no METABOLISM 



(Hertzog, 1902). [Bernard (1904. Beih. z. bot. Centrbl. 6) was unable to 

 confirm Friedel's results, which must be considered as still requiring proof. 

 Compare also Molisch, Bot. Ztg. 62, I, i.] 



In the third place, the part played by sunlight in the process of assimilation 

 must be emphasized. Each of the methods mentioned above shows in the 

 clearest possible manner that the evolution of oxygen takes place only in the 

 presence of light and only in regions which are illuminated. The bubble method 

 demonstrates in the most self-evident way how assimilation ceases when a plant 

 is gradually removed from the window to the back of a room, and how the 

 process comes to a standstill even in light of relatively high intensity, which to 

 our eyes would appear still sufficiently intense for the purpose. At the present 

 moment we can only emphasize the fact that light is essential to the assimilation 

 of carbon-dioxide, and postpone till later a consideration of the question of 

 the quality and intensity of the light. We must first determine what is 

 the exact significance of light in carbon-dioxide assimilation, and what sub- 

 stances are produced from the carbon-dioxide, or in other words, we must 

 study the nature of the first products of assimilation. 



The result of the gas-analysis method of investigation, viz. that the volumes 

 of the carbon-dioxide operated upon and of the oxygen evolved are equal, pro- 

 vides us with a certain basis on which to work. The equality of the volumes 

 suggests, for example, that carbon-dioxide is decomposed into carbon and 

 oxygen. All experience, however, is contrary to the supposition that free 

 carbon is produced ; carbon never occurs as such in the plant, and it cannot 

 be built up into organic material when it is artificially supplied in that form 

 to the plant. All organic bodies contain in addition to carbon, at least hydro- 

 gen, and this can have been obtained only from the water which is present 

 everywhere in the plant. If we now assume that hydrocarbons, which are the 

 simplest organic substances, are those which originate in carbon-dioxide assimi- 

 lation, then the oxygen must have been derived not from the carbon-dioxide alone 

 but from the water also. If that be so, however, much more oxygen should be given 

 off free than is, as a matter of fact, found to be the case. The experiments 

 of BoussiNGAULT (1868) moreover showed that hydrocarbons cannot be further 

 worked up. On the other hand, the proportion observed to exist between the 

 carbon absorbed and the oxygen given off agrees entirely with the manufacture 

 of carbohydrate. Thus if we express the formation of starch or glucose schema- 

 tically we obtain the following equations : — 



6CO2 + 6H2O = QHjjOe + 6O2 



Carbon dioxide. Water. Glucose. Oxygen. 



or, 6COa + 5H,0 = C,Hi„05 + 60^ 



Starch. 



In both cases the relation is-p^ = i, as analysis itself also shows. The 



variations from unity are in many plants quite inconsiderable ; thus Bonnier and 

 Mangin (1886) found it to be in the ivy, i-o8, and in the horse-chestnut and 

 in Syringa, i-o6. These numbers tell us that a little more oxygen is formed 

 than can he accounted for by the above formulae. This excess of oxygen 

 reaches, however, a quite noticeable amount in other cases, for the same authors 



give the value of the fraction ^^ in Ilex as 1-24. We shall take another oppor- 

 tunity of returning (Lecture XVI) to these cases, and consider, in our pre- 

 liminary investigations into the origin of carbohydrates, the first-mentioned 

 plants only. The development of carbohydrates as a result of carbon assimi- 

 lation in green plants has, as a matter of fact, been established beyond all 

 doubt. 



Among the carbohydrates which result from the assimilation process, 



