PHOTOPERIODIC EFFECTS IN WOODY PLANTS 235 



photoperiods. These results show that, whatever substances photo- 

 periodic treatments cause to be produced in woody plants, they can be 

 stored in twigs and branches. Are these growth-promoting or growth- 

 inhibiting substances? If we recall Spiegel's work which gives evidence 

 for the existence of rooting inhibitors, then it might be difficult, at this 

 point, to decide whether the substance which regulates rooting in the 

 cited experiments is a growth-promoting or a growth-inhibiting one. 



Another experiment by Waxman (1957), however, gave evidence 

 that growth-promoting substances must be formed under long days and 

 stored in stems. A series of large plants of Cornus florida rubra were 

 subjected to 9-, 12-, 15- and 18-hr days during the year 1955. In 

 November 1955, the plants from all treatments were placed in the 

 same greenhouse under natural short days and at 5°C. After a while, 

 all the plants dropped their leaves. The low temperature broke the 

 dormancy of the buds in all cases and, in May 1956, when the tem- 

 perature reached a suitable level in the greenhouse, buds started to 

 open. No trace of dormancy remained in the plants which had been 

 grown previously under short days; as a matter of fact, these very 

 plants were the first ones to open their buds. Nevertheless, the average 

 shoot growth produced by short-day plants was far below the growth 

 produced by plants previously subjected to long days. As shown in 

 Fig. 8B,C, the average shoot length of plants subjected to 18-hr days in 

 1955 was nearly 6 times greater than that of plants previously grown 

 under 9-hr days. Thus, when differences due to varying degrees of 

 dormancy had been erased by the cold treatment, there remained a 

 difference in the amount of growth-promoting material stored in the 



plants. 



In summary, the various examples given in this section point toward 

 the photoperiodically controlled production of factors which some- 

 times inhibit growth, sometimes promote it. To clarify this dilemma, 

 it is necessary to find out what substances are actually produced in 

 the plant as a result of photoperiodic treatments. 



PRELIMINARY STUDY OF AUXINS AND INHIBITORS 

 IN PHOTOPERIODICALLY TREATED PLANTS 



Ethylacetate extracts of fresh shoot tips of Cornus florida rubra 

 grown under long and short days were made by Waxman (1957), 



