CHAPTER IX 

 SOME EXTERNAL FACTORS AND SIZE 



The influences upon size of a number of factors 

 have now been reviewed ; these factors may be con- 

 veniently grouped as internal factors. Those con- 

 sidered were: reproduction, growth, regeneration, 

 transplantation, inheritance, conjugation, and other 

 reorganizations. In a subsequent chapter some men- 

 tion will be made of the effects of various cell struc- 

 tures such as the nuclei. 



With a knowledge of what results and variabilities 

 are to be expected from factors which, so far as now 

 known, are internal, it is possible to analyze the effects 

 of some definite external influences. In very few in- 

 stances have the effects of a controlled influence upon 

 single individual unicellular organisms been studied. 

 Practically all the results are therefore of a statistical 

 nature, obtained by comparing many individuals, some 

 of which are under experimental conditions and some 

 under control conditions. 



1. Temperature 



Variations in temperature produce striking changes 

 with respect to body size. The changes were measured 

 adequately by Popoff ('08) and Rautmann ('09) upon 

 three species of infusoria. Their procedure was to 

 measure the sizes of many individuals taken from a 

 culture at room temperature, then to set the same cul- 

 ture at another temperature for 24 hours or more, 

 finally measuring many other individuals from the cul- 

 ture. Their mean results, in which each point was 

 based on ten to a hundred individuals, are represented 



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