230 



CHEMICAL AGENTS AND GROWTH 



ROOTING 



5 s 100% 



u. 3 60 



2 2. 40 



DAYLENGTH 



%CUTTINGS ROOTED 

 eo D \--D 1^ 



ROOTS/CUTTING 



TIME OF THE YEAR 



Fig. 3. Rooting behavior of cuttings taken from the same tree {Popuhis 

 canadensis Moench) at various dates between July and September. White 

 circles, average number of roots per cuttings (10 replicates). Squares, 

 percentage of cuttings rooted after 3 weeks. Black circles, daylength at 

 Ithaca, New York. 



This phenomenon, which was described by Moshkov and Kocher- 

 zhenko (1939), has been studied in some detail by Waxman (1957), 

 who showed that when cuttings are taken from Weigela or Cornus 

 florida plants growing under long days and rooted under various 

 photoperiodic treatments, the number of roots produced per cutting is 

 lower under short-day than under long-day treatments (Fig. 4). As a 



24 9 9*1 18 24 



HOURS OF LIGHT 



Fig. 4. Number of roots produced after 30 days on similar cuttings of 

 Weigela florida and Cornus florida subjected to various photoperiodic 

 treatments while rooting. Each figure is the average of 10 measurements. 

 9 4-1 means: 9-hr day plus one hr of light (25 ft c) in the middle of the 

 dark period (from Waxman's data, 1957). 



