CHAPTER II 

 THE EFFECT OF LIGHT ON METABOLISM 



It is well known and iniiversally recognized that the general 

 behaviour of many organisms is regulated by light ; the contrast 

 between the activities of nature by day and its stillness by night needs 

 no stress. This is a widespread characteristic of vegetable life which 

 exists so much more closely to the sun and the earth than do animals, 

 but even among the latter dramatic changes are frequently evident, 

 particularly in the lower forms. Thus among Protozoa, some Rhizopods 

 change their form, contracting under the influence of light (Engelmann, 

 1882 ; Verworn, 1889) (Figs. 2 and 3), many species are activated by 

 light (such as flat-worms, Loeb, 1893-94), while other creatures become 

 inactive under its influence (maggots, Herms, 1911 ; and many insects, 

 such as cockroaches, Gumi, 1940). Among the higher forms of life, in 

 addition to a number of basic metabolic functions, the reproductive 



Fig. 2. Fig. 3. 



Figs. 2 and 3. — Pelomyxa pahistris at rest (Fig. 2), and contracted 

 under the influence of light (Fig. 3). 



cycle and secondary features such as colour changes and behavioural 

 habits are similarly regulated by light although in many cases other 

 factors such as temperature, humidity and nutrition exert sometimes 

 contributory, sometimes more potent effects. In this way the alterna- 

 tion of day and night has imposed a rhythmic diurnal cycle upon a 

 number of the activities of living organisms (photoperiodism) ; and 

 it is to be remembered that in many of the phenomena thus involved 

 darkness seems to be as important a stimulus as light. Indeed, in 

 many cases the rhythm has become so fundamental that if the organism 

 is placed in experimental circumstances wherein the natural alternation 

 of light and darkness is changed to become out-of-phase, or if it is 

 exposed to continuous light or darkness, many of these cyclic changes 

 continue as if the normal 24-hour rhythm still persisted ; the rhythm 

 originally imposed by external circumstances has eventually become 

 autochthonous. 



