80 • Floral Hormones and the Induced State 



of induced leaf tissue, of which there are many examples in the 

 literature. Xanthium is striking in this regard. Several double- 

 branched plants grafted together in series can all be brought to 

 flower by short-day treatment of a single leaf on one of them (see 

 Naylor, 1953). Khudairi and Hamner (1954a) found that a total 

 leaf area of less than one square centimeter was enough to bring 

 about flowering from a single 16-hour dark period. Xanthium may 

 be more extreme in this regard than most species, but the idea 

 that induced leaves simply supply an inhibitor-free stream of 

 assimilates is hard to reconcile with such results. However, some 

 form of inhibitor hypothesis is still favored by certain investiga- 

 tions, of which a few should be considered. 



In annual Hyoscyamns (LDP), removal of all the leaves brings 

 about flower formation, which then takes place at the same rate 

 irrespective of light or dark conditions. Presumably, then, the 

 effect of long days on an intact plant is to prevent an inhibition 

 of flowering exerted by the leaves under short-day conditions. Since 

 clearly in the defoliated plant the floral stimulus is present or can 

 be formed in the stem or roots, leaves on short days apparently not 

 only fail to produce it themselves, but also destroy it, or inhibit its 

 production, or produce an inhibitor of flowering. The latter hypoth- 

 esis can be avoided either by adopting the first or by suggesting 

 a mechanism for the second— for example, that the short-day leaves 

 remove some substance that could otherwise act as a precursor for 

 production of the stimulus. So far, there is no clear evidence in any 

 direction (see Lang, 1952) . Whatever the explanation, such effects 

 may be responsible for some of the ambiguous results obtained 

 from grafting experiments, as in the following example taken from 

 Zeevaart (1958). 



Defoliated scions of the LDP Nicotiana sylvestris grafted on 

 stocks of the SDP Maryland Mammoth will flower on short days, 

 suggesting florigen transfer from the induced stock. However, such 

 scions also flower on long days, noninductive for Maryland Mam- 

 moth, although similarly defoliated but ungrafted Nicotiana sylves- 

 tris fails to flower on long days. Does Maryland Mammoth then 

 produce florigen under, for it, noninductive conditions? The ex- 

 planation may be that defoliated Nicotiana sylvestris, like Hyoscya- 

 mus, has the capacity to flower if sufficient assimilates are present. 

 In Hyoscyamus these come from the large storage root, whereas in 



