GROWTH SUBSTANCES FOR PHOTOTROPISM 



157 



in a very reduced form . If the amount of growth substance given 

 off by the tip, in darkness, is adjusted to equal 100 per cent, then, 

 on this scale, the growth substance given off by the shaded side of 

 a unilaterally illuminated tip is increased from the expected 50 to 

 67 per cent. By another method of calculating the results given 



Table 10. — Relative Amounts of Growth Substance Recovered from 



AVENA-COLEOPTILE TiPS PlACED UPON AgAR BlOCKS AND KePT 



Either in Darkness or Exposed to Unilateral Light 

 See Fig. 2 (Went) 



in Table 10, Went (1935) adjusted to 100 per cent the total 

 amount given off into agar by the illuminated tip; then 65 per 

 cent of this is dispersed into the block beneath the shaded side, 

 and 35 per cent into that under the illuminated side (Fig. 2) 

 (Went). There is some doubt whether one can conclude from 

 these experiments that the amount of growth substance given 

 off on the shaded side is increased by unilateral illumination. 

 This method does not give entirely satisfactory evidence for the 

 displacement of growth substance in the tip. 



It does not seem possible to present proof for the displacement 

 of growth substance in the coleoptile tip, since new substance is 

 continually being dispersed from there. In the basal portion of a 

 decapitated coleoptile, however, none is present for some time 

 after decapitation, and it should be possible here to decide 

 whether a displacement of growth substance can take place. 

 Dolk (19296) showed that in horizontal coleoptile cyHnders of 

 Avena which were supplied with growth substance at one end, 



