Compounds in Chloroplasts of Green Cells of Plants. 5 57 



and once an accumulation of organic material has been reached, even the 

 fats and their allies can easily be synthesised by the combination of linked 

 exothermic and endothermic reactions by the living cell acting as a trans- 

 former, without the use of external energy such as that of sunlight.* 



Once organic matter has been synthesised, the living cell can oxidise one 

 portion of this to reduce still more another portion. Iu this manner the 

 animal cell can oxidise carbohydrate, for example, and use the energy so set 

 free to build up another portion of carbohydrate into fat which weight for 

 weight contains double as much chemical energy as the carbohydrate, without 

 in the process making use of an external source of energy. 



At the commencement, however, when only water and carbon dioxide are 

 the available materials, it. is indispensable that an external source of energy 

 such as sunlight should be available, and a suitable mechanism, or chemical 

 system, for the transformation of this store of energy into the chemical 

 energy of organic compounds. 



Such a transformer has been recognised for a long period in the chloroplast 

 or chlorophyll-granule of the green cell of the higher plant. 



Since the days of de Saussure/f now over a century ago, the green colouring 

 matter of the leaf, chlorophyll, has been regarded as the fundamental agent 

 for this world-wide photo-synthesis. But it is remarkable how completely 

 this view is based upon indirect or circumstantial evidence, and how little, 

 if any, direct observation can be cited in its support. 



Chlorophyll is known by the biochemist to be one of the most complex of 

 substances, comparable to haemoglobin in its molecular structure, and yielding 

 a host of disintegration constituents themselves complicated substances of high 

 molecular weight. Between the simple colloidal molecules of inorganic iron 

 salts in solution or suspension and such a highly complex organic substance 

 as chlorophyll there is a wide hiatus, and it was with the view of discovering 

 some intermediate links or finding some explanation for the gap that the 

 present experiments were commenced. 



Before describing the experiments in detail it is desirable to touch upon 

 present views as to photo-synthesis in the green cell as far as these bear upon 

 our investigation, in order to give an appropriate setting to the new facts, and 

 show how present knowledge regarding the absolute necessity for the presence 

 of iron in the green leaf, which has been hitherto devoid of all explanation, 

 led up to these experiments. 



Photo-synthesis with production of oxygen only occurs in the chlorophyll- 



* See Moore, 1 Kecent Advances in Physiology and Biochemistry,' edited by Leonard 

 Hill, pp. 135, 138, Arnold, London (1906). 



t "Recherches Chimiques sur la Vegetation" (1804), 'Ostwald's Klassiker,' Nos. 15-16 

 VOL. LXXXVII. — B. 2 U 



