Roots and its Influence on the Ecesis of Plants in Swamps . 29 
not furnish enough available water for plants on account of the great water- 
retaining power of Sphagnum . For this reason plants with roots above water 
may show more or less severe wilting on the hotter days. If the drought 
becomes too prolonged or too intense some of these plants may perish. 
Plants, such as the Ericads, which are not protected by taller vegetation 
may be subjected to very great water loss. The roots of these plants are 
generally not extensive. They ordinarily grow above the water-level or 
not far below it. They may, however, endure submergence for some time 
without apparent injury. During the summer the Sphagnum on which the 
bog heaths grow may have but little available water because of the lower- 
ing of the water-level and the great ability of Sphagnum to retain water, or 
for other reasons. Therefore, water in sufficient amounts for the needs of 
the plants might be difficult to obtain. Gates (8, p. 451) states that in such 
cases ‘ The xerophytic adaptations . . . materially aid by lessening the 
demand upon root absorption ’. In winter, with continued water loss from 
the leaves at a time when replacement by absorption is very difficult if not 
impossible, the xerophytic structures are very valuable in reducing the rate 
of transpiration. 
The character and position of the roots with reference to the water- 
level has been shown to be directly correlated with the need of aeration. 
In securing adequate aeration the plant may undergo structural modifica- 
tion by the development of aerenchyma, or the roots may develop above 
the water-level. In the latter case, however, the amount of water available 
is involved. If the plants are more or less protected by taller vegetation, 
or if the substratum does not become too dry, sufficient water is available to 
replace the loss by transpiration in mesophytes, and they are able to persist. 
On the other hand, a lowering of the water-level, a cold substratum, or other 
factors which retard absorption, combined with atmospheric factors which 
bring about a high rate of transpiration, make it impossible for mesophytes 
to persist. Only the Ericads with their strongly xerophytic leaf-structures 
are able to endure such conditions. Accordingly the presence of hydro- 
phytes, mesophytes, and xerophytes in swamps is to be explained by 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. 
The Relation of Aeration to Ecesis . 
Many plants are able to establish themselves in habitats with an excess 
of water. Fruits of Alnus incana , Betula pumila , Panicularia americana , 
Rumcx britannica, and Scirpus cyperinus germinated, and the seedlings were 
able to establish themselves on Sphagnum when the water-level was kept 
just at the surface of the Sphagnum . Seedlings of Rumex were able 
