GROWTH MOVEMENTS 



1081 



Table 2. — Data Showing the Phototkopic P^ffectiveness ok Restricted 



Regions of the Visible Spectrum 

 That for the Hg line 4358 A is taken as unity. {From Johnston, 23) 



5secxlOOM,C 



Note. — SG, Sextant green (1.94 mm.); NC, Noviol "C" (4.15 mm.); HRN, heat-resistant Noviol 

 (3.04 mm.). 



SENSITIVITY 



In much of the work with coleoptiles the entire length of the organ 

 was exposed to Ught. Such experiments are not strictly comparable 

 with those in which only the tips are 

 illuminated. Lange (28) has shown 25 

 that the first 50 /i of the tip is the '^ 

 most sensitive. Toward the base the ^ 

 sensitivity decreases. He reports that 25 

 the first millimeter zone is 160 times as '5 

 sensitive as the second millimeter zone 5 

 and 1800 times that of the third. 

 Lange's idea that only the tip is the 

 region of light perception is not in 

 agreement with the work of Went (42) 

 who determined the difference in the 



25 

 15 

 5\ 



1020304050 102030405050 90 120 

 Fig. 6. — Graphs showinglight-growth 

 response of Avena. The ordinate repre- 

 light-growth response of the tip (first sents growth in microns per minute, the 

 ^ _ , 1,11 /.n !• abscissa time in minutes, the arrow the 



2.5 mm.) and the base (6 mm. from moment of illumination. Graph 3, 



tip) of coleoptiles illuminated on three illumination of entire coleoptile (l 2 mm. 



. 1 „, , . r J^ ■ . ii j^ long); Graph 4, illumination of the tip 



Sides. Went is of the opinion that (1.25 mm.); Graph 5, illumination of 



there are two distinct growth responses, base (9 mm.) while the top (:i mm.) is 



, , , . 1 .1 X kept in darkness. (From Went, 42.) 



the tip response and the base response. 



These differences are perhaps best seen in Fig. 6 (his Graphs 3 to 5). 

 Their interpretation should be of interest in connection with Dillewijn's 

 experiments and other work where positive and negative curvatures are 

 obtained with different light intensities. 



