12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



darkness. Spectrum sensitivity data obtained in studies on photo- 

 tropism are in general accord with similar information about the 

 formative influence of different wave lengths of light upon the 

 growth of illuminated v/hole plants (cf. Burkholder, 1936; Duggar, 

 1936; Johnston, 1934). The formative action of light in relation to 

 growth-promoting substances has been emphasized in a recent paper 

 by Katunskij (1936). Students of phototropism at present usually 

 explain differential growth under conditions of unilateral illumination 

 as being a result of unequal growth substance concentration on the 

 lighted and shaded sides. This concept, which had its origin many 

 years ago, is supported by a considerable body of evidence (cf. 

 Boysen Jensen, 1936 a). The manner in which unequal distribution 

 comes about is not agreed upon by all investigators. 



Went's (1928) experiments with excised and unilaterally illumi- 

 nated Avena coleoptile tips placed on agar blocks have seemed to 

 indicate that the amount of growth substance given off by the lighted 

 side is appreciably lessened, while that of the darkened side is slightly 

 increased above the amount in tips kept in darkness. If the total 

 amount recovered from unilaterally lighted tips is adjusted to 100, 

 then 68 percent of this comes from the shaded side while 32 percent 

 is recovered from the lighted side. Recalculation of Went's data in a 

 different way shows an increase of only 7 percent in the shaded half 

 tips as compared with similar half tips kept in the dark. Van Over- 

 beek (1933) obtained evidence for cross transfer of growth substance 

 (in some experiments, not in others) with Raphaniis hypocotyl seg- 

 ments which were illuminated unilaterally. It seems certain now that 

 less growth substance occurs in the illuminated half of an organ than 

 in the shaded half during phototropic activity (Went, 1928; Van 

 Overbeek, 1933; Boysen Jensen, 1933, 1936; cf. DuBuy and Nuern- 

 bergk, 1932-1935), but whether this difference always comes about 

 by lateral displacement seems open to reasonable doubt. 



Some investigators have reported decreased amounts of growth 

 substance in tissues exposed to light. Went (1928) observed a de- 

 crease of 18 percent in Avena coleoptile tips which had been exposed 

 to 1 ,000 meter candle seconds ; various results were obtained with 

 larger and smaller dosages of light where events were apparently 

 complicated by modified rates of transport. Although weak blue light 

 and even strong white light with the infrared and some red removed 

 is reported to have no diminishing effect upon growth substance, 

 DuBuy (1933) found that strong white light plus infrared decreased 

 the supply in Avena, and direct sunlight eliminated it completely. It is 

 of considerable interest that the effect of light upon growth response 



