LIFE AND LIGHT 31 



day and night periods; this would indicate that the first steps at 

 least in the synthesis are more purely chemical and out of control 

 of the cell and the plant of which it is a part. 



Serious consequences to the plant, however, occur if it is kept 

 assimilating day and night without its normally recurring periods 

 of darkness and rest. Thus it has been shown by Siemens and 

 others that plants exposed continuously to electric light such, 

 for example, as decorative plants in winter gardens soon fade, 

 dropping their leaves and showing other pathological changes. 

 The formation of green in the etiolated plant occurs with a lower 

 intensity of light than that which just suffices to give a credit 

 balance of photo-synthesis over respiratory production of carbon 

 dioxide, and induce an increase in organic dried weight. 



All rays of the spectrum are not supposed to be equally 

 effective in inducing photo- synthesis. It has been suggested by 

 Lommel, and afterwards by Timiriaseff, who threw a spectrum 

 upon a starch- free leaf and then determined the relative amounts of 

 starch produced under different spectral regions, that the greatest 

 activity lies where most light is absorbed by a chlorophyll solution 

 viz., in the red between the B and C lines. Other optima lie 

 under the positions corresponding to the other absorption bands of 

 chlorophyll. 1 



By its green fluorescent action on the light, chlorophyll, more- 

 over, possesses the property of reducing a good deal of the light of 

 higher frequency of vibration to a colour and speed of vibration 

 at which it produces a greater photo- synthetic effect in the 

 plant. 



The spectrum of fresh chlorophyll is very characteristic in the 

 plant world, just as is that of haemoglobin in the animal world, 

 although the two spectra are entirely different from each other. 

 These two pigments show many interesting similarities in chemical 

 constitution. Both are lecithides in their relationships in chloro- 

 plastid and in blood- corpuscle respectively one contains iron, the 

 other magnesium in colloidal form in the molecule; one causes 

 photo- synthesis accompanied by reduction, the other the com- 

 plementary process of breaking down of the synthesised products 



i This is the orthodox view, but recent experiments by the author and 

 Mr. Edward Whitley have shown that all regions of the visible spectrum very 

 effectively photo -synthesise even in regions where there exist no absorption 

 bands of chlorophyll. 



