GROWTH OF DUCKWEED 415 



growth. The significance of these differences cannot be determined 

 without further investigation. 



DISCUSSION 



Differences in growth rates among populations of S. oligorrhiza 

 were expected. Several investigators suggested that variations in 

 grow^th rates of L. minor populations may be related to genetic or 

 environmental differences (Wangermann and Ashby, 1951), age of 

 the original parent plant (Claus, 1972), annual cycles (White, 1936), 

 or other cyclic variations (Dickson, 1938). Because of the observed 

 differences among the five S. oligorrhiza populations in this study, 

 discussion is restricted to within-population comparisons across 

 temperature treatments. 



Only limited frond mortality was observed in the S. oligorrhiza 

 plants grown during the study. Although little information is 

 available on longevity in this species, Rejmankova (1973a) indicated 

 that fronds of Lemna species in Czechoslovakian lakes usually live 

 about 4 weeks. Frond length of life is related to environmental 

 conditions, especially temperature (Ashby, Wangermann, and Winter, 

 1949; Ashby and Wangermcinn, 1951). This study was designed to be 

 completed within the projected lifetime of a single plant. 



Spirodela oligorrhiza demonstrated highest population growth 

 rates at a constant temperature of 25° C. Only during the final week, 

 as crowding in the culture chambers became apparent, was growth at 

 this temperature reduced. Similar curves consisting of an exponential 

 growth phase, a linear growth phase, and a later steady-state stage of 

 population maintenance were described by Clatworthy and Harper 

 (1962) in laboratory populations of Lemna species. At the lower 

 extreme, the 10°C constant-temperature regime severely restricted S. 

 oligorrhiza frond multiplication. This pattern is consistent with the 

 report by Jacobs (1947) that optimum growth of S. polyrhiza 

 occurred at 25° C and the population deteriorated at 7°C. 



The relationship between temperature and growth in other 

 members of the Lemnaceae has been well established. In an early 

 study of L. minor. Hicks (1934) reported an exponential increase in 

 frond number at temperatures between 15 and 30°C. Above 35° C, 

 the growth rate declined rapidly. Similarly, Ashby and Oxley (1935) 

 reported that the relative multiplication rate of L. minor fronds 

 increased linearly with temperature up to 20° C and confirmed that 

 temperatures above 35° C were deleterious to frond increase. At a 

 light intensity of 500 ft-c, frond production in L. minor was twice as 

 fast at 30° C as at 20° C (Ashby and Wangermann, 1951). More 



