3 



The Siihstratiim 



As a factor of the environment the substratum is only shghtly less ele- 

 mental than the medium. The substratum is the surface upon which 

 the organism rests or moves, or the solid material within which it 

 lives in whole or in part. Some ecologists have not distinguished be- 

 tween medium and substratum. Another possible source of con- 

 fusion is the use by bacteriologists of the term "substratum" for the 

 nutrient medium used for growing microorganisms. The important 

 point is to distinguish between the concepts, and it seems most logical 

 to use "medium" exclusively for the material which immediately sur- 

 rounds the organism, and "substratum" only for the surfaces or solid 

 materials of the environment on which or within which the organism 

 lives. 



SIGNIFICANCE OF THE SUBSTRATUM 



The substratum is not inevitable as is the case with medium. Every 

 organism has a medium, either air or water, but some organisms can 

 do without a substratum. In the aquatic environment the permanent 

 plankton and many pelagic fish have no substratum at any time, but 

 terrestrial organisms must have a substratum for at least part of their 

 lives since no animal or plant can live permanently suspended in the 

 air. Another general difference between the medium and the sub- 

 stratum is that, whereas the medium is rarely changed from air to 

 water, or vice versa, by the activity of the organism, the substratum 

 can be profoundly modified by many of the animals and plants which 

 live on it or in it. 



Needs Provided by the Substratum 



Fundamental needs of the organism which may be provided by 

 the substratum are purchase, attachment, shelter, and nourishment. 



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