PHOTOPERIODIC EFFECTS IN WOODY PLANTS 231 



matter of fact, in the case of Cormis florida, a light break of 1 hr of 

 low-intensity, incandescent light, given in the middle of the dark 

 period, increased the number of roots per cutting when compared with 

 a similar short-day treatment with uninterrupted night. In other words, 

 photoperiodic treatments influence the number of roots produced on 

 leafy cuttings. 



It is well known (see, for example, Went and Thimann, 1937) that 

 the number of root primordia produced on cuttings is linked to the 

 level of auxins. It has also been demonstrated that factors other than 

 auxin and sometimes grouped under the name "rhizocaline" (Went, 

 1938) modify the extent of rooting obtained. The fact that photo- 

 periodic treatments influence the formation of adventitious roots sug- 

 gests that these treatments may change the level of growth substances 

 in the cuttings, whether auxins or rhizocahne. Spiegel (1954) demon- 

 strated the existence of natural rooting inhibitors, however, which 

 opens a different explanation for the results presented in Figs. 3 and 

 4. Perhaps in these cases also, the results of the photoperiodic treat- 

 ments may be explained in terms of inhibitors formed under short days. 



Effect of Gibberellic Acid 



Gibberellic acid has been reported to break dormancy of the buds of 

 Fagiis sylvatica (Lona and Borghi, 1957) and of various other species 

 in early spring (Marth et al, 1956). Also, it has been shown to re- 

 place the cold treatment necessary to break the dormancy of embryos 

 of Mains amoldiana (Barton, 1956). Similar results have been ob- 

 tained by Nitsch (1957a) in the case of peaches. If peach embryos of 

 the variety Lowell are dissected out of the seed and germinated at room 

 temperature, they give rise to seedlings which remain dwarf, the 

 terminal buds being dormant (Fig. 5, right). If the terminal buds of 

 these seedlings are treated with about 5 fig of gibberellic acid in lano- 

 lin, they start to elongate, and the seedlings become undistinguishable 

 from those derived from cold-treated seeds (Fig. 5, left). 



Gibberellic acid was also applied to sumac seedlings in the following 

 manner. Plants growing vigorously under long days were divided into 

 four equal groups. At time zero, two of these groups, one treated with 

 10 iLig of gibberellic acid in lanolin, the other untreated, were placed 

 under short days of 10 hr of light; the two other groups, one of them 



