BEN'THOS (Gk. pevOos, ben-thos, the depth of the sea ). In botany, the fixed vegetation of the sea or of great lakes, in contrast with the floating vegetation or plankton (q.v.). Benthonic plants are in large part attached to rocks, in which case they are called lithophytes. The lithophytic members of the benthos are chiefly algfe, and it is here that the wonderful display of marine plants of our ocean coasts is found. The benthos may be subdivided into the regions abote and below the low-tide line. Above this line the vegetation is relatively sparse, and is composed of plants which are able to endure strong expo sure during the period of low tide. Along the North Atlantic coast, the rockweed (Fuchs) is one of the most characteristic plants of this zone.
Below low tide is the zone of constant sub mergence, and here the benthos finds its richest development. Red, brown, and green algle. and sometimes other plants, grow in vast profusion. In the upper parts of this zone the brown and green alga: often dominate, while red algae domi nate at greater depths. Deeper in the water all of these large plants cease.
The ecology of the benthos has been studied by Berthold and Kjellman. In the Mediterranean there are seasons which are determined by light, surface forms vegetating early in the year, while the alga of deeper waters vegetate in summer. One of the surprises of the northern seas is the rich development of plant life, the brown alga: being particularly common. Here the seasons depend rather upon heat than light. Most of the benthonic lithophytes have well-developed hold fast organs (bhapterre) attaching them to rocks, and many forms are provided with bladders which permit free floating in the water.
Less conspicuous than the rock plants are the plants attached to loose soil. A few algae like Caulerpa are classed here, but perhaps the most prominent plant of this group is the eel-grass (Zest era), which often forms vast submarine meadows. The benthos of fresh waters is not well developed, or, if present, has a different ecology. See HYDROPHYTES ; ECOLOGY.