504 RADIATION BIOLOGY 



at a concentration of 5 ppm, or 0.25 mg per plant, were adequate to 

 cause floral initiation in any month of the year. Distribution of free 

 and bound auxin in vegetative Cabezona pineapple plants was measured 

 by Van Overbeek et al. (1947); the free auxin content was found to be 

 greatest in the apex of the stem axis, and the base of the youngest leaves 

 had the highest amount of bound auxin. On the basis of these data the 

 hypothesis was advanced that flowering under natural conditions was 

 brought about by an increased level of free auxin in the apical meristem, 

 resulting from an increased rate of conversion of bound auxin from the 

 young leaf bases into its free form. Low night temperatures were 

 observed by Van Overbeek and Cruzado (1948b) to cause unseasonable 

 flowering of Red Spanish pineapple, a variety reported as insensitive 

 to photoperiodic control. Geotropic stimulation (Van Overbeek and 

 Cruzado, 1948a) also resulted in floral initiation on the Cabezona variety 

 but was inefl"ective on the Red Spanish and Smooth Cayenne varieties. 

 These observations indicate that floral initiation in Ananas comosus might 

 be caused by a common effect of various treatments on the physiology of 

 the plant. Van Overbeek pointed out that auxin level can be an impor- 

 tant internal factor, which in itself need not be causative but may inter- 

 act closely with other processes that influence floral initiation. This 

 may be similar to the functioning of the pituitary in photoperiodically 

 sensitive animals. 



The auxin economy of the plant thus has been demonstrated to inter- 

 act with floral initiation and flower development. How causative it 

 might be as a control factor, however, remains to be established. 



INHERITANCE OF RESPONSIVENESS TO PHOTOPERIOD 



The genetics of photoperiodism might serve as a guide for physiologi- 

 cal and possible biochemical work. Action of a single gene is thought to 

 control a particular reaction even though the reaction involves function- 

 ing of many genes. If a single gene is involved in inheritance of 

 photoperiodic response, it might possibly affect the photochemical 

 reaction. 



The most extensive investigations of inheritance of photoperiodic 

 response have been with tobacco. Allard in 1919, before the discovery 

 of photoperiodism, described crosses of Maryland Mammoth tobacco 

 with White Burley, Connecticut Broadleaf, and several other varieties of 

 Nicotiana tabacum which are now known to be indeterminate. In all 

 cases the nonflowering characteristic of Maryland Mammoth as grown 

 under field conditions was inherited as a unit character. A total of 

 1820 Fo plants, of which 439 were of- the Maryland Mammoth type, 

 were grown. Lang in 1948 made crosses between Maryland Mammoth 

 and the indeterminate variety Java. Of a total of 467 F2 plants, 128 



