78 • Floral Hormones and the Induced State 



latter not in the axil of the leaf but on the opposite branch of an 

 otherwise debudded two-branched plant. The rate of stimulus 

 translocation can then be calculated by the difference in the first- 

 llowering-node values of the shoots in the plants with the receptor 

 buds close and the receptor buds far from the induced leaf. An 

 example from one experiment may make this clear. In the "close" 

 series, in which the average distance from leaf to bud (mainly 

 through petiole tissue) was 90 mm, the mean first flowering node 

 was 3.4 if the dark treatment was given 24 hours alter decapitation. 



4.5 with it given 48 hours after, and 5.3 with it given 72 hours 

 after. In the "far" series, the distance between leaf and bud was 

 about 235 mm, through both branch and petiole tissue. Here, 

 inductive treatment started immediately after decapitation gave a 

 first-flowering-node average of 4.6. By interpolation from the pre- 

 ceding figures, it is as if the inductive treatment for the "close" 

 series had been delayed some 55 hours. Since the difference be- 

 tween "far" and "close" is about 145 mm, this difference of 55 

 hours represents the movement of the stimulus at 145/55, or about 



2.6 mm per hour. 



Such experiments of course give an average value for the 

 transport through a petiole, then both down and up a branch: 

 other experiments suggested that upward transport may be faster 

 than downward. Also, the transport rate in plants so mutilated 

 may well differ from that in intact plants. In any case, all experi- 

 ments with Pharbitis gave values of the order of 3 mm per hour. 

 This represents a considerably slower movement than that observed 

 for carbohydrates in phloem tissue (often exceeding 200 mm per 

 hour), but rates of virus transport in the phloem sometimes fall 

 in this low range (see Esati et ai, 1957). 



FLOWER PROMOTION OR FLOWER INHIBITION? 

 THE SPECIFICITY OF FLOWERING STIMULI 



The florigen hypothesis in its simplest form postulates a single 

 substance, common at least to many plants, uniquely responsible 

 lor flower initiation. Much of the evidence so far presented is 

 consistent with this hypothesis, but some investigators, on the con- 

 trary, have concluded that flowering is controlled by an inhibitory 



