PHOTOPERIODIC EFFECTS IN WOODY PLANTS 229 



branch was completely defoliated while the other was kept intact. The 

 branch with all the leaves on stopped growing under short days. The 

 defoliated branch produced one more node, then stopped growing also, 

 suggesting that some of the inhibitory principle made in the intact 

 branch was reaching the defoliated one. 



NATURE OF GROWTH-REGULATORY PRINCIPLE 



The experiments which have just been presented suggest that some 

 growth-regulating principle is formed in the leaves and that its produc- 

 tion is controlled photoperiodically. The question is now to find out 

 what type of a principle this may be. Fundamentally, long days favor 

 continuous growth, short days bring growth to a stop. At first sight, 

 one could try to explain this pattern in two different ways: ( 1 ) under 

 short days, an inhibitor is produced, which causes growth to stop; or 

 (2) a growth-promoting substance is formed under long days, but not 

 under short days. In order to avoid making too quick a judgment on 

 this alternative, let us examine various bits of evidence pertaining to 

 the general problem of growth and dormancy. 



Evidence from Leaf Removal Experiments 



The experiments presented under "Receptor Organ" point toward 

 the inhibitor hypothesis. In the case of Weigela, it is clear that the 

 removal of leaves, under short days, promotes growth. The simplest 

 explanation would be to postulate that, under short days, leaves manu- 

 facture an inhibitory substance which causes vegetative growth to stop. 



Evidence from Rooting Experiments 



Horticulturists know from practical experience that cuttings root 

 more readily at certain times of the year than at others. If we make 

 poplar cuttings, for example, on July 10, when the day length is 15 

 hr at Ithaca, New York, we get 80% rooting after 3 weeks and an 

 average of 5.8 roots per cutting. If we take the same type of cuttings, 

 from the same tree, when the day length is 14, 13, and 12 hr, then we 

 observe that the number of roots per cutting decreases progressively to 

 less than one root per cutting (Fig. 3). In other words, as days become 

 short in the fall, the rooting capacity of the poplar branches decreases. 



