Vernalization and Photoperiodism —94— A Symposium 



been found that two or more long nights are necessary for the subsequent 

 development of pistillate flowers. 



New Evidence: — Changes in anatomical structure have appeared 

 consistently with or prior to blossom induction when usual conditions of 

 environment are used. They have also been well in advance of blossom 

 primordia. There remains the question of the anatomical history when 

 blossoms are initiated under such unusual environmental treatments as 

 reversed night temperatures (22), "temperature girdles," banding, and 

 grafting (29). 



When plants which come to flower early under relatively warm night 

 temperatures as, Cannabis saliva (hemp). Datura stramonium (Jimpson 

 weed). Euphorbia pulcherrima (poinsettia). Glycine max (soybean, var. 

 Biloxi), Nicotiana tabacum (tobacco var. Maryland Mammoth), Panicum 

 milaceum (millet), Phaseolus vulgaris (kidney bean), Solanum capicastri 

 (ornamental pepper), Xanthinin echinatum (cocklebur) and Zea mays 

 (corn) are transferred to an environment with a warm (75°F) dark period 

 of 13 hours and cool light period (55°F) of 11 hours, they soon become 

 nearly etiolated or at least produce new growths with very little green 

 .color (23). Such plants are of particular interest from the standpoint of 

 the physiology of sexual reproduction. They differentiate and develop 

 blossoms and may even set fruits at almost the same rates as normally 

 green plants in cool nights and warm days. Long continued development 

 is not usual with such pale plants but the initiation of sexual reproduction 

 is not inhibited and only slightly delayed by a lack of green color. 



Examination of the stems of plants grown with warm nights and cool 

 days discloses that they have an anatomical history parallel to those grown 

 in cool night and warm day temperatures. While they have the thin-walled 

 cells with slight inclusions which are typical of partly etiolated or shade- 

 grown samples, reduced cambial activity preceded the appearance of 

 primordia when the proper photoperiod was used to induce reproduction. 

 (Figs. 1-4.) 



Three unusual methods of inducing blossoming also resulted in plants 

 with characteristic changes in cambial and cellular condition prior to and 

 during blossom bud development. The short-day plants, poinsettia 

 (Euphorbia pulcherrima) and Maryland Mammoth tobacco (Nicotiana 

 tabacum) do not blossom in short days at warm night temperatures of 

 75°F (30). They will blossom in this environment if a current of cool 

 air is passed through a short chamber placed about the stems some 3 to 4 

 nodes below the tips (29). The structure of the stems above the "tempera- 

 ture girdles" becomes typical of those induced to blossom by usual induction 

 techniques (Figs. 5, 6). Borthwick, Parker and Heinze (2) report 

 that cooling of the petiole of the leaf responsible for providing the flowering 

 stimulus prevented the transfer of the stimulus and inhibited blossoming 

 of Biloxi soybean. 



Poinsettia plants in short days, but too warm at night to initiate blossom, 

 can also be brought into blossom by wrapping taut rubber bands about 

 the stems near the tips. When blossoming is induced in this manner, 

 characteristic changes in the anatomical structure were found to occur 

 above the point of banding. 



