216 Light 



it is necessary to analyze the reactions involved. Many practical 

 applications of such knowledge suggest themselves, such as the control 

 of insect pests. 



In attempting to interpret the locomotory reactions of organisms 

 and to apply them to natural situations, the following considerations 

 should be kept in mind: (1) Responses to individual stimuli are 

 usually limited by the influence of other conditions. Thus the posi- 

 tively phototactic moth does not attempt to fly to the sun. (2) The 

 observed distribution may be the result of movement without direct 

 orientation. The reduced locomotory activity of wood lice in situa- 

 tions where moisture is high results in a tendency for these animals 

 to congregate in moist places although they are not specifically 

 oriented to them, just as the traffic on a main highway becomes 

 denser in a bad stretch of road because the speed of the cars is 

 reduced. (3) For an oriented response the orienting factor must 

 provide a stimulus above the threshold of sensitivity, and either the 

 direction of the flux of the orienting force or the direction of a gradient 

 produced by it must be perceptible to the organism. The tempera- 

 ture gradient in the ocean, for example, may be shown to be below 

 the threshold for the response of zooplankton (Clarke, 1934). (4) 

 Factors whose gradients are below the threshold often exert an in- 

 direct effect by controlling the sign or the speed of the response to 

 another factor. (5) Responses may similarly be altered under 

 changed internal physiological conditions, as before or after feeding, 

 or breeding. Within the same species the males, females, and various 

 immature stages sometimes orient quite diversely. Thus, in gen- 

 eral, we see that the distribution of plankton in the water, of microbes 

 in the soil, or of insects in the vegetation is the result of a complex 

 interplay of the orienting factors of the environment acting directly 

 and indirectly on the changing physiological state of members of the 

 population. 



Periodicity 



Diurnal Periodicity. A good number of the fundamental rhythms 

 in nature are related to the light factor. Many animals and plants 

 exhibit a 24-hour cycle in their activities; this has long been known 

 as diurnal periodicity. However, the term diel periodicity may be 

 substituted if confusion arises from the fact that diurnal is also used 

 for daytime activity as opposed to nocturnal for nightime activity. 

 The most fundamental diurnal rhythm is that of photosynthesis itself, 

 which necessarily fluctuates because of the daily change of light. 

 Many plants exhibit other more specialized reactions to the alterna- 



