THE SYNTHESIS OF FLOWERING HORMONE 161 



and this has been done in Fig. 9-3. The general shape of this "rate" 

 curve is similar to the flowering curve, leading to the conclusion that 

 the most rapidly expanding leaf is the one most sensitive to photo- 

 periodic induction. The most rapidly growing leaf on a plant is by 

 no means the largest one. It is only a little more than half expanded, 

 but it is expanding more rapidly than any of the others. Perhaps this 

 means that its chemistry is more active, making it more efficient in 

 synthesizing flowering hormone. 



Since all of the reasoning in the above paragraph is based on the 

 assumption that the S-shaped curve of Fig. 9-3 is a truly representa- 

 tive leaf expansion curve for cocklebur, it seemed appropriate to 

 check this assumption by actual measurement of cocklebur leaves. 

 In initial attempts during the spring and summer of 1962 we found 

 that the leaves do not seem to grow normally when they are being 

 measured. Figure 9^ shows the results of our most recent, very 

 careful attempt to obtain a growth curve by daily measurement of 

 midrib length. Although plants were handled only for a few seconds 

 each day, the leaf being measured never grew to the usual 11 or 12 cm 

 length, but turned yellow and died after reaching about 8 cm. Thus 

 we were confronted with the remarkable discovery that one can kill 

 a cocklebur leaf simply by touching it for a few seconds each day! 

 This came as a real surprise, since we have always assumed that 

 cocklebur plants could stand a great deal of abuse with no ill effects. 

 Such results also have some interesting ecological implications. We 

 are presently considering camera techniques in the measurement of 

 cocklebur leaf growth. 



Another interesting result of this experiment is evident in the 

 growth rate curve. Although the leaf area curve is approximately 

 S-shaped, there is enough of a deviation from a true S-shape to 

 produce two peaks and a trough in the rate curve. This could be 

 another result of handling, but it might also be the result of changing 

 weather conditions during the period of measurement.^^ If this is 

 true, then we might have some measure of explanation for the 

 difficulties encountered in exactly duplicating experimental results. 

 If sensitivity to a dark period is a function of leaf expansion rate, 



" Examination of weather data (greenhouse and outside temperatures, daily 

 light conditions) failed to reveal any clear cut correlations with the growth rate 

 curve, although there was a cloudy day with very low light intensity about 4 days 

 before the trough in the growth rate curve. 



