452 EFFECT OF TEMPERATURE UPON FACET NUMBER 



Examples in Which a Specific Structure Depends upon a Definite 

 Environmental Stimulus. 



In three well marked cases, specific temperatures determine the 

 character of the organism. 



Baur cites a case of Primula sinensis which at ordinary temperatures produces 

 red flowers. If a plant is subjected to a temperature of 30-35°C. a few weeks 

 before blooming, the flowers will be white. If the plant is returned to 15-20°C., 

 the buds opening immediately will still be white but those developing later will 

 be red. As Baur points out, white cannot be said to be inherited, red cannot 

 be said to be inherited, but rather the capacity to produce red flowers at 15° 

 and white ones at 30° is the thing inherited. 



Hoge found a race of Drosophila in which one or more legs showed reduplica- 

 tion. Under ordinary cultural conditions only about 10 per cent of the individu- 

 als from a pure reduplicated mating showed the condition. It was later dis- 

 covered that by subjecting the eggs to 9-10°C. the percentage of offspring showing 

 the character could be raised to practically 100. 



Roberts found that a temperature difference of 4-5° C. had a direct effect 

 on the length of the wing in the mutant "vestigial" of Drosophila, much greater 

 than twenty-nine generations of selection had produced. 



Bar eye is a sex-linked factor that reduces the number of facets in 

 the adult Drosophila from 1,000 to 70. Its Mendelian behavior is 

 clear cut and regular. As shown in this paper, however, the number 

 of facets produced, when this factor is present, is dependent upon 

 the specific temperature at which a definite stage in larval develop- 

 ment is passed. 



In the first three examples only two temperatures are involved. 

 Obviously no quantitative measure can be appHed although the re- 

 sults are very definite. 



In bar eye, observations have been made at close intervals over a 

 range of temperature from 15-31°C. The temperature relations 

 have been shown to approximate closely those of many physiological 

 reactions. 



In certain cases other environmental factors may be said to deter- 

 mine form. Morgan has shown that a definite amount of moisture 

 is necessary for the development of the abnormal abdomen in Droso- 

 phila. Metz has several mutants that depend on specific cultural 

 conditions for their recognition. 



