Vernalization and Photoperiodism — 56 — A Symposium 



pineapple by means of ethylene and acetylene and synthetic growth sub- 

 stances, naphthaleneacetic and 2,4-dichlorophenoxyacetic acids (Over- 

 BEEK, 1945). Then, too, these and related chemicals have been widely used 

 to stimulate fruit setting and size in tomatoes, beans and a few other plants. 

 There is some evidence that the growth substances increase the auxin con- 

 tent of plants and that ethylene and acetylene, by acting on the mature 

 leaves, may release a flower forming substance (Traub et al., 1939). 



Tests have indicated that the flowering hormone is not identical with 

 the following substances: Vitamins Bi, B2, Bg, ascorbic acid, nicotinic 

 acid, pantothenic acid, or theelol, inositol or indoleacetic acid (Hamner 

 and Bonner, 1938). 



Judging from the results frequently obtained, the production of florigen 

 has certain quantitative aspects. With increasing amount of foliage, Hght 

 intensity, number of photoperiods and change in temperature more of it 

 or less may be formed, as has been pointed out already. When there is a 

 lack of sufficient quantity of the hormone, flower production may be sup- 

 pressed or their development may be incomplete (Murneek, 1939). 

 Morphological and histological variations of flowers because of insufficient 

 photoperiodic dosages or correlated disturbances, leading in extreme cases 

 to their anomalous development, the formation of so-called "vegetative" 

 flowers, has been described by Biddulph (1935), Murneek (1936-1940), 

 Greulach (1942) and Harder et al. (1942). 



Harder undertook an extensive study of the effects of a reduced flower 

 hormone supply on the formation of inflorescences of Kalanchoe bloss- 

 feldiana by the following experimental means : a) Limited number of 

 effective photoperiods (short days) ; b) the use of intermediate photo- 

 periods ; c) lowering of temperature at night; d) interference with trans- 

 port of hormone by cutting of midribs of donor leaves and e) variations 

 in distance of transport of hormone. The results showed that all the 

 above procedures, properly executed, reduced the formation of flowers 

 and changed to various degrees inflorescences into vegetative shoots. He 

 makes the interesting observation that the vegetative organs near inflo- 

 rescences, because of the latter's reduction in size or complete absence, were 

 changed, and expresses the belief that these vegetative organs are even 

 better indicators than the flowers themselves, of the amount of hormone 

 present.* 



The transport of the hormone from the leaves to the apical meristems 

 and elsewhere seems to be of the nature of diffusion, involving living cells. 

 Hence the rate of transfer both through the leaf petiole and the stem is 

 comparatively slow. When the main vein of a leaf is cut, there does not 

 appear to be an interference in hormone movement from the leaf to the 

 stem. It does not seem to be translocated either through the xylem tissue 



* Gertrud Meyer (Biol. Zentrbl. 66: 1-20, 1947) has demonstrated that in Sedum kamtscha- 

 ticutn an exposure of as small a leaflet as J^ cm2 to a long photoperiod will affect to some extent 

 the development of other parts of the same plant kept in a short photoperiod. Hence in Sedum 

 (long-day plant) a relatively small leaf surface area has the same or similar effect on flower hor- 

 mone production and/or transfer as a much larger leaf area in Kalanchoe (short-day plant). There 

 does not seem to be enough evidence for the special catalytic substance ("Metaplasin") assumed to 

 be produced by the photoperiod. and to function in modifying the vegetative parts of the plant, which 

 is postulated by Harder et al,, and agreed with by Meyer (1947). The morphogenetic effects on 

 vegetative organs are most likely the results of photoperiodically induced sexual reproduction (Mur- 

 neek, 1936, 1939, 1940) (cf. p. 90). 



