ECOLOGY 617 | 
water relation. ‘The latter method appears to be simpler, and 
will now be considered. 
According, therefore, to the relation plant communities have 
assumed in regard to water, they may be grouped as follows: 
Hydrophytes or water plants. 
Helophytes or marsh plants. 
Halophytes or salt plants. 
Xerophytes or desert plants. 
Mesophytes or intermediate plants. 
.. Tropophytes or alternate plants. 
Fivbroravide —The effect of an aquatic environment on 
the structure of water plants is most striking. The root systems 
_are reduced both in length and number of branches. The 
root hairs of those immersed in the water are absent. The 
supportive action of the water is such that the fibrovascular 
elements of the stems, which usually function both for support 
and for the conduction of crude sap, are greatly reduced in size 
and strength. The leaves, stems and roots possess large air- 
spaces. ‘The mesophyll of the leaves is spongy and the chloro- 
plasts motile. Stomata are entirely absent from leaves that are 
submerged and only present on the upper surface of floating 
ones, where they are nearly always open. Some of these plants 
have broad floating leaves and dissected submerged ones, often 
with thread-like divisions. ‘The submerged parts are devoid of 
special protective walls, e.g., those containing cutin or suberin. 
The cell sap has a low osmotic pressure. The submerged leaves 
often absorb more water than the roots. The free floating 
microscopic plants (blue-green algze, bacteria, diatoms, desmids, 
etc.) form the plankton of our ponds, rivers and lakes. The free- 
swiming higher plants (the plewston) comprise certain liverworts 
like Riccia and Ricciocarpus, water-ferns and such seed plants as the 
water-lettuce and water-hyacinth. The aquatic plants, includ- 
ing the algae, mosses, and flowering plants which live attached to 
rocks comprise the /ithophilous benthos. Another class of aquatic 
plants (benthos) include those with true roots, which attach the 
plant to the substratum, and at most possess floating leaves. 
This type includes the water-lilies, the water-chestnut, the splatter 
docks, the floating-heart and the pondweeds. 
An wOn es 
