_ GROWTH AND METABOLISM IO9 



proportion of the protoplast, contains a high proportion of 

 materials such as pigments and other lipoids, which are inert 

 in so far as they are directly concerned in growth. The rela- 

 tive growth rate of C. pyrenoidosa^ for example, depends on 

 the protein content of the cells, not upon lipoid.^" Thus, 

 whereas under optimum conditions the substances assimi- 

 lated by a non-photosynthetic cell are largely used for the 

 synthesis of more material capable of growth (i.e. 'autosyn- 

 thetic', see ref. 145), a large part of the synthetic capacity 

 of an alga is diverted to the production of the non-growing 

 material of the chromatophore and it may be because of this 

 that the alga has the lower rate of growth under otherwise 

 comparable circumstances. However, were the situation as 

 simple as this, it would be expected that colourless algae 

 would show higher growth rates than the pigmented species 

 from which they are derived but this does not appear to 

 be so. 



The chemical kinetics underlying the growth sequence in 

 simple algae are presumably similar to those which have 

 been postulated to occur in bacteria.^^^ In material which 

 is not actively growing, enzymes may have denatured and 

 the concentrations of essential metabolites may have fallen, 

 so that a period of reconstitution is necessary before expo- 

 nential growth can begin. In agreement with this view it 

 has been found that in Anabaena cylindrica a marked in- 

 crease in the amount of nitrogenous substances per unit 

 h volume of algal material occurs during the lag and that the 

 length of this phase increases with the age of the inoculum. ^^ 

 The duration of the lag in A. cylindrica is dependent upon 

 light intensity, being longer the higher the intensity. ^*^ It is 

 possible that the establishment of the level of concentra- 

 tion of nitrogenous constituents necessary for exponential 

 growth is directly retarded as carbon assimilation becomes 

 more intense but other explanations of this phenomenon 

 are possible, e.g. photo-oxidation of some essential meta- 

 bolite may occur at higher light intensities. Cases in which 

 exponential growth of algae evidently awaits the develop- 

 ment of enzymes adapted to particular substrates have 

 already been mentioned (p. 56). 



