338 TEXTBOOK OF BOTANY 



ditions by which it can make these two kinds of foods. Flowers and 

 fruits will grow no better than leaves, stems, or roots without these 

 foods. Nor will they grow well when conditions are favorable to the 

 synthesis of onlv one of them. Many facts of both practical and scientific 

 interest were discovered. Some of them have been included in previous 

 chapters. 



Most of these investigators think that these foods are not a direct 

 cause of the initiation of flower primordia. When cultivated plants in 

 gardens, orchards, and fields are exposed to conditions favorable to both 

 photosynthesis and protein synthesis, they usually grow well and bear 

 an abundance of flowers and fruits. It is possible, however, that other 

 processes essential to the initiation of flower primordia also occur best 

 under these same conditions, and that the foods are important because 

 they are a necessary source of material and energy in the growing cells 

 of the primordia and of the expanding flowers and fruits. 



Following the definite proof of hormones in plants about 1928, several 

 botanists became interested in testing Sachs' idea that the initiation of 

 flowers is dependent upon specific hormones made in the leaves. From 

 the results of experiments performed within the last five years it appears 

 reasonably certain that hormone-like substances are made in the leaves 

 which promote the initiation of flower primordia, and that probably 

 others inhibit it. The evidence back of this conclusion and the differences 

 discovered in the plants investigated cannot be presented in this short 

 chapter, but a few facts selected from two of the reports are too inter- 

 esting to be omitted. 



No initiation of flower primordia occurs in plants of cocklebur (Xan- 

 thium pennsijlvanicum ) growing in an ordinary greenhouse if the plants 

 are exposed to continuous hght for 16 hours each day. Under these con- 

 ditions they remain vegetative indefinitely. If the daily light period is 

 shortened to 9 hours, microscopic flower primordia are evident within 5 

 days, and the plants are in bloom in about 2 weeks. If only one branch 

 bearing mature leaves is exposed to a daily light period of 9 hours, and 

 another branch of the same plant is exposed to the 16-hour daily light 

 period, flowers develop on both branches. Finally, if only one mature 

 leaf on the plant in the 16-hour day is exposed to a short day of 9 hours, 

 flowers develop on all branches of the plant. 



Whatever it is that passes from the leaves exposed to the short days 

 and promotes the initiation of flower primordia on the part of the plant 

 exposed to the 16-hour day, it passes both up and down the stems. It 



