PHOTOPERIODISM 707 



length of day and night naturally suggest that photoperiodism is often 

 a factor of considerable importance in the varying behavior of a particular 

 species or variety at different seasons of the year and in different latitudes 

 as well as in the natural distribution of plants generally. Various angles 

 of the subject have received the attention of investigators. However, 

 approach to the problem by way of direct experimentation is made 

 difficult by the fact that in nature the photoperiodic response is subject to 

 modification by various other factors of the environment complex. 

 Garner and Allard have emphasized the significance of the photoperiodic 

 effect as explaining why some varieties of such warmth-loving summer 

 plants as soy beans consistently are relatively early in maturing while 

 others mature later. In soy beans changing day length apparently 

 exercises a selective action on the early and late forms whereas differences 

 in temperature seem to have much the same effect on both forms. It is 

 considered that the chances for successful reproduction in this type of 

 plant in a given region will depend largely on whether the day-length 

 requirements are such as will permit completion of the reproductive 

 process before frost in the fall. 



From extensive observations on species differing widely in habitat 

 Lubimenko and Szeglova (42) conclude that specific photoperiodic 

 adaptation has developed under natural conditions of illumination during 

 the period of vegetation and is dependent on the geographical latitude 

 of the habitat. In general, tropical and subtropical plants are adapted 

 to a short day and normally develop within a range of day length of 

 about 10 to 14 hr.; Arctic species growing above a latitude of about 60° 

 are adapted to a very long day; species of the temperate zone may be 

 adapted to a relatively wide range in day length extending from the short 

 day of spring and autumn to the moderately long day of summer. These 

 authors remark that from the standpoint of general biology hereditary 

 adaptation to length of day is extremely interesting, as a result of the 

 direct action of the medium on the plant organism. This adaptation 

 manifests itself as an entirely negative character, for it serves only to 

 diminish the plasticity and cannot give any advantage to the plant. 



Doroshenko (17) and Doroshenko and Rasumov (19) have made an 

 interesting study of the day-length requirements of a variety of forms of 

 wheat, barley, oats, rye, flax, beans (Phaseolus), peas (Pisum), and other 

 species in relation to the geographical latitude of their origin. It was 

 established that in general a definite relationship exists between response 

 of a given form to day length and its origin with reference to latitude. 

 Southern forms, which normally grow in days of shorter duration, for the 

 most part are less retarded in attaining the reproductive stage and suffer 

 less reduction in yield of seed when exposed experimentally to a shortening 

 of the day than the northern forms which are accustomed to longer 

 days. Wheat, oats, barley, and rye show these characteristics. In 

 Phaseolus southern forms act as short-day plants and northern forms as 



