56 The Medium 



tached plants and those animals with streamlined forms and special 

 hooks, sucking discs, or other devices for clinging to the bottom can 

 exist in turbulent mountain streams (Nielsen, 1950; Welch, 1952, 

 Ch. 17). Currents of moderate velocity, such as are encountered in 

 the lower reaches of streams and along the shores of lakes and of the 

 ocean, produce a force that must be resisted or avoided by the animals 

 and plants attempting to maintain a foothold in such habitats. Mov- 

 ing sand and silt continually threaten to abrade or to smother the 

 organisms present. A current with a speed of only 1.4 m per sec 

 (2% knots), for example, will move stones and gravel up to 2.5 cm in 

 diameter and thus would grind up all unprotected forms. 



At the margin of lakes and particularly on the ocean beach the force 

 of breaking waves, and of the sand, gravel, or ice carried by the water, 

 produces an extremely serious abrasive action. Where exposure to 

 such molar action of waves is particularly severe, the shoreline is often 

 barren except for especially adapted species. However, in zones of 

 very considerable wave action, a surprisingly large number of species 

 of plants and animals have developed methods of maintaining them- 

 selves. 



Devices for withstanding the serious molar action of the water 

 medium are extremely varied. Anyone who has caught and cleaned a 

 bass, a scup, or other fish inhabiting turbulent coastal waters knows 

 how much protection the tough skin and thick scales provide for the 

 surface of the animal. This resistant integument is in sharp contrast 

 to the tissue-thin skin of the deep-sea fish. These latter aniiuals living 

 deep in midocean have no wave action to contend with, nor are they 

 subject to chafing against rocks or gravel by currents. The inverte- 

 brates of the shoreline as well as the fish are protected by a resistant 

 outer surface. The shells of mollusks and of crustaceans inhabiting 

 such situations are characteristically thick and hard. 



During the course of evolution many species in the littoral zone 

 have developed holdfasts, cementing organs, suckers, etc., which pre- 

 vent their owners from being washed away. Instead of evolving 

 with more and more rigid outside surfaces, certain inhabitants of the 

 surf zone solved the problem in the opposite direction. The sea- 

 weeds, such as the kelp and the rock weeds, are composed of flexible 

 tissue with a leathery exterior. These forms give with the surge of 

 the waves rather than attempting to resist their force. The form 

 and structure of these plants present a sharp contrast to the brittle 

 nature of the typical land vegetation. Imagine what would happen 

 if your flower garden were suddenly transplanted into the middle of 

 the breaker zone! 



