GROWTH AND METABOLISM II9 



is available. High lipide contents have been recorded for 

 representatives of the Xanthophyceae^* and the Bacilla- 

 riophyceae^^^ whereas the Phaeophyceae,^^^' 2'^» ^^"^ Rhodo- 

 phyceae^^^ and Myxophyceae^* rarely have a total lipide 

 content of more than 5 per cent. In view of the results 

 obtained with Chlorella pyrenoidosa and since various 

 species of Chlorophyceae and a diatom have been found to 

 contain approximately equal amounts of lipide after growth 

 under similar conditions,^^'' it has been suggested that the 

 accumulation of lipide which is regarded as characteristic 

 of classes such as the Bacillariophyceae is dependent more 

 on the environmental conditions under which such species 

 habitually grow than on genetically determined peculiari- 

 ties of metabolism.^^^ That inherent differences in capacity 

 to accumulate lipide do, nevertheless, exist among the algae 

 is shown by the fact that certain Myxophyceae do not 

 accumulate lipide even under conditions of nitrogen 

 deficiency.^* 



The general explanation of the phenomena just described 

 lies in the fact that carbon assimilation may continue in an 

 alga after cell division has been brought to a standstill. 

 This much was pointed out by Beijerinck in 1904^^ but it 

 is now possible to put forward more detailed evidence in 

 support of this idea and to suggest mechanisms for some of 

 the observed effects. 



Many of the factors acting to bring about the cessation of 

 exponential grov^h are without direct effects on photo- 

 synthesis or on the chemotrophic assimilation of carbon. 

 Thus, synthesis of fresh protoplasm cannot occur in the 

 absence of a sufficient supply of nitrogen whereas chemo- 

 trophic assimilation of carbon can continue under these 

 conditions. Photosynthesis is reduced but not totally in- 

 hibited by nitrogen deficiency.^**' ^^^' ^^^ Similarly defi- 

 ciency of most other nutrient substances except carbon 

 dioxide or the organic carbon source affects protoplasmic 

 synthesis more directly than carbon assimilation. Photo- 

 synthesis has been found not to be directly affected by 

 changes in reaction within widely separated limits, '^^' ^^^' ^^ 

 whereas the growth of many algae can only take place within 



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