Light 317 



though kept under a photoperiod unfavorable for flowering, will then 

 flower. The age of the plant may change its photoperiodic response. In 

 Kalonchoe, for example, young plants 3 months old flower only in short 

 days, but after 5 months they have become day-neutral (Harder and 

 von Witsch, 1940« ) . 



Even though the initiation of floral primordia has begun under a given 

 photoperiod, the later differentiation of the various structures can be 

 greatly altered by changing the length of the period. Thus in the stami- 

 nate inflorescence of maize, after the initiation of primordia, later growth 

 under longer photoperiods will cause the flowers to be infertile and 

 even to show progressive changes toward a vegetative condition. The 

 glumes develop ligules and the lemmas differentiate into blade, ligule, 

 and sheath until the spikelet becomes much like a vegetative shoot and 

 can be propagated as such (Galinat and Naylor, 1951). When trans- 

 ferred to a photoperiod unfavorable for flowering, buds which would 

 have produced flowers will sometimes grow into abnormal vegetative 

 shoots (phyllody), as reported by Behrens (1949) and others. Skok and 

 Scully (1955) present evidence that floral development is associated 

 with a dark-dependent mechanism and the elongation of the main axis 

 with quite a different and light-dependent one. 



The length of the photoperiod may affect the differentiation of the 

 sexes. This is well shown by the work of Schaffner ( 1931) on sex reversal 

 in staminate plants of hemp, Mercurialis annua (Fig. 13-5). He planted 

 seeds in the greenhouse every 2 weeks from July 15 until May 15 and 

 found that in the beginning, when days were long, the flowers were all 

 staminate but that the percentage of pistillate ones steadily increased 

 up to the plantings of Nov. 1 and 15 (which came to flower during the 

 shortest days) and that the percentage of these flowers gradually de- 

 creased after this until in the long days of spring the plants were all 

 staminate again. Long days obviously favor the production of staminate, 

 and short days, of pistillate, flowers. Similar results were obtained by 

 others, as by Jones ( 1947 ) in Ambrosia. 



In Cannabis sativa under a 16-hour day flowering takes place in from 

 4 to 6 weeks, the leaves become more complex (with up to nine leaflets), 

 and the plants are about half males and half females. Under 8-hour days, 

 however, development is more rapid, flowering is reached in 3 or 4 

 weeks, and the plants are about half hermaphrodites and half females 

 (Petit, 1952). 



Day-length also affects reproduction in the lower plants. In the alga 

 Vaucheria sessilis (League and Greulach, 1955) the production of sex 

 organs was earlier and more abundant under 18-hour days than under 

 8-hour ones. Addition of glucose and peptose to the culture medium 

 hastened their formation under short days. Sex organs were not produced 



