CHLOROPHYLL 15 



red. The substance, if viewed through a spectroscope, either 

 when undergoing combustion or in the gaseous state, would 

 produce a dark band in the red section of the spectrum. Further, 

 light falling on a coloured substance is not completely absorbed. 

 A red substance reflects red light and absorbs the other rays. 

 Only light corresponding to the spectral bands is absorbed. 

 Grotthus proved that no effect could be produced by light unless 

 it was absorbed. He showed that red iron-thiocyanate was 

 bleached by exposure to green light, yellow chloride of gold and 

 blue starch-iodine complex by blue and yellow light respectively. 



1. A mere glance at the absorption spectrum of the green leaf 

 is sufficient to show that the light best absorbed is that having 

 a wave length less than 500 /x^i, the amount absorbed becoming 

 greater as the wave length becomes shorter, i.e. the chloroplasts 

 absorb the actinic rays (violet and a small amount of the ultra- 

 violet rays). There is also a well-marked absorption band in 

 the red portion of the spectrum between 665 and 685 JJ./JL. The 

 figure for the maximal energy of solar radiation is given by 

 S. P. Langley as 650 to 666 /m/m. for high sun, so that the green 

 leaf is able to (a) utilise the actinic rays, and (b) absorb light of 

 that wave length (red) which is emitted by the sun in greatest 

 amount. The pigments of the chloroplast do not utilise green- 

 yellow light, nor do they absorb the heat (infra-red) rays at all. 



2. Consider next the physical (and chemical) changes brought 

 about by the absorption of light. Although this is the primary 

 problem in Biology and has attracted many investigators, it 

 remains unsolved. Research has made it more apparent that 

 the mechanism for converting solar radiation into bound energy 

 is not so simple as was at first thought. Certain facts, however, 

 have been brought to light. 



(1) Matter is assimilated. Elements taken from the environ- 

 ment are built into organic compounds. Boysen- Jensen has 

 shown that in July the accumulation of matter (dried) may reach 

 16-5 per cent, of the total dry weight of the plant. A large 

 proportion of this matter can be shown to be carbohydrate by 

 a very simple experiment. It is only necessary to screen a por- 

 tion of a leaf from the light, leaving a certain portion exposed, 

 and then observe any differences between the normal and the 

 darkened portions. If a leaf, like sunflower or fuchsia, is chosen 

 and previously kept overnight in the dark, exposure to light for 

 15 minutes is sufficient, and one hour is more than ample for our 

 purpose. The leaf is then bleached with warm alcohol, and 



