vni. c. 1 Bro7vn: Growth of Elodea 19 
of the experiment; while the plants in the jars through which 
CO2 was passed showed no signs of dying at the cut ends. 
This again emphasizes the important influence which variations 
in the percentage of CO^ exert on the growth of this plant. 
Results essentially similar to those given in Tables VIII to 
XI were obtained in another series of experiments which were 
practically identical with those just described. The details are 
omitted as they show nothing new. 
Pond« found that several species of submerged aquatics 
grew much better when rooted in a good soil than when anchored 
oyer the same soil and concluded that they could not survive a 
single season, if denied a substratum of soil. These results are 
in harmony with those given in Table VIII. The explanation 
would seem to be that the roots act as anchoring organs holding 
the plants near the soil which serves as a source of CO2 rather 
than that they are necessary for the absorption of nutrient 
salts, the rooted condition proving of no advantage to the plants 
when the substratum did not give off CO,. It is of course 
possible that roots may be of advantage as absorbing organs 
under other conditions than those tried in these experiments 
or for other submerged aquatics, but it would seem probable 
that their chief function, in the case of plants similar to Elodea, 
is that of anchorage. The rooting of the plants of Elodea cer- 
tainly does not seem to be essential for their healthy growth 
when they are furnished with an abundant supply of CO2. 
Kofoid ^« after making careful quantitative determinations 
of the plankton in a number of lakes concluded that *'the amount 
of plankton produced by bodies of fresh water is, other things 
being equal, in some inverse ratio proportional to the amount 
of its gross aquatic vegetation of the submerged sort." He at- 
tributes the scarcity of plankton in lakes containing submerged 
vegetation to a number of causes, but chiefly to the removal 
from the water, by the larger aquatics, of a great part of the 
available food material. 
In a discussion of Pond's experiments by Reighard and Pond ^ 
these authors concluded that, if Pond's observations are correct, 
submerged aquatics when rooted not only act as a mechanical 
support for algae, but also play an important nutritive role by 
'Rept. U. S. Comm. Fish and Fisheries 29 (1905) 483-526. 
"Kofoid, C. A. The Plankton of the Illinois River, 1894-1899, with 
Introductory Notes on the Hydrography of the Illinois River and its Basin. 
Part I. Quantitative investigations and results. Bull Illinois State Labo- 
ratory of Natural History 6 (1903) 95-629. 
