79° 



ZONES AND REGIONS [Pt. Ill, Sect. V 



graphical differentiation ; many species living in the depths, for instance 

 Florideae, approach the surface in shady places. The strength of move- 

 ment is also important. Thus many Corallineae grow in very agitated 

 water, whereas the Cystoseireae and Padina Pavonia are confined to calmer 

 spots, and species peculiar to the sand and mud occur only in quite calm water. 

 A change in the salinity of water, for instance near river mouths, occasions 

 important differences — new forms appear, others, much more numerous, 

 disappear. Organic impurities in the water of drains and canal mouths 

 effect like differences. 



ii. THE BENTHOS OF TROPICAL SEAS. 



In opposition to the terrestrial vegetation tropical marine vegetation 

 is less luxuriant and apparently less rich in forms than is that of the 



Fig. 479. Navicula Grevillii, Ag. (Diatomaceae). A Branched miniature-tree- 

 like colony. B Some ends of the tubes with cells. C Single cells. After 

 Schutt in Engler und Plantl, Die natiirlichen Ptlanzenfamilien. 



temperate and polar zones. Only a few usually small groups of forms 

 are exclusively or mainly tropical, for instance the marine Hydrocharitaceae 

 (Halophila, Enhalus, Thalassia), the species of Halodule and most of those of 

 Cymodocea among Potamogetonaceae, also the Valoniaceae, Dasycladaceae, 

 Caulerpaceae, and Codiaceae, among Chlorophyceae. Rhodophyceae are 

 richly represented, Phaeophyceae poorly so. Yet to the last-named class 

 belongs a genus — Sargassum — of extremely large, richly differentiated, sea- 

 weeds, common in the tropical seas and including many forms familiar 

 to all seafarers by the occurrence of severed yellow branches floating 



