416 SHARITZ AND LUVALL 



recently Hodgson (1970), after observing the growth of L. minor at 

 constant temperatures of 12.5, 17.5, 22.5, and 27.5°C, reported that 

 the population leaf area was highest at the 27. 5° C exposure but the 

 maximum net assimilation rate was achieved at 17. 5° C, with a 

 marked decline at higher temperatures. These results disagree with 

 those of Ashby and Oxley (1935), who indicated that the assimila- 

 tion rate in L. minor was independent of temperature between 18 

 and 29° C. Hodgson attributes the discrepancy to differences in 

 experimental conditions, especially illumination. There appears to be 

 general agreement, however, that population growrth (frond multipli- 

 cation) in the Lemnaceae under constant-temperature regimes 

 increases with temperature to a maximum at approximately 30° C. 



Combining the data from populations 1 through 5 in Table 1 

 yields mean frond densities of 3, 47, 54, 62, and 178 for the 10°C, 

 cyclic, 17.5°C, acyclic, and 25°C regimes, respectively. Because the 

 individual populations demonstrated different responses, these com- 

 bined means cannot be taken as predictions of response for any given 

 population, but they can be used to evaluate the relative importance 

 of constant and cyclic regimes. According to Tukey's HSD test (Kirk, 

 1968), two of the combined means must differ by at least 12 to be 

 significantly different at the 5% level. Thus the densities at constant 

 temperatures of 10, 17.5, and 25°C are significantly different, but 

 the densities under cyclic, 17.5°C, and acyclic regimes are not. The 

 interval of 12 required for significance was computed from a 

 mean-square error term that may have been inflated by inclusion of 

 the highly variable replicates from the 25°C regime. An interval 

 computed from a mean-square error term involving only the cyclic, 

 17.5°C, and acyclic populations would be smaller and might indicate 

 significant differences among means. This would not alter the 

 conclusion that cyclic and acyclic regimes had relatively little effect 

 on the growth response of duckweed, however. 



Although few efforts have been made to examine effects of 

 fluctuating temperatures on growth in the Lemnaceae, Rejmankova 

 (1973a; 1973b) demonstrated seasonal changes in growth of field 

 populations of L. minor and L. gihha and suggested that frond 

 multiplication is regulated by temperature during early and late 

 periods of the growing season. 



The effects of cyclic temperatures on growth of other organisms 

 have been much more carefully examined. Shimo (1977) demon- 

 strated the influence of diurnal thermoperiodicity on the growth of 

 sporelings and fronds of Porphyra and reported greater elongation of 

 plantlets grown at temperatures of 18 to 22° C when night 

 temperatures were lowered 4 to 8°C below day temperatures. Heath 



