24 LIFE AND LIGHT 



or in darkness synthesis and reduction fall off, and oxidation pre- 

 dominates with output of carbon dioxide. To set the balance 

 towards carbon dioxide assimilation in excess with oxygen produc- 

 tion a considerable light intensity is requisite ; thus moonlight is 

 insufficient, but slight oxygen production has been noticed by algae 

 in the twilight, and in electric light of, say, 1,000 candle-power the 

 balance is turned at a distance of about half a metre to a metre 

 and a half from the source of light. 



Turning our attention now to the minute energy transformers 

 by which the radiant light energy is converted into chemical energy, 

 we find that these are contained in hundreds of thousands in the 

 cell in the form of what are called chloroplastids or chloroplasts. 



The shape of these microscopic chloroplasts varies in different 

 plants, as also their dimensions, but in all cases they consist of two 

 kinds of material a colourless material, probably consisting of pro- 

 tein and being bioplasmic in nature and origin, which forms a stroma 

 or spongework, and a green- coloured material, oily as to physical 

 character, which fills up the meshes of the stroma without mixing 

 with it, and is particularly developed in a network of varying forms 

 upon the surface of the plastid. The colourless part of the plastid 

 can develop independently of the action of the light, and colourless 

 plastids occur in plants which have been grown in the dark, and hence 

 are etiolated or colourless. These colourless plastids on being ex- 

 posed to light rapidly commence to turn green in colour from develop- 

 ment of the green- coloured oily matter, and now photo-synthesis 

 commences. After a certain amount of the green oily matter has 

 been synthesised by the action of the light, further manufacture of 

 it ceases, and it then acts as an agent or transformer for conversion 

 of the radiant energy into other forms of chemical energy, such as 

 sugars, starches, or oils, which accumulate in the cell. 1 



There is no doubt that botanical chemists have laid too much 

 stress upon the synthesis of sugars and starches only by the plastids, 

 as if this synthetic activity was confined to one line of synthesis. 

 It is known that fats may appear very early in the process and be 



the synthesis is nearly all carbohydrate synthesis, and here, as in the animal, 

 the quotient approximates to unity; but in fat formation in other parts 

 hydrogen in excess is laid down in the molecule and the quotient exceeds 

 unity just as it falls below it in fat oxidation in the animal body. 

 Compare with succeeding chapters. 



