Abrasive Action of Medium 53 



Other examples of the destructive action of currents may be on a 

 smaller scale but of considerable economic importance. In many 

 coastal regions the planktonic larvae of cod, haddock, and other com- 

 mercially important fish are normally carried by currents from the 

 areas where they are spawned to offshore banks suitable for the feed- 

 ing of the juvenile stages when they take to the bottom. There is 

 evidence that occasionally unusual eddies carry the young stages 

 beyond the banks into the open ocean. When the developing fish 

 are ready for bottom life, they find no suitable bottom within reach, 

 and whole populations may perish as a result. A practical applica- 

 tion for an understanding of these ecological relations is involved 

 in the intelligent establishment of hatcheries both on the coast and 

 inland. Before large amounts of the taxpayer's money are spent, 

 some assurance should be gained that the young fish released will be 

 carried by the existing currents to areas suitable for their further 

 development. 



ABRASIVE ACTION OF MEDIUM 



The abrasive action of the medium and of the material carried 

 by it, sometimes referred to as "molar" action, is another aspect of the 

 ecological influence of the medium. If the medium is air, this action 

 means the mechanical force of the wind and the grinding action of 

 sand, dust, snow, and other materials driven by it. In the water, even 

 stronger abrasive action is produced by waves, currents, and particu- 

 larly by stones, sand, ice, and the like, carried by the water. 



Even the wind by itself can influence the growth form of plants in 

 exposed regions. The buttresses at the base of the ceiba trees in the 

 flat country of Cuba have been shown to develop to the greatest extent 

 in directions tending to support the trunk against the most frequent 

 winds (Fig. 2.13). In the Texas Panhandle it is said that on a normal 

 day a man can expectorate a mile and a quarter! The sand carried 

 by winds in such exposed areas produces an abrasive effect which can 

 be resisted only by plants with tough cuticle like the cacti and many 

 grasses. In mountain regions the amount of strong wind to which the 

 vegetation may be subjected is not always appreciated. For example, 

 on Mt. Washington, only 1920 m high, the weather station recorded 

 a wind velocity of more than 120 km per hr (75 miles per hr) on 

 85 days between October, 1940, and March, 1941. The average wind 

 speed for the period was 61 km per hr, with a maximum of 219 km 

 per hr. The all-time high for Mt. Washington was reached during 



