80 PHOTOCONTROL OF GROWTH 



paper, with 80% aqueous isopropanol and 1% ammonia as a run- 

 ning solvent. After drying, the chromatogram was cut up into 10 

 equal strips, each of which was placed in a small petri dish and 

 moistened with water. Ten isolated birch embryos were then planted 

 on each of the filter paper strips and observations made on the growth 

 inhibitory activity of the different regions of the chromatogram. It 

 was found that there was a powerful growth inhibitor present on the 

 chromatogram in the region Rf 0.7-0.9. The same inhibitory zone 

 was found in a sample of birch achenes which were entirely lacking in 

 the embryos. Thus, it would seem that the inhibitor is located pri- 

 marily in the pericarp. The presence of a growth inhibitor in the 

 pericarp does not, of course, necessarily imply that it constitutes the 

 sole basis of the inhibitory effect of the pericarp on the embryo, and 

 further experiments were carried out in an endeavor to obtain decisive 

 evidence on this question. 



First, it was found that when birch embryos were planted on the 

 inhibitory zone and then exposed to different photoperiodic treatments 

 a high proportion of embryos germinated when maintained under long 

 days, but gave only a low germination percentage under short days. 

 That is to say, whereas the isolated embryos planted on filter paper 

 moistened only with water germinated equally well in both light and 

 dark, when they are planted on filter paper containing the inhibitor 

 their photoperiodic behavior is restored. In a further experiment, intact 

 seeds were slowly leached in water in darkness for 3 weeks. They were 

 then sown and held under long-day or short-day conditions, and their 

 germination was compared with that of unleached seeds. It was found 

 that in the leached seeds germination was high not only under long 

 days but also under short days. The two foregoing experiments thus 

 indicate that ( 1 ) the photoperiodic behavior of the intact seeds can be 

 largely restored if isolated embryos are planted on the inhibitor, and 

 (2) if the inhibitor is leached out of intact seeds, their photoperiodic 

 behavior is largely lost. These results are, therefore, consistent with 

 the hypothesis that the inhibitory effect of the pericarp arises pri- 

 marily from the presence of the growth inhibitor. It was shown that 

 at the low light intensities used, there is very little photodestruction of 

 the inhibitor on the filter paper, and it would seem that the effect of 



