Mar. I, 1920 



Effect of Length of Day on Plant Growth 



571 



DISCUSSION OF RESULTS 



The results of the experiments which have been described show clearly 

 that both the rate and extent of the growth attained by the plants under 

 study and the time required for reaching and completing the flowering 

 and fruiting stages are profoundly affected by the length of the daily 

 exposure to sunlight. The behavior of some of the plants under the differ- 

 ent exposures would seem to indicate that the action on the vegetative 

 phase of development is more or less independent of that on reproduction, 

 but only tentative conclusions can be drawn on these points at the pres- 

 ent time. The effects of the different light exposures on these two phases 

 of plant development can best be discussed separately. 



IvENGTH OF DAILY LIGHT EXPOSURE IN RELATION TO VEGETATIVE 



DEVELOPMENT 



Under the conditions of the tests it was not possible to secure quantita- 

 tive data on the various details of vegetative growth and development, 

 but measurements of height and the photographic records will clearly 

 indicate some of the differences resulting from the various light exposures. 

 In general, the extent of growth was proportional to the length of the daily 

 exposure to light; and this held true when the plants received two daily 

 exposures to light, with an intervening period of darkening, as well as 

 when there was only a single daily exposure to the light. Under the 

 shorter exposures the plants were shorter and less stocky, and there were 

 someindications of etiolation or chlorosis. HistologicaleJtaminationof the 

 test plants was not undertaken, but in most species no very striking dif- 

 ferences in gross anatomy resulted from the different exposures. Broadly 

 speaking,- the extent rather than the character of growth and vegetative 

 development was chiefly affected. Table II is intended to bring out the 

 relationship between size of plant and length of the exposure to the light 

 for soybeans and the aster. This relationship is strikingly brought out 

 for the Biloxi soj^bean in figure 3, which shows the decreasing heights of 

 progressively later plantings. How length of exposure affects the Man- 

 darin is shown in the foreground of Plate 78, B. 



Table II. — Effect of length of daily exposure to light on the height of soybeans and aster 



oThe relatively greater heights in proportion to the number of hours in the total daily exposures under 

 this treatment are due to the fact that in this case the length of the growing period wa^ not materially shor t- 

 ened by forced earliuess in blossoming; they are not to be ascribed to an increased rate of growth. 



