PHOTOPERIODISM 679 



at the expense of root growth in sugar beets, and by its use he obtained 

 earUer flowering in tomatoes. Stimulation of growth was observed in 

 several other plants. 



It will be observed that, with the exception of Bonnier, who dealt 

 only with anatomical effects, all of the above mentioned investigators 

 found that accelerated growth and more rapid development can be 

 obtained in several species by increasing the daily light period even 

 up to continuous illumination. In several instances the conclusion 

 was reached that plants generally do not require a daily rest period. 

 Although both conclusions may be regarded as sound so far as concerns 

 the particular species in question and the conditions under which they 

 were studied, as will later appear, it is now known that neither conclu- 

 sion can be made to apply to all plants. In the work of Tournois (75) 

 with Cannabis sativa and Humulus japonicus, published in 1911, there is 

 to be found the first definite suggestion that attainment of the flowering 

 stage may be hastened by a relatively short rather than a long daily 

 light period. Tournois demonstrated that a precocious type of flower- 

 ing which occurs in very early spring plantings of these species can be 

 reproduced by allowing the plants to receive sunlight for only 6 hr. daily. 

 Apparently this investigator did not extend his researches in this direction. 



In a comprehensive attempt to ascertain the relation of external 

 factors to reproductive processes in Sempervivum Klebs (36) made obser- 

 vations on the effects of varying the daily duration, the composition and 

 the intensity of light. It is considered that attainment of the "ripe-to- 

 flower" stage is conditioned primarily on an intensive carbon assimila- 

 tion, accelerated transpiration, and a limited absorption of nutrient 

 salts. Temperature is of special importance as it affects disassimilation. 

 Light is essential for the laying down of flower primordia and it functions 

 in the formation of the inflorescence somewhat as in the first phase of the 

 reproductive process. It was found that 10 to 12 hr. of light per day 

 were not suJB&cient to induce flowering and continuous illumination was 

 more effective than 18 hr. of light. Though recognizing that there can 

 be no simple relation between intensity and duration with respect to 

 light requirements, Klebs finds a complementary relationship within 

 Umits and regards the quantity of the light energy as a decisive factor 

 in the initiation and completion of the reproductive process. 



DISCOVERY OF THE LENGTH-OF-DAY EFFECT 



In a paper published in 1920 Garner and AUard (22) presented exten- 

 sive data demonstrating the importance of relative length of day and 

 night as a factor in the sharply contrasted growth and development of 

 different species and varieties which are seen at different seasons of the 

 year and in different latitudes. It was shown that not only does the 

 seasonal and latitudinal change in length of day quite commonly affect 



