PART III 



GENERAL CONSIDERATION OF REACTIONS 



TO LIGHT 



CHAPTER XI 



ADAPTATION, FORMATION OF AGGREGATION IN REGIONS 



OF A GIVEN LIGHT INTENSITY AND DIFFERENT 



METHODS OF RESPONSE IN ATTAINING THIS 



REGION AND REMAINING IN IT 



I. Introduction Showing that Reactions in General 



are Adaptive 



Thus far we have dealt almost exclusively with the mech- 

 anism of orientation. In this and the following chapters 

 we shall deal with reactions to light from a much broader 

 point of view. We shall consider not only the various differ- 

 ent methods of response leading to a general classification, 

 but also the various factors which control the different 

 responses, leading to a discussion of the nature of stimula- 

 tion and the cause of reactions. 



Under natural environmenta conditions organisms are 

 usually found in places well suited for the continuance of 

 their life processes. Sometimes they crowd together and 

 form dense aggregations in such regions. This is especially 

 true in case of unicellular organisms. Euglena, Chlamydo- 

 monas, Volvox and other similar forms collect in the more 

 highly illuminated regions of their environment during dark, 

 cloudy days, early in the morning and late in the evening, 

 and in shaded places when the sunlight is very intense. A 

 certain amount of light is necessary for the well-being of 

 these organisms since they depend upon photosynthesis In 



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