution while ferric citrate decreased it. 
The pH value of certain solutions was determined also after the growth of 
plants over the 3-day period ending April 28. It was found that, when the two 
higher planes of Fe 2 (S0 4 )3 were added, the pH value increased from 3.0 to 
3.2 and from 3.6 to 4.0 for the 50-milligram and 10-milligram planes 
respectively. In all other cases the change of pH value was inappreciable. 
The plants were so small, however, that they could hardly be expected to 
exercise much effect upon the composition of the nutrient solution. Cer¬ 
tainly, as regards form and appearance of the roots, the increased acidity 
from the use of Fe 2 (S 0 4 ) 3 was injurious to the plants. 
Photographs of selected cultures were taken on May 1. These are 
reproduced in figures 2-4. On May 2, after 18 days of growth, the cultures 
were harvested and the usual separation of tops from roots was accom¬ 
plished. The data of growth measurements are given in table 3. 
