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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