284 



PRINCIPLES OF ANIMAL BIOLOGY 



Another influence that may be indirectly important for animals is 

 the effect of daily duration of light upon the reproductive processes of 

 plants. Many plants mil come to flower only if they are exposed to a 

 certain number of hours of light each day. . A certain range of duration 

 is always permissible. If the daylight period is longer or shorter than 

 this required range of hours, the plant may grow vegetativel}'^ even more 

 vigorously than usual but will not bloom. Unless a plant has some 

 satisfactory asexual method of propagation, it cannot maintain itself 

 in a region not affording the right duration of daylight. Probably no 

 plant whose required amount of daylight is yet known is the sole food 



l"'iu. 258. — Shore vegetation of lake, which is too deep elsewhere for rooted plants. {Photo- 

 graph by F. C. Gates.) 



of any species of animal, but the possibility exists that the range of some 

 animal is thus limited by the length of day. 



Of structural changes induced in animals b.y light, the most significant 

 ecologically is probably the production of wings in aphids. In some 

 sti-ains of aphids, light has an e\c\\ more important influence than has 

 temperature, the effect of which is described above. In one of these 

 strains, alternating light and darkness caused nearly every individual to 

 be winged, provided the dark period was at least 12 hours long. Shorter 

 periods of darkness, including continuous light, made most of the aphids 

 of this strain wingless. Since temperate regions in summcn- do not have 

 12-hour nights, wing production must be considerably cui'tailed in such 

 strains. Other strains respond differently to light, some of them directly 

 reversing the behavior just described. The importance of wings in 

 the migration of these insects from one host plant to another has already 

 been mentioned. 



