[Chap. XXXI INITIATION OF FLOWERS 339 



also diffuses across a graft between two branches of the same or dif- 

 ferent plants, even if direct contact in the graft is prevented by the inser- 

 tion of lens paper. 



Approach grafting was employed in these experiments; that is, por- 

 tions of the sides of two stems were shaved to the cambium, and the two 

 shaved surfaces were bound together without removing either stem from 

 the plant. The leaves above the graft on one stem were then exposed to 

 a 9-hour daily light period. Those on the other stem were exposed to the 

 16-hour dav. Flowers developed on both branches. 



As in the cocklebur the initiation of flowers in soybean plants also 

 occurs only during relativelv short davs, but there is one striking differ- 

 ence. If the leaves on one part of a plant are exposed to short days and 

 those on the remainder of the plant to long days, flowers develop only 

 on the branches exposed to short days. 



This localized effect has also been reported as occurring in other 

 plants, such as cosmos, chrysanthemum, tobacco, and poinsettia. But it 

 has been shown that in soybeans if the leaves are removed from the 

 branches exposed to long days when the experiment is started, flowers 

 develop on these branches also. If the leaves are removed from the 

 branches exposed to short days, no flowers develop. From these facts 

 one might infer that there are at least two substances that influence the 

 initiation of flowers in soybeans: one made in leaves exposed to short 

 days which induces the initiation of flowers, and the other one made in 

 leaves exposed to long days that inhibits it. 



Environmental factors. By this time it is probably evident that en- 

 vironmental factors, through their effects upon processes and conditions 

 within the plant, may affect the initiation of flower primordia. Of these 

 factors light and temperature are often the most important and will be 

 discussed further. 



Light. When light is mentioned as a factor, one should know whether 

 intensity, quality, or daily duration is being considered. From investi- 

 gations made to date it appears that in the initiation of flower primordia, 

 the so-called red rays of light are more effective than the blue, and 

 green is least effective of all. 



Many plants have been known to grow and die in light of low inten- 

 sity without bearing flowers and fruits. Some of them may live in this 

 purely vegetative state for many years. Whether the failure of flower 

 formation in deep shade is the result of deficient photosynthesis, hor- 

 mone synthesis, or of some other internal condition is unknown. The in- 



