PHOTOPERIODIC EFFECTS IN WOODY PLANTS 



233 



treated group, on the contrary, continued to grow as if the dormancy- 

 inducing effect of the short days had been completely overcome. How- 

 ever, the growth rate of the latter group started to decline 2 weeks after 

 the treatment (Fig. 6). 



The experiments reported here show that the addition of a growth- 

 promoting substance, gibberelHc acid, overcomes, at least temporarily, 

 the growth-inhibiting effect of short days. 



Photoperiodic After Effects 



It has been shown above that photoperiodic treatments given to 

 cuttings while they are being rooted can influence the number of roots 

 produced. A comparable effect can also be obtained in a slightly dif- 

 ferent way. Let us divide into 4 groups a lot of poplars growing in pots 

 under 24 hr of light. At various dates, we will move one of these lots 

 to a short-day treatment ( 10 hr of light), so that, on a given day, lot A 

 will have been exposed to short days for 13 weeks, lot B for 6 weeks, 

 and lot C for 4 weeks, lot D being still under continuous light. On that 



4 6 

 WEEKS 



10-HR. 



Fig. 7. Average number of roots (10 replicates) produced after 3 weeks 

 by Popidus canadensis cuttings rooted under the same photoperiod but 

 which had been taken from stock plants previously subjected to 0, 4, 8, 

 and 13 weeks of short, 10-hr days. 



