394 



RADIATION BIOLOGY 



1. DESCRIPTION OF THE NATURALLY OCCURRING PROCESS 



Seeds of almost all angiosperms germinated in complete darkness pro- 

 duce seedlings that contain no chlorophyll. These seedlings become 

 green when brought into light of proper intensity and wave length and 

 under favorable conditions of temperature, nutrition, and ambient atmos- 

 phere. As is well known, the green color is attributable to chlorophyll. 



Spectroscopic examination of extracts of unilluminated and briefly 

 illuminated dark-grown seedlings reveals differences in the spectral 



020 



420 



460 



500 



620 



660 



700 



540 580 

 WAVE LENGTH, m// 



Fig. 7-1. Absorption spectra of methyl alcoholic extracts of dark-grown oat seedlings: 

 curve a, from unilluminated seedlings, showing the carotenoid peaks at 440 and 470 

 m;u; curve h, long-wave-length region of curve a enlarged so as to show the absorption 

 maximum of protochlorophyll at 630 m/n; curve c, from illuminated seedlings, showing 

 the chlorophyll absorption maximum at 665 mp. {Frank, 1946.) 



absorption of the extracts. In Fig. 7-1 are shown the absorption spectra 

 of methanol extracts of dark-grown oat seedlings. The extract of unillu- 

 minated seedlings, curve a, shows marked absorption in the violet end of 

 the spectrum owing to the presence of carotenoids and a very small 

 absorption maximum at about 630 m^t, characteristic of protochlorophyll. 

 On an enlargement of the scale, curve 6, this absorption band is empha- 

 sized. After the leaves are illuminated, a new absorption band appears 

 in the extract at 665 m/x, curve c, which is attributable to chlorophyll a. 

 The lesser absorption band of curve c at about 630 m^, although in the 

 same position as the absorption band of protochlorophyll, is also largely 

 due to chlorophyll a. 



