VIII, c, 1 Brown: Growth of Elodea n 
torn of which was covered with a layer of good soil and through 
which CO2 was passed. The plants in the corresponding jar, to 
which CO2 had not been added, had made a healthy growth; but 
this was much less than in any of those through which it had 
been passed. The greater growth made by the plants, in the jars 
containing the good soil, was probably due to the CO, given off 
by the soil. In the case of the jar, to which CO. had been added, ' 
the soil furnished a constant supply in addition to the inter- 
mittent one from the generator, and thus probably gave the 
plants in this jar an advantage over those which obtained CO. 
only from the generator and air. 
Judging from the previous experiments, it is probable that 
had this and the last been run long enough death would have 
ensued in less than twenty days in the case of all of the plants 
which were dependent for their supply of CO^ on that which 
diffused from the air into the water. All of the experiments, 
therefore, agree in showing that at Baltimore, during the winter 
months, and under the conditions of this experiment, sufficient 
CO2, to keep the plants growing or even ahve, does not diffuse 
from the air, into the water; and that unless CO2 is added from 
some other source, grovd;h stops in less than three days and 
death ensues in less than twenty. Elodea is therefore very 
sensitive to a diminution in the supply of CO^. These results 
are what would be expected after a consideration of the amount 
of CO2 which water absorbs from the air, and would seem to 
show that the quantity of CO2 given off by different soils would 
markedly affect the growth of submerged plants rooted in them. 
EFFECT OF VARIATIONS IN AMOUNT OF NUTRIENT SALTS 
L 
An examination of Tables III and IV shows very little differ- 
ence between the growth of Elodea in tap water and in tap water 
diluted with an equal volume of distilled water. This would 
indicate that wide variations in the concentration of the salts in 
the water had little effect on the growth of Elodea. It seemed 
advisable, therefore, to try the effect of additions of nutrient 
salts to tap water. This was done by making up various 
strengths of Knop's solution with tap water. The basis of the 
additions was a Knop's solution made by taking 1.5 cc N. CaNOg, 
0.4 cc N. KNO,, 0,4 cc N. MgSO,, 0.4 cc K^HPO,, and adding 
sufficient tap water to make a liter. This solution was used in 
one half strength, full strength, two and four times full strength. 
In tables 5 to 7 these strengths are designated as 0.5, 1, 2, and 4 
standard. 
