FOOD DIGESTION AND RESPIRATION 55 



although this element is not a part of the molecule of chlorophyll, 

 yet if a plant be grown from the seed in absence of iron, no green 

 pigment is developed until iron is supplied. Chlorophyll, or rather 

 the important green part of the molecule is, briefly, a number of 

 pyrrol derivatives, four to be exact, united by magnesium. Thus 

 the magnesium is in organic combination. On the other hand, the 

 red colouring matter of the blood, haemoglobin, is a similar pyrrol 

 derivative in which iron takes the place of magnesium. The 

 properties of haemoglobin are as remarkable in another way as 

 those of chlorophyll are, as we shall see presently. The structure 

 of pyrrol is that of a ring of four carbon atoms andjcme_ nitrogen 

 atom, each united to hydrogen, and is produced from proteins by 

 destructive distillation. In those derivatives which form chlorophyll 

 and haemoglobin, two of the hydrogens are replaced by methyl and 

 one by ethyl. The magnesium may be removed by the action of 

 acids, without destruction of the green colour, a fact which makes 

 its presence somewhat puzzling, if the function of chlorophyll is 

 merely that of absorbing light of a particular wave-length. 



On the whole, it must be admitted that we know little about 

 the mechanism. The system is a very complex one, and photo- 

 chemical reactions, even of a simple kind, are still obscure in many 

 respects. 



The way in which oxygen is produced is still more difficult to 

 explain. We can only point out how it might happen, on the 

 basis of certain facts which are known. It is not an uncommon 

 action of radiant energy to bring about the formation of peroxides, 

 raising the chemical potential of oxygen. Peroxides are oxides 

 containing more oxygen than the simple oxides. Thus, water being 

 H.,O, the peroxide of hydrogen is H. 2 O 2 . This may be represented 

 either as H O O H, or if the rise in' potential of oxygen implies 



H-0 

 its becoming quadrivalent, as ', In either case, the extra 



H-,0. 



oxygen atom is readily available for oxidising other substances, or 

 being set free from two molecules together, given off in the form of 

 gaseous molecular oxygen. If, in any way, peroxides wer$ pro- 

 duced in the leaf under the action of light, oxygen could be 

 obtained from them under appropriate conditions. There is 

 evidence that peroxides are formed in chlorophyll systems by light, 

 although it is not certain that they do not arise from destructive 

 oxidation of the pigment itself. Such organic peroxides give rise 

 to the production of hydrogen peroxide by interaction with water. 

 Further, there is an enzyme, called catalase, present in all green 1 

 leaves, which decomposes hydrogen peroxide with the evolution 

 of gaseous oxygen. This is about as far as we can go at present. 



