The Morphology of Ruppia Maritima. 117 
both oxygen and carbon dioxide are necessary for the life of the 
water plant just as in the case of the land plant. Of the gases in 
‘solution in water, both oxygen and carbon dioxide are present, indeed, 
in a greater proportion than in air. The diffusion of these gases, 
however, takes place much more slowly than in the air, so that 
the larger the area of leaf surface, the more readily will the re- 
-quisite amount of gases be absorbed (Warming, 1902, p. 127 and 
Goebel, 1893, pp. 248 ff.). 
It is obvious, therefore, that for the purpose of carrying on its 
absorption both of gases and nutrient mineral solutions in the best 
manner possible, the long, slender leaf of Ruppia is an ideal form. 
In the salt-marsh creeks and ditches where it makes its home, 
Ruppia is constantly subjected to the action of fairly strong tidal 
currents. On this account also it is advantageous to the plant to 
have slender leaves; for such easily accommodate themselves to 
these alternately inflowing and outflowing tidal currents. This ac- 
commodation becomes the easier from the fact that the leaves are 
not dorsiventral, but alike on both flat surfaces and, therefore, bisym- 
metrical ; a condition which may be referred to the diffuseness of the 
light in the water, as well as the constant moving about of the leaves, 
resulting in a tendency to equality of conditions on both sides of the leaf. 
A condition of general ecological significance, which affects all 
of the vegetative organs, but particularly the shoot system, has to 
do with the tidal currents mentioned above. At times these currents 
are even so strong as to detach portions of plants, which I have 
often observed floating up or down stream, according to the tide. 
These plant segments become entangled in grass, &c. along the 
stream edge, or collect in some miniature cove and then begin an 
independent existence, at perhaps a considerable distance from the 
parent plant. This is probably a common mode of vegetative 
reproduction among such water plants. 
2. Internal Structural Adaptations. 
a. Epidermal modifications. 
In many respects the epidermis of stem and leaf clearly reflects 
environmental influence. 
In contrast to the thick outer or free walls of the epidermal cells 
of land plants, we meet here with a thin wall. Surrounded by 
water, there is no danger of the drying up of the plant by evaporation 
from the epidermal cells—a process which, on the other hand, 
commences immediately on exposure of the plants to the air—and 
consequently a thick wall would here be superfluous. The fact 
that even in Ruppia the outer wall of the epidermal cell is slightly 
