ROOT FORMATION 



193 



e.g. Cinnamonum, root better if the twigs are etiolated 

 ("blanched") for 2 weeks before taking cuttings (Blackie, 

 Graham, and Stewart, 1926). On the other hand it is well- 

 known that leafy cuttings need light in order to root. 



A number of experiments have been carried out (Went, 

 1935a) on the effect of different colored lights on root forma- 



rools er ten planfs 



60 1- 



40 



20 



g 



^ 



g 



violet white 

 400-460 400-700 

 1.2 X 100% 



color dark red red orange yellow green blue 



wavelengtil 610-700 580-700 520-700 480-700 480-570 400-520 



energy o 93% 6|% 6i% 60% I.4X 22% 



Fig. 54. Root formation on pea stems exposed for 14 days to light of the 

 color and intensity specified. Shaded columns, one leaf present; open columns, 

 no leaf present, but treated with indole-acetic acid (20 mg. per liter for 20 

 hoiu's). 100% energy = about 100 erg/cm. ^/sec. 



tion. Etiolated pea cuttings, without leaves and treated 

 with auxin in the standard way, were placed in a series of 

 chambers illuminated through calibrated color filters for 15 

 days. The results (Figure 54) show that light of any wave- 

 length reduces the number of roots formed below the number 

 formed in darkness, white light having the greatest effect 

 and blue the least. (The intensities of blue and green light 

 used were, however, much lower than the others.) If, how- 

 ever, the leaves are left on and no auxin applied, the opposite 

 result is obtained (Figure 54, shaded columns). Here the 

 dark controls produce almost no roots, and the white light 

 controls a maximum. Blue appears to be more active than 



