362 



GROWTH FACTORS AND FLOWERING 



Fig. 1. Stem tips of representative Xanthium plants showing response 

 to treatments with various numbers of photoinductive cycles (figures in 

 lower left-hand corners) and gibberellin. In each pair of tips the controls 

 are on the left, the gibberellin-treated plants on the right. 



is concerned. Mann (1940), Naylor (1941), Salisbury (1955), and 

 otliers have shown that while one long dark period is sufficient for 

 photoperiodic induction of Xanthium, additional long dark periods 

 increase the rate of reproductive development. Gibberellin can sub- 



