PHOTOPERIODfSM 683 



sive research in the past decade and it will not be practicable to include 

 in the present survey all of the work which has appeared in this field. 

 In general, there has been remarkable agreement in the results obtained 

 by various investigators with regard to the basal principles of the subject, 

 though difference of opinion has developed as to the correct interpreta- 

 tion and the significance of some of the experimental data. General 

 reviews of the subject of photoperiodism at different stages of its develop- 

 ment have been presented by Maximov (44), Kellerman (34), Redington 

 (60), and Schick (65). These authors have included fairly extensive 

 bibliographies, as have also Lubimenko and Szeglova (43) and Tincker 

 (71) in their research papers. Ramaley (55) has recently published a 

 bibliography of day-length and artificial illumination as affecting seed 

 plants. 



LONG-DAY AND SHORT-DAY PLANTS 



As pointed out by Garner and Allard, some plants are not particu- 

 larly sensitive to differences in length of day. Such plants flower more 

 or less readily in either a long or a short day, the effects of differences in 

 the light period being of a quantitative rather than a qualitative char- 

 acter. There is a larger group of plants, however, in which length of 

 day may produce definite formative action, particularly with respect to 

 sexual reproduction. This group may be logically separated into two 

 subdivisions, in one of which under suitable conditions flowering is readily 

 induced by exposure to a relatively short day, while in the other flowering 

 is favored by a relatively long day or even continuous illumination. 

 The former are designated as "short-day plants" and the latter as 

 "long-day plants." In setting up this classification, which now has come 

 into rather general use, the authors recognize that for both the long-day 

 and short-day types there is in each case a critical length of day for 

 flowering. Short-day plants flower readily under day lengths below the 

 critical, but with day lengths in excess of the critical there is extensive 

 stem elongation without flowering. In long-day plants, on the other 

 hand, there is elongation of the axis followed by flowering with day lengths 

 in excess of the critical, but there is no flowering with day lengths below 

 the critical, and most typical plants of this group tend to remain in the 

 leaf rosette stage. In the less sensitive group, which now has come to 

 be known as the indeterminate or neutral group, there is no definite 

 critical day length for flowering. 



The foregoing classification rests primarily on differences in response 

 to length of day with respect to reproductive processes, although typically 

 there is also definite contrast in the alternative types of vegetative 

 activity in the long-day and short-day types. Separation into the two 

 groups is not based on any particular day length as the dividing line (21a). 

 In most cases short-day plants flower more or less readily through a wide 



