32 Bergman . — Relation of Aeration to Growth and Activity of 
necessity is met by structural modifications or merely by a change in the 
level at which the roots develop. 
(16) The presence of hydrophytes, mesophytes, and xerophytes in 
swamps is due to local differences in the habitat. These differences, 
according to the nature of the adjustment of the roots to the water-level 
which influences the amount of water available, affect the ratio of absorption 
to transpiration and determine the character of the plant. 
(17) Ecesis is possible for many plants even when submerged. Ecesis 
can occur only when the oxygen requirements are satisfied. 
The writer wishes to express his sincere thanks to Dr. F. E. Clements, 
who was at the time Head of the Department of Botany at the University 
of Minnesota, for many helpful criticisms and suggestions. Thanks are also 
due to Dr. I. H. Derby, of the School of Chemistry, University of Minne- 
sota, for criticisms and suggestions on the chemical work in connexion with 
gas analysis. 
l. 
2. 
3 . 
4. 
5. 
6 . 
7 . 
B. 
9 . 
10 . 
11 . 
12 . 
13 . 
14 . 
15 . 
16 . 
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