PHOTOPERIODISM 697 



though it was somewhat less injured when sunUght entered into the 

 ilhimination. It is thought that the injury which involves defohation is 

 produced by a breaking down in the process of photosynthesis rather 

 than excess accumulation of photosynthetic products. Pelargonium, 

 Coleus, and Nicotiana Tahacum also showed definite injury in the con- 

 tinuous light. Furthermore, some species which showed no definite 

 injury were unable to benefit in growth from the continuous light as 

 compared with an 18- or 19-hr. day-length. 



Reference was made in the introductory paragraphs to the work of 

 Bonnier (9) who obtained important modifications in leaf and stem struc- 

 ture in plants exposed to continuous artificial light. However, in a 

 comparison of the anatomical effects of continuous light and various day- 

 lengths on several species grown under controlled conditions Pfeiffer (53) 

 was unable to confirm Bonnier's results. Several other investigators 

 before and after PfeifTer's work have reported a similar experience, so 

 that apparently the effects observed by Bonnier were due, at least in part, 

 to other factors. 



INTERRELATIONSHIP OF OTHER ENVIRONMENTAL FACTORS 



The manner and extent to which normal response to the day-length 

 factor may be influenced by other factors of the environmental complex 

 recently have been the subject of considerable study. As perhaps would 

 be expected, it has been found that the relations are complex and under 

 certain conditions the day-length effect may be variously modified or 

 interfered with through the intervention of other supporting or opposing 

 factors. Temperature perhaps is the most important factor involved. 

 Suboptimal temperature, of course, will tend to slow down the rate of 

 growth. Temperature also may have important effects on the rate and 

 the character of development. Again, the natural seasonal change in 

 length of day commonly is followed by a corresponding change in mean 

 temperature. 



In a study of Xanthium pennsylvanicum, a short-day type, exposed to 

 different combinations of temperature and day length Gilbert (30) found 

 temperature to be a determining factor in influencing the time of forma- 

 tion of flower primordia, but this effect was closely associated with a 

 response to length of day. The interrelation of day length and tempera- 

 ture w'as found to be as follows: A high temperature, short-day exposure 

 gave quick flowering; high temperature and long day resulted in definite 

 delay in flowering; low temperature and short day decidedly prolonged 

 vegetative activity; low temperature and long day also prolonged the 

 vegetative period. In investigating the physiological factors which may 

 determine the duration of the vegetative period in plants Maximov (44) 

 finds that in addition to winter annuals some of the late-maturing forms 

 of summer types may be much hastened in completion of the life cycle 



