Hamner — 65 — Hormones 



long, dark period seems to inhibit flowering while with short-day plants, 

 a long, dark period seems to stimulate flowering. In either case, if the 

 dark period is interrupted with a short period of illumination, it is non- 

 effective ; in long-day plants, it does not inhibit flowering and with short- 

 day plants, it does not stimulate flowering. The dark period may also be 

 rendered ineffective by very low intensities of illumination. In both 

 plants rather high intensities of illumination are required for the stimu- 

 lation of flowering. While it is true that certain long-day plants seem to 

 be stimulated to flower by the use of low intensities of illumination at 

 night, it seems apparent that this illumination has merely served to shorten 

 the dark periods. 



With short-day plants the changes which occur during the dark pe- 

 riod and which lead to initiation of flowers should receive more careful 

 study. With Xanthium, for example, dark periods of Syi hours dura- 

 tion are ineffective in stimulating flowering whereas dark periods of 9^,4 

 hours are almost as effective as dark periods considerably longer. It 

 would appear, therefore, that changes occur in the leaves after a period of 

 8^ hours of darkness which result in the production of a stimulus for 

 flowering. Whatever these changes are, in some short-day plants they 

 seem to be of a rather permanent or irreversible nature (this is indicated 

 in the work with Xanthium pennsylvanicum and Perilla ocymoides) . Such 

 chemical investigations as have been made to date have not given a clear 

 indication as to just what these changes are. Apparently during the entire 

 dark period a progressive change is taking place, and this change passes 

 a certain critical point after a very definite length of time. Whatever this 

 change is, it is dependent upon a previous exposure to bright light and the 

 complete absence of light while the actual changes are taking place. 



Transmission of the Photoperiodic Stimulus : — The evidence to 

 date indicates that the stimulus is transmitted from the leaf to the stem 

 rather slowly. In Xanthium (16), one short day is sufficient to cause the 

 initiation of flowers. If plants receive two short days and are then de- 

 foliated, no flowers result. On the other hand, if a small portion of a 

 mature leaf remains attached to the plant, flowering will result. Thus, 

 the stimulus from only a small portion of one leaf is sufficient to cause 

 flowering but not enough stimulus is received from all of the leaves dur- 

 ing the first two days of treatment. Cailahjan (8) has also concluded 

 that the transfer of the stimulus is slow. One of his arguments for a 

 special flower hormone is based upon his conclusion that the transmission 

 of the stimulus is much slower than the translocation of organic nutrients 

 and growth hormones. Both Moskov (27) and Cailahjan (6, 8) con- 

 clude that the transfer of the flower-producing substances from the leaves 

 is through living cells. Cooling the petioles (3, 8) greatly reduces or 

 completely stops the transmission of the stimulus also indicating its pas- 

 sage through living cells. 



The leaves of some short-day plants apparently continue to supply a 

 stimulus even after the short-day treatment has been discontinued, while 

 with other plants such is not the case. Cailahjan (6) with Perilla found 

 that large leaves of a plant growing under short-day conditions would 

 induce flowering in a plant continuously exposed to long day when grafted 



