THE SYNTHESIS OF FLOWERING HORMONE 153 



1. The Leaves Respond to the Proper Combinations of Light and 

 Darkness 



This is quite easy to show by covering the leaf with a black bag of 

 some sort and leaving the rest of the plant under continuous light. 

 Such an experiment will result in flowering of a short-day plant. 

 Covering the rest of the plant and leaving the leaf exposed will result 

 in the flowering of a long-day plant. It is quite obvious, then, that 

 some response in the leaf is being transmitted to the tip where flowers 

 are formed. I can think of three possible explanations: First, a 

 nervous or electrical impulse of some sort may bring about the 

 transmission. This seems quite unlikely, since such impulses are 

 virtually unknown in plants, and anyway such an impulse is incon- 

 sistent with the following two evidences. Second, the nutritional 

 conditions may be upset in such a way that flowering is induced. 

 Perhaps it is a matter of how much sugar gets transmitted from the 

 leaf to the bud. This also seems unlikely in view of the next two 

 evidences, and indeed so far no treatments with sugar, amino acids, 

 or other nutrient compounds has resulted in flowering. Third, a 

 hormone must be involved. 



A special committee reported in 1954 with a definition of a hormone 

 which seems quite suitable: Plant (growth) hormones are organic 

 substances formed in one tissue or organ and then translocated to 

 another site where special (growth) control is produced. They must 

 be active in very small amounts, and nutrient elements and energy 

 sources are excluded. If we can eliminate from our thinking a nervous 

 impulse and the upset in nutritional balances, a hormone must be 

 the only alternative left to account for the stimulus sent from the 

 leaf to the bud. 



2. The Flowering Stimulus Will Pass a Graft Union From One Plant 

 to Another 



Induced cocklebur plants, for example, are approach grafted to 

 vegetative cockleburs as described in Chapter 5. After a week or two 

 the plant which has never received an inductive dark period begins 

 to flower anyway. It is then possible to separate the two plants and 

 graft another vegetative plant onto the one which had been induced 

 by grafting. The third one will then flower. It is reported that this 

 has been carried on through 8 or 10 graft "generations". The long 



