CHAPTER VI. 

 THE PHOTOSYNTHETIC SYSTEM. 



/. GENERAL CONSIDERATIONS. 



The term assimilation is used by plant physiologists sometimes in a 

 wider and sometimes in a narrower sense. It may be defined so as to 

 comprehend all the metabolic processes whereby organic or inorganic 

 food materials are incorporated in the living organism. Schleiden, 

 Pfeffer and Wiesner, among others, employ the term in this wide 

 sense, and the same usage prevails among animal physiologists. 

 Sachs, on the other hand, limits its application to the most funda- 

 mental and remarkable of all assimilatory processes, namely, that in 

 which organic materials are synthesised from carbon-dioxide and water, 

 while oxygen is evolved as a by-product. Every vegetable organism, 

 whether green or otherwise, "assimilates" in the wide sense of the term, 

 whereas assimilation in the narrower sense is almost exclusively 

 confined to plants that are provided with chloroplasts. 



If the comprehensive definition of assimilation be maintained, then 

 obviously no special assimilatory system exists ; this statement holds 

 good even if attention is strictly confined to the incorporation of those 

 food-materials that are employed for the synthesis of carbohydrate 

 and proteins (to the exclusion of the " ash-constituents," which are left 

 as a residue when the other components of the plant-body are destroyed 

 by combustion). Since it has been proved by experiment that the 

 presence of chlorophyll is not an essential condition for the formation of 

 protein compounds, we cannot a priori assume any living cell of a plant 

 no matter to what tissue-system it may belong to be incapable of manu- 

 facturing protein substances. Whether all the living cells of a plant do 

 participate to an equal extent in this process is another question. The 

 principle of division of labour may quite conceivably hold good in this 

 respect, as it does in the case of so many other functions ; in other 

 words, the synthesis of protein or, in more general terms, the 



