President's Address. 343 



of the process, as opposed to the older one of Liebig that they are 

 organic acids, is founded upon such considerations of gas interchange 

 supported by anatomical facts. 



But although assimilation theories are based upon this assumption 

 (it may be noted in passing how completely the nitrogenous con- 

 stituents of the chlorophyll-corpuscles are shut out from any influ- 

 ence in the process, and there is, indeed, at present no reason for 

 supposing that they have any), it is not a necessary consequence. 

 Any such conclusions draAvn from the constancy of the gas volume 

 in gas interchange would hold if green organs only assimilated and 

 did not respire in light. But with respiration taking place the 

 conditions must be different, because oxygen is thereby inhaled ; and 

 although this oxygen enters into combinations, one of the products 

 of which is carbonic acid, yet the volume of the gases in this inter- 

 change are not equal More oxygen is inhaled than carbonic acid 

 exhaled. Germinating seeds rich in starch cannot, as has been 

 pointed out, be simply or fairly compared with green organs of adult 

 plants in respect of their respiration, but in germinating oily seeds 

 more oxygen is evidently inhaled than carbonic acid exhaled ; and 

 in green organs this inhalation of oxygen increases considerably in 

 light. Respiration, then, in green organs exposed to light in a 

 limited atmosphere necessitates a diminution in the gas volume 

 thereof. 



If, now, the gas volume around an assimilating and respiring 

 plant remains constant, the immediate product of the reducing 

 process must be a substance poorer in oxygen than a carbo-hydrate, 

 and poorer by that amount of oxygen used up in the respiratory 

 process. This conclusion is inevitable if the carbon compounds are 

 directly formed from the carbonic acid and water. 



But, it might be supposed that the constancy of gas-volume ob- 

 served under certain conditions only occurs if there is a definite 

 amount of respiration, only if the primaiy reduction product combin- 

 ing with oxygen is transformed almost entirely to a carbo-hydrate, 

 which then persists as a stable reserve substance in the chlorophyll- 

 corpuscles resisting further oxidation in light. The amount of 

 respiration in a green tissue would in that case influence not only 

 the observed gas interchange, but also determine the character of 

 the formed compounds deposited within the chlorophyll-corpuscles. 

 The function of these corpuscles is a double one, they assimilate 

 and they respire ; and one is naturally led, on this account, to the 

 hypothesis dealt with in the next chapter, that it is the accumulation 

 of colouring matter which brings about the formation of difi"erent 

 construction products in the chlorophyll-corpuscles, that is to say, 

 through respiration in the chlorophyll-corpuscle a primary rich-in- 



TKANS. EOT. SOC. VOL. XIV. 2 A 



