Vernalization and Photoperiodism — 58 — A Symposium 



possibly other environmental factors (Sinnott, 1938). And if we con- 

 sider Gregory's idea that the problem of photoperiodism in plants is not 

 really concerned with factors that determine flower formation but those 

 that lead to failure of flowering, then other later phases of development of 

 floral organs, as influenced by the photoperiod, must likewise be taken into 

 detailed consideration. 



One should recognize the following stages in flower inception, develop- 

 ment and function (Murneek, 1937, 1939) : 



o) Terminal meristems or determined loci where the floral hormone is received 

 and condition of "ripeness to flower" established physiologically. Usually there are 

 far more such meristematic points than there is available hormone or other indispens- 

 able substances. Some of them, perforce, cannot initiate reproductive tissues. This 

 is the first elimination of certain meristems as flower producers. 



b) With a large number of meristems made "ripe to flower" and having formed 

 floral primordia, it is very probable that many of tliem are eliminated early because of 

 lack of supply of building material of one sort or another. Organic nitrogen com- 

 pounds most likely play an important role here. Possibly certain catalytic substances 

 or other hormones, necessary for the early development of floral organs, should be 

 considered also as factors. 



c) Further elimination of a large proportion of the developing flowers undoubtedly 

 occurs because of competition for available food supply during their growth. Far 

 more floral primordia are usually formed than can possibly develop into functional 

 flowers. The greater the limitation in reserve food supply or synthesis of organic sub- 

 stances (lack of light or extreme temperature) the fewer flowers will be brought to 

 completion (Murneek, 1926). 



d) Not all flowers that reach anthesis are able to function normally, i.e. to form 

 gametes and participate successfully in fertilization. There may be various reasons 

 for this condition. Some flowers, though appearing normal on casual observation, are 

 abnormal in many essential morphological structures, such as incomplete development 

 of stamens or ovaries or the micro- or macro-gametophytes. It is a common knowl- 

 edge that plants often produce an enormous number of flowers of which only a small 

 proportion participate in fertilization (Loehwing, 1938; Nielsen, 1942). 



In the delicate balance between vegetation and reproduction the growth 

 rate and photoperiodic or other inhibition of the main axis and lateral 

 branches should be taken into account (Murneek, 1936, 1939, 1940). 

 Stem elongation is promptly retarded in the Biloxi soybean, Rudbeckia 

 bicolor, R. hirta and many other plants by exposure to a short photoperiod 

 (Murneek, 1936, 1940; Greulach, 1942). In the soybean, var. Biloxi, 

 this treatment probably effects independently induction of reproduction and 

 inhibition of vegetative growth, although it is possible that a special in- 

 hibiting substance may be produced by the floral primordia. The writer 

 found that growth of the main axis of R. bicolor, a long-day plant, cannot 

 be induced but only maintained on a long photoperiod. Growth in height 

 stopped promptly when the plants were moved from long- to short-day 

 exposures. It was found possible to combine photoperiodic induction and 

 photoperiodic inhibition in various ways in R. bicolor plants by exposing 

 them to certain number of long and short light periods. Size and form of 

 the vegetative parts and amount and character of flower development 

 thereby was changed.* Stem elongation has been found to be inhibited by 



•Another example is reported by J. C. Sen Gupta and S. K. Payne (Nature 100: 510, 1947) 

 who show that leaf heteroniorphism in Sesamum orientate evidently is related to the photoperiod. 

 With increasing length of day, from 10 to 16 hours, there was a reduction in the number of linear 

 lanceolate leaves produced and an increase in number of ovate entire and ovate serrate leaves. More- 

 over, plants that received a 14-hour or a longer photoperiod produced larger and more variable leaf 

 forms than plants given a 10-hour photoperiod. 



