7. LIGHT 



Visible light represents a small fraction oi 

 the whole gamut of radiation. As might be 

 expected, this fraction shares many of the 

 properties of other wave lengths, especially 

 of those just longer or just shorter than the 

 visible band. Although it is a restricted pari 

 of radiation, light is a compUcated environ- 

 mental factor. Organisms are affected di- 

 rectly by energy values, intensity, and 

 wavelength, including associated aspects of 

 color. Certain of these factors vary in a reg- 

 ular way in diflFerent parts of the spec- 

 trum; heat energy is relatively greater at 

 the red end, and photo-chemical activity is 

 greater at the violet end. To the normal 

 human eye, brightness is greatest in the 

 orange for high and in the green for low 

 intensities. As a result, the effect produced 

 varies gieatly for different organisms and 

 for different processes. 



Photonegative animals are frequently not 

 disturbed by exposure to red light and will 

 collect at the red end of an experimentally 

 imposed spectrum. The opposite tendency 

 is exhibited by those light-positive animals 

 that react primarily to photochemically ac- 

 tive wavelengths of Hght. These collect near 

 the blue end of the spectrum. Animals that 

 respond primarily to brightness frequently 

 collect in the yellow-green. Photosynthesis 

 of carbohydrates by green plants, the most 

 important chemical synthesis of the world 

 through long geological ages, proceeds most 

 rapidly under red hght, although with heat- 

 energy values equalized, it goes forward at 

 the same rate in violet light. 



Light may be important in development 

 and toleration as well as in response phys- 

 iology. The fundamental polarity of the 

 egg of the marine alga Fuciis is environ- 

 mentally determined, and light is one of a 

 number of potent determining agencies. 

 Other things being equal, the lighted end of 

 the algal egg becomes the growing apical 

 point of the plant, while the shaded end 

 develops into a holdfast (Hurd, 1920; 

 Whitaker, 1931). 



It has long been known that hght retards 

 the rate of elongation of green plants. 

 Bachmetjew (1907) reviewed critically the 

 effect of light on the rate of development of 

 insects and decided that the absence of 

 light retards the development of insect lar- 

 vae that normally hve in the light, while 



the presence of light has a retarding effect 

 for those that normally hve in darkness. 

 This generahzation can be extended to 

 population physiology since marine sur- 

 face plankton are hkely to contain Hght- 

 resistant forms, while those from the deeper 

 waters may all be hght-sensitive. Tested 

 animals include lobster larvae, a northern 

 coral, amphipods, ascidians and shrimps 

 (Huntsman, 1924). With temperature con- 

 trolled, animals as diverse as the marine 

 mussel Mytilus and certain salmon larvae 

 in their early stages grow larger in darkness 

 than in Ught. On the other hand, hghted 

 salmon larvae show earlier differentiation 

 and shghtly better survival (Smith, 1916). 



PHOTOPERIODICITY IN ANIMALS 



The hterature concerning the effect of 

 length of day upon animals is large, com- 

 plex, and rapidly growing. The relation be- 

 tween increased length of illumination and 

 egg laying of birds or their seasonal migra- 

 tion has a long history (Chapter 2), but 

 has flowered, so to speak, since the work of 

 Garner and AUard (1920) on photoperio- 

 dicity in plants. We now know of short-day, 

 long-day and indifferent day-lengths proc- 

 esses among animals as well as among 

 plants. These include: 



Gonadal Activation 



Some sheep, deer, and goats correspond 

 to short-day plants and may be brought to 

 sexual activity by a decrease in the length 

 of exposure to dayhght. Spring-breeding 

 birds and mammals— ferrets, starUng, juncos, 

 turkeys— and many others become sexually 

 active with lengthening days; they corre- 

 spond to long-day plants. Brook trout show 

 a similar reaction. Stickleback fish, ground 

 squirrels, guinea pigs, and guinea fowl are 

 little or not at all affected by the length 

 of exposure to daylight, and house spar- 

 rows breed both with increasing and de- 

 creasing length of day. (Bissonnette, 1936, 

 1936a). 



Migration 



The evidence collected by Rowan (1931) 

 and others, including Wolfson (1945), 

 though incomplete, supports the suggestion 

 that certain birds migrate toward the north 



121 



