ACTION OF CHLOROPHYLL 443 



When carbon dioxide is exposed to the influence of the chloro- 

 plastid in the presence of light and moderate warmth, it under- 

 goes a decomposition which may probably be expressed by the 

 equation 2C0., = 200 + O,, splitting up into carbon monoxide 

 and oxygen. At the same time, water is decomposed, probably 

 in the way denoted by the equation SH.^O = 2H„ + Oo. The two 

 molecules of oxygen unite and are evolved, the quantity being 

 the same as the quantity of CO.j undergoing decomposition. The 

 two residues, 2C0 and 2H,,, are then thought to unite to form 

 2CH„0, a body known as formic aldehyde. It is a property of 

 the aldehydes to undergo readily what is known as polymerisa- 

 tion or condensation of several molecules together. Such a 

 condensation of formic aldehyde would lead to the formation 

 of sugar thus, — 6CH,j0 = C,;H,.^0„ ; and it is probable that this 

 process takes place. At any rate, sugar is present in the meso- 

 phyll cells very speedily after the absorption of the C0._, and 

 the beginning of the exhalation of oxygen. 



These processes are carried out by the ohloroplastid under 

 the conditions set forth. It is evident that such changes as the 

 decomposition of G0._, and water cannot be accomplished withoiit 

 a good deal of energy being expended upon them. In this need 

 we have the explanation of the composite nature of the plastid. 

 The chlorophyll absorb ^ certain of the rays of light that fall upon 

 it, and the energy so acquired is applied by the protoplasm of 

 the plastid to effect the decompositions that take place. Sugar 

 is the result, though what kind of sugar is first formed is still 

 open to experiment. There is a fairly copious and continuous 

 production of it, indicating, as in the case of the raw materials 

 and the cells which absorb them, that there must be a removal 

 of it as it is formed. Otherwise the cell-sap would be saturated 

 with sugar and its formation would cease. The quantity which 

 is in excess of the immediate needs of the cell may either be 

 transported away from it by diffusion, or some of it may be con- 

 verted into an insoluble form and be temporarily deposited in 

 the cell. The latter operation is carried out by the plastid, 

 which quickly forms small granules of starch in its own sub- 

 stance, at the expense of some of this surplus sugar. The starch 

 can readily be detected in situ by treating the leaf with iodine, 

 after bleaching it by soaking it in alcohol. This appearance of 

 sta.rch was formerly held to be the last stage in the ordinary 

 anabolic processes, but as starch must be re-converted into sugar 

 before protoplasm can use it as food, its occurrence is now held 

 to be a case of what we shall see is a very usual thing in plants, 



