A QUA 7 '1C VEGETATION. 621 



into two groups which correspond with the divisions: hydro- 

 phytes, which grow in fresh-water streams, lakes, ponds, etc.; 

 and halophytes, or those which grow in the ocean or in salt lakes. 

 The fresh-water plants are sometimes called limnetic, and the 

 marine forms pelagic. In both the limnetic and pelagic floras 

 there are plants which are fixed to some place of support (to 

 the rocks, lithophytes, as in case of pelagic forms; to the soil, as 

 in case of most limnetic forms; while algae, etc., attached to 

 other plants are epiphytes), while others are free-floating or 

 swimming.* 



1 109. Different regions of light in deep water. Aquatic plants 

 are distributed at different depths in the water, and it is sur- 

 prising the depth at which some can live. It is obvious that 

 plants located at different depths will have different light rela- 

 tions, those near the surface being well lighted, while those lower 

 down will receive light of diminished intensity. Light is the 

 controlling factor, and three regions are recognized, ist. The 

 bright-light region, or photic region, in which the light intensity 

 is favorable for the normal development of the larger plants, 

 (macrophytes). It is the region also of most of the microphytes 

 which have photosynthesis, especially the green algae. 2d. The 

 dimly-lighted region, or dysphotic region, where most of the 

 macrophytes reach only a stunted growth or fail, and where a 



* Schimper uses the term Benthos or benthonic (/3e'0oS= depths) for 

 those forms which are fixed. Plankton is the term applied to the free- 

 swimming or floating forms, but perhaps its best expression is now for 

 the swimming microscopic plant and animal forms of deep water. Plank- 

 ton was first used to signify pelagic animals. The best examples of plant 

 plankton are found in the Protococcoideae, floating and swimming green 

 algae, as Volvox, Pandorina, Eudorina, Chlamydomonas, etc., the desmids, 

 the blue-green algae, the Peridineas, and the diatoms especially. The 

 diatoms have a siliceous external skeleton. They occur in great numbers 

 in fresh water, but in vastly greater numbers in the ocean. In geologic 

 times they were so abundant for long periods that their skeletons have built 

 up great rock formations. Hemiplankton is a term applied to those plants 

 floating and swimming in the shallow water of the coast or in the shallow 

 waters of inland lakes and ponds, where they are mixed with benthonic forms. 



