THE CHLOROPHYLL APPARATUS 161 



decomposed by an enzyme into water and oxygen. The 

 first decompositions were held to be effected by the light and 

 the colouring matter, the body of the plastid taking part 

 only in the subsequent constructive processes. 



The most recent theory of the process of photosynthesis 

 was advanced by Harvey-Gibson in 1907. He suggested 

 that the rays of solar energy absorbed by the chlorophyll 

 are transformed into electric energy, and that by means 

 of the consequent currents . of electricity, moist carbon 

 dioxide is decomposed, the result being the formation of 

 formaldehyde and oxygen. The process takes place in the 

 intercellular spaces of the leaf; the formaldehyde is sub- 

 sequently absorbed into its cells, where it undergoes poly- 

 merisation, as Baeyer suggested. 



Though the production of starch is apparently not the 

 ultimate aim of the photosynthetic processes, its ready 

 occurrence affords us an easy method of demonstrating the 

 activity of the chlorophyll apparatus. If a leaf is partially 

 covered by a piece of opaque material, and is then exposed 

 to the light, starch rapidly appears in the illuminated 

 portion, as can be shown by bleaching the leaf with boiling 

 alcohol, and then immersing it in iodine, which forms a 

 blue colour with starch. The blue tint only appears where 

 the light has reached the chlorophyll apparatus. 



These processes are carried out by the chlorophyll apparatus 

 under the conditions set forth. It is evident that such 

 changes as have been described cannot be accomplished 

 without the expenditure of a considerable amount of energy. 

 In this need we have the explanation of the composite 

 nature of the chloroplast. The chlorophyll absorbs certain 

 rays of light which fall upon it, and the energy which is 

 liberated by the extinction of their vibrations is taken up by 

 the protoplasm of the plastid and applied by it to effect the 

 decompositions that take place. A very ingenious method 

 of demonstrating that the energy is derived from the rays of 

 light absorbed by the pigment was devised by Engelmann. 

 He observed that certain_bacteria were excited to active 



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