28 Bergman . — Relation of A Ration to Growth and Activity of 
assumed to be due to the lack of one or more mineral constituents. In view 
of these facts it seems very probable that the much greater reduction in 
growth of plants in peat or Sphagnum , with roots submerged, was due to 
a lack of aeration. This conclusion is further confirmed by the fact that 
analyses show the oxygen content of the water of peat and Sphagnum sub- 
strata to be much lower than that of lake or tap water. The reducing 
power of peat and Sphagnum also operates to prevent a replenishment of 
the oxygen supply from the air. It has also been shown that, if aeration is 
provided, plants grown with the roots submerged in either swamp or tap 
water show little or no reduction in growth and no essential difference in 
behaviour as compared with that of plants grown in moist soil. 
The Relation of Roots to the Water-level with Reference 
to the Character of the Plant . 
Plants growing in swamps may be hydrophytic, mesophytic, or 
xerophytic in character. Many writers regard Scirpus , Equisetum , J uncus 
and similar plants as bog xerophytes on account of the absence of leaves 
and general external appearance. The ratio of transpiration to absorption 
is the important factor in determining the character of a plant. Scirpus has 
been shown by Sampson and Allen ( 21 , p. 49 ) to be a typical hydrophyte 
in its rate of transpiration. And as Groom ( 10 ) has shown, in the case of 
Larix decidua , some so-called xerophytes transpire more rapidly than some 
mesophytes. The only plants which are unquestionably bog xerophytes 
are the Ericads. Gates ( 8 ) concludes that winter evaporation is fundamen- 
tally responsible for their xerophytic character. 
Proximity of location in a swamp does not necessarily mean similarity 
or identity of conditions either for the roots or for parts above ground. In 
fact, conditions for different layers are very different, as both Yapp (27) and 
Sherff (23) have pointed out. The conditions for root growth above and 
below the water-level are also very different. Hydrophytes with their 
extensively developed air-conducting systems are able not only to with- 
stand root submergence, but to make better growth than in moderately 
moist soil. Absorption is not retarded, and however great the* water loss 
from aerial parts it is readily replaced by absorption of water by the roots. 
Hence there is no need for structural modifications of the aerial organs to 
prevent water loss. 
Plants with roots above the water-level are usually low-growing, and 
consequently more or less protected from excessive water loss. The 
absorbing capacity of the roots is little or not at all reduced ordinarily, 
since good aeration is provided and other factors are not very adverse. 
Such plants are usually mesophytic in character. However, in dry years 
or during dry periods of the -summer they may be subject to more severe 
conditions. The upper layers of the substratum become dry, or at least do 
