132 GROWTH HORMONES IN PLANTS 



coleoptile tip is diminished by only one-tenth when the back side 

 is not shaded by the primary leaf. In other words, the shaded 

 side receives nine-tenths of the light which falls upon the side 

 nearest the source of light. In the basal region, however, the far 

 side obtains only one-twentieth to one-fiftieth the amount of the 

 light received by the front side. Van Dillewijn (1927a) found, 

 in rather good agreement with these measurements, that the 

 back side receives about one-thirtieth as much light as does the 

 front. 



Nuernbergk (1927) since has found lower absorption values in 

 unilaterally illuminated coleoptiles than were indicated by earlier 

 workers. When the broad side was illuminated, the light was 

 decreased to about one-seventh in the basal region; when the 

 narrow side was illuminated, the light value on the far side was 

 reduced to one-tenth of the incident amount. The decrease in 

 intensity in the apical zones depended somewhat upon whether 

 or not the primary leaf was within the coleoptile. With the leaf 

 present, the front side received 4 to 8 times as much light as the 

 back. With the leaf not acting as a screen, the front side received 

 1.3 to 1.5 times as much light as the back when illuminated on the 

 broad side and 2.6 to 3 times as much when illuminated on the 

 narrow side. This difference in the absorption values for the nar- 

 row and wide dimensions of the organ is probably due to the 

 difference in thickness of the tissues through which the light 

 passes. As shown in Fig. 21, the walls of the coleoptile are much 

 thicker in the narrow portions containing the bundles than in the 

 other parts of the organ. 



Bergann (1930) also determined the decrease of light in the 

 Avena coleoptile by microphotometric measurements. It was 

 found that the blue rays passing through the coleoptile tip from 

 the broad side were 2.2 times and from the narrow side 3.0 times 

 more intense on the front than on the back side. In the basal 

 portion of the coleoptile the intensity was 33 to 37 times greater 

 on the exposed side. However, the stimulating effect of light 

 arises mainly by its action upon the apical portion of the tip. 



It should be mentioned that the height of the primary leaf in 

 the hollow cylinder of the coleoptile has in itself no significance 

 for the course of phototropic curvature, although the presence of 

 the leaf, acting as a light screen, has a decided influence upon the 

 hght gradient across the coleoptile. Du Buy (1934) studied the 



