502 RADIATION BIOLOGY 



contrast continues for many generations to produce flightless forms par- 

 thenogenetically and still can be quickly induced to produce winged 

 migrant forms (Rissonnette, 1932a). This apparently is equally the case 

 for other aphids that reproduce parthenogenetically for long periods and 

 then can be induced to produce sexual forms (Marcovitch, 1924). 



FLOWERING IN RELATION TO AUXINS 



Floral initiation in long-night plants can be suppressed by continued 

 applications of auxins and enhanced by use of antiauxins (Bonner and 

 Bandurski, 1952). Bonner and Thurlow (1949) found that indoleacetic 

 acid and a-naphthaleneacetic acid at a concentration of 500 mg/liter 

 suppressed floral initiation of Xa7ithmm canadense (X. chinense) when 

 sprayed on the foliage. Cuttings placed in nutrient solutions of a-naph- 

 thaleneacetic acid at concentrations of 10 100 mg/liter and held in short 

 photoperiods were inhibited in floral initiation. It was necessary to 

 apply the auxins only during the dark period, and they were most effec- 

 tive at the beginning of darkness. Xanthium canadense leaves given an 

 external supply of auxin showed little increase in auxin by the Avena 

 test. Cajlachjan and Zdanova (1938) concluded that auxins as meas- 

 ured by the Avena test were formed more intensively on long photo- 

 periods for all types of plants. Existence of antiauxins and inhibitions, 

 however, confound the Avena test. 



The effects of a-naphthaleneacetic acid on the short-night plant Wintex 

 barley and the long-night plant Chalco teosinte were studied by Leopold 

 and Thimann (1949). Growth regulators did not induce floral initiation 

 when applied to either of these plants maintained on unfavorable photo- 

 periods. Wintex barley maintained on photoperiods favorable for floral 

 initiation responded to very low concentrations of a-naphthaleneacetic 

 acid by an increase in the number of flower primordia as contrasted with 

 control plants. Higher concentrations of a-naphthaleneacetic acid, how- 

 ever, inhibited the formation of floral primordia. Applications of growth 

 regulators to teosinte maintained on photoperiods favorable to floral initi- 

 ation resulted in inhibition of flowering at all concentrations tested. 

 Increased growth of Wintex barley was strongly correlated with increased 

 number of flower primordia. Stem elongation of barley has also been 

 shown to be closely correlated with spike development (Borthwick, Hen- 

 dricks, and Parker, 1948). 



Denfter and Grundler (1950) observed that growth in height of Sinapis 

 alba, Fagopyrum sagittatum, Impaliens balsamina, and Calendula officinalis 

 was reduced and blooming time appreciably delayed following treatment 

 of leaves with very low concentrations, 0.005-0.02 per cent, of a-naph- 

 thaleneacetic acid and a-indoleacetic acid. In all these species except 

 C. officinalis, however, flowers formed at approximately the same node 



