GROTTIIUS' LAW 15 



of the plastid. The existence of chlorophyll in fossil vegetation 

 shows that it is of prinuiry importance in the e\ohition and 

 maintenance of life. Man, lord of the earth, depends for his very 

 existence on the presence of an organic magnesium compound in 

 the humble grass of the field. 



The light that falls on the leaf may be {a) reflected, {h) refracted, 

 or {c) absorbed. It is obvious that only absorbed energy can have 

 any effect on the metabolism of the leaf. This stateinent is known 

 as Grotthus' Law, and can quite readily be proved. For example, 

 a substance appears red to us because it is reflecting light whose 

 mean frequency is 375 X 10^^ cycles per second. It will, therefore, 

 absorb light of other frequencies. Grotthus showed that red iron 

 thiocyanate imderwent no change on exposure to red rays, but was 

 bleached when green light fell on it. Similarly the blue complex 

 formed by the action of iodine on soluble starch is decolourised by 

 yellow light and yellow gold choride by blue light. 



Absorption Spectrum of the Green Leaf. 



A mere glance at the absorption spectrum of the green leaf 

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

 a wave-length less than 500 aye, the amount absorbed becoming 

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

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

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

 the red portion of the spectrum between 665 and 685 /x/^. The 

 figure for the maximal energy of solar radiation is given by 

 S, P. Langley as 650 to 666 /x/x 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. 



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 ))y 

 a very simple experiment. It is only necessary to screen a portion 



