6o4 



FACTORS AFFECTING GROWTH 



is not the same for all species but for most lies within a range of 12-14 hours. 

 In this range of day-lengths many species of both the long-day and short-day 

 types will flower, although maximum reproductiveness is not usually attained 

 in photoperiods of this duration. According to Garner (1933) a day-length 

 of 14 hours comes closest to a dividing line between plants of these two groups. 

 In order to determine in which of the three groups a species belongs it 

 usually is sufficient to expose some individuals of that species to a relatively 

 short day-length of about 10 hours, and others to a decidedly long one of 



Fig. 135. Eflfect of length of the photoperiod on flowering of buckwheat, an indeter- 

 minate species. Photograph from Arthur, et al. (1930). 



about 18 hours. If flowering fails to occur or is markedly retarded under the 

 short day-length but occurs under the long day-length the plant belongs to the 

 long-day group. If the contrary is true the plant is a short-day species. If 

 blossoming occurs under both day-lengths the plant can be assigned to the 

 indeterminate group. 



In many species a difference of one hour in the length of the daily photo- 

 period, or sometimes even less, determines whether the plant will blossom or 

 will be restricted to the vegetative phase of development. Garden balsam 



