The Gibberellins • 101 



Hyoscyamus, carrot (Daucus carota), and several other biennials, 

 all under long-day conditions (see Fig. 6-1). Several LDP kept on 

 short days, including annual Hyoscyamus, Samolus pannftorus, and 

 Silene armeria, also flowered in response to such treatment. No 

 promotion of flowering occurred in the SDP Xanthium and Biloxi 

 soybeans kept on long days. These experiments were conducted 

 with gibberellins of fungal origin. Similar results on both Samolus 

 and biennial Hyoscyamus were later obtained with extracts of 

 wild-cucumber (Echinocystis) seeds, known to be rich higher-plant 

 sources of gibberellins (Lang et ah, 1957). Evidently, then, gibberel- 

 lin can substitute for the cold requirement of certain vernalizable 

 plants and for the long-day requirement of certain LDP, but not 

 for the short-day requirement of SDP. This general conclusion still 

 appears valid, but requires expansion. 



Vernalization or long-day requirements have not been suc- 

 cessfully replaced by gibberellin in all plants tested. One reason 

 for this may be the known difference in activity, for a given plant, 

 among the various gibberellins themselves (see Phinney and West, 

 1960) well illustrated by Fig. 6-2. Possibly plants that have not 

 responded so far will do so when other gibberellins are tried. In 

 the "classical" experimental objects for vernalization studies, the 

 winter cereals, gibberellic acid can hasten flowering in unvernalized 

 seedlings, but only when applied at a particular stage; in addition 

 to flowering, which is often abnormal or abortive, other changes in 

 meristem development occur (Caso et al., 1960; Koller et al., 1960; 

 Purvis, 1960). Further lack of exact correspondence between gib- 

 berellin effects and vernalization is found in the work of Sarkar 

 (1958), discussed in the next chapter, showing that optimum 

 sensitivity to gibberellin or to cold treatment need not occur at 

 the same stage of development. Moore and Bonde (1958) have 

 observed that gibberellic acid actually devernalizes or prevents 

 vernalization in a variety of Pisum, depending on whether it is 

 applied after or before the cold treatment. 



It is important to realize that, at least so far, all the LDP in 

 which gibberellin does replace long days are those in which flower- 

 ing is associated with "bolting"— the rapid elongation of the axis 

 from the almost stemless "rosette" of leaves characteristic of the 

 vegetative condition. In caulescent LDP, having elongated stems 

 even when vegetative, gibberellin apparently cannot bring about 



