The Morphology of Ruppia Maritima. 117 



both oxygen and carbon dioxide are necessary for the hfe 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 easity 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 fiat 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 particular!}^ 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 fioating 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 clearl}^ 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 



