The Gibberellins • 105 



species Chrysanthemum morifolium. In those varieties requiring 

 only cold treatment to flower, irrespective of daylength, gibberellic 

 acid can cause flowering. In those that are SDP, however, it does 

 not (Harada and Nitsch, 1959b). In Kalancho'e, gibberellin reduces 

 the flowering of plants kept on short days, although it promotes 

 vegetative growth. In spite of this, the effect is not identical with 

 that of long days since it makes no difference whether or not the 

 gibberellin-treated leaf lies between the short-day (induced) leaf 

 and the growing point (Harder and Biinsow, 1956, 1957). 



At least two detailed studies on Xanthium have appeared. 

 Both agree that gibberellic acid cannot cause flowering under long- 

 day conditions; it can, however, increase the flowering response to 

 a limited number of short-day cycles. Greulach and Haesloop 

 (1958) obtained such results with intact plants; Lincoln and 

 Hamner (1958), on the other hand, found this effect only in de- 

 budded plants, and concluded that the compound acted by 

 increasing the capacity of the young leaves to store the flowering 

 stimulus. 



Flowering in a strain of the duckweed Lemna perpusilla may 

 take place under any daylength or may require short days, depend- 

 ing upon factors to be discussed later. In both situations, however, 

 gibberellin can completely abolish flowering at levels that promote 

 vegetative growth, although other associated morphogenetic effects 

 prevent this from being considered a specific inhibition of flower^ 

 ing (Hillman, 1960). 



In summary, the gibberellins have already contributed greatly 

 to the study of flowering: they are the first compounds discovered 

 with which many kinds of plants can be brought to flower almost 

 at will. Further understanding of the way in which they fully or 

 partially satisfy requirements for long-day or cold treatments, at 

 least in rosette plants, will be of great value. The closeness of their 

 relation to flowering, as compared with other developmental 

 processes such as stem elongation, is still in doubt, and the results 

 with SDP indicate that no gibberellin so far tested can be con- 

 sidered a florigen. However, there is good preliminary evidence 

 that native gibberellin levels in certain plants increase as a result 

 of treatments leading to flowering, and such changes may be part 

 of the normal mechanism involved (Chapter Five). 



