GROWTH AND METABOLISM I07 



this and other reasons most conveniently considered 

 separately. 



GROWTH AND METABOLISM IN SIMPLE ALGAE 



Many studies have been made of the growth of simple 

 algae in laboratory culture (e.g. ^^o, 225, 88, 254, 302, i48) and it 

 is possible to give a generalized picture of the growth cycle 

 under these conditions. Following inoculation of an appro- 

 priate medium with the alga and its exposure to suitable 

 conditions of light and temperature, growth in cell nuni- 

 bers may begin immediately, as is generally the case in 

 Chlorella spp. and other Chlorophyceae,^^*^. 225 qj. after a lag 

 period, as in Anabaena cylindrica.^^ Growth is usually expo- 

 nential in the first few days, the cell numbers per unit 

 volume of medium (««) at any time {t) being given by the 

 expression: 



tit^riQe'^. . . . (24) 



where n^ is the number of cells per unit volume of medium 

 at zero time, e the base of natural logarithms, and r the 

 relative growth constant. This expression may be converted 

 to the form: 



log««=logwo+^^ • • • (25) 



and the accuracy with which it represents experimental 

 results may be demonstrated by the closeness of fit of the 

 points to a straight line when log rit is plotted against t. 

 Following the exponential phase is a period in which the 

 relative growth constant declines continuously and finally a 

 stationary phase is reached in which there is no further 

 increase in cell numbers (Fig. 15). When other measures 

 of growth, such as bulk or dry weight of alga per unit 

 volume of medium, are used, the pictur^es obtained are 

 generally similar to that just described but differ in detail 

 since cell volume and relative dry weight are not constant 

 throughout the growth cycle (Figs. 12 and 15). 



Algae growing under natural conditions show phases of 

 growth similar to that observed in culture, for example the 

 growth of phytoplankton populations during the spring 

 'outburst' in lakes generally follows a nearly exponential 



