GROWTH AND METABOLISM 121 



corresponds to the formation of carbohydrates as the prin- 

 cipal photosynthetic products. Because of this it has been 

 supposed that Hpide formation does not take place directly 

 from the primary products of photosynthesis^^ but, as we 

 have already seen (p. 44), this does not appear to be so. 

 Measurements of photosynthetic quotients appear never to 

 have been made using material known with certainty to be 

 capable of accumulating lipide in quantity and it seems 

 likely that the explanation of the apparently discrepant 

 values is that diatoms transferred from growing cultures to 

 a medium deficient in nitrogen are not in a condition for the 

 immediate rapid synthesis of lipide. It is to be expected 

 that in a growing organism the enzymic equipment will be 

 adapted to produce protoplasmic constituents and that this 

 system, if transferred to conditions under which growth 

 cannot take place, will not immediately be reorganized and 

 will meanwhile produce such temporary reserve products 

 as can be synthesized with the enzymes available. This sup- 

 position is not necessarily inconsistent with the idea that the 

 high respiratory quotient of growing cells transferred to 

 nitrogen deficient conditions indicates fat formation (p. 112). 

 Fat formation might proceed at the same rate in photo- 

 synthesizing cells but at high light intensities its effect on 

 the photosynthetic quotient would be slight. On prolonged 

 incubation under nitrogen deficient conditions or when the 

 transition to conditions under which growth cannot occur is 

 less abrupt, as during the ageing of a culture, there is oppor- 

 tunity for the enzymic equipment to be reorganized for the 

 production of high proportions of lipide from excess primary 

 products of photosynthesis. 



This hypothesis receives support from the finding that 

 different nitrogen fractions, including enzymes, in micro- 

 organisms are affected to different extents by nitrogen- 

 starvation^^^ and from determinations of the relationship 

 between the lipide and total nitrogen contents of various 

 algae. ^* In Fig. 18 the fatty acid contents of six species have 

 been expressed as a percentage of the total content of re- 

 serve material Ctaken as dry material other than protein) 

 and plotted as a function of nitrogen content. From this it 



