Evidence for Flowering Hormones • 69 



PRELIMINARY EVIDENCE FOR THE EXISTENCE 

 OF FLOWERING HORMONES 



The clearest early investigations indicating the existence of 

 floral hormones were by Chailakhyan in Russia. One of his major 

 experiments (1936a) showed that if the upper portion of the SDP 

 Chrysanthemum indicum were defoliated, it would initiate flowers 

 if the lower (leafy) portion received short days, even if the de- 

 foliated part were kept on long days. With the conditions reversed 

 —if the upper defoliated part were kept on short days and the 

 lower leaves on long days— no flowering occurred. He interpreted 

 these results as indicating that under the proper photoperiodic 

 conditions the leaves could form a hormone that moved to the 

 apex and brought about flowering. From subsequent work he 

 concluded also that this hormone, which he named "florigen" 

 (flower-maker), could move either up or down the stem and could 

 be transferred from one plant to another through grafts (Chail- 

 akhyan, 1936b, 1936c, 1937). 



Several investigators at first obtained data suggesting that 

 florigen, like the auxins, could pass through a nonliving connec- 

 tion, but these proved to be illusory. Moshkov (1939), for example, 

 soon reported his inability to repeat his own earlier experiment in 

 which the Chrysanthemum floral stimulus had apparently passed, 

 through a thin film of water, and he concluded that such move- 

 ment could take place only through living tissue. A similar en- 

 couraging but false start was made by Hamner and Bonner (1938). 

 They showed that a photoperiodically induced Xanthium plant 

 grafted to a noninduced plant could bring about flowering in the 

 latter. They further observed that interposition of a piece of fine 

 lens paper between the stock and scion would still permit this 

 effect. This suggested that florigen could move from the induced 

 plant (the donor) to the noninduced plant (the receptor) without, 

 direct tissue contact. When this work was repeated by Withrow and 

 Withrow (1943), using various kinds of membranes including lens 

 paper between the cut surfaces of donor and receptor, it appeared 

 that the original interpretation was mistaken. Anatomical studies 

 showed that tissue union could occur by the growth of cells through 

 the lens paper; the transmission of florigen took place only when 



