GROWTH OF DUCKWEED 413 



TABLE 1 



NUMBER OF ADULT FRONDS OF Spirodela oligorrhiza GROWN 



IN 4 WEEKS FROM ONE ADULT FROND IN EACH OF FIVE 



TEMPERATURE REGIMES* 



♦Population 1, beaver pond; 2, sewage-effluent pond; 3, roadside ditch; 4, 

 chemical-effluent pond; and 5, natural stream. Values are X + standard error for N = 24 

 replicates for each temperature and population treatment. 



tValues in each row with like superscripts do not differ significantly (a = 0.05) 

 (Tukey's HSD test; Kirk, 1968). 



is given in Table 1. Significant differences (multiple significant 

 difference; Morrison, 1967) {a = 0.05) in population (clone) growth 

 occurred at the extremes of the temperature treatments. The greatest 

 number of fronds was produced by each population at a constant 

 temperature of 25° C. Except in the sewage-effluent population, 

 which showed reduced growth under all treatments (Table 1), the 

 increased growth at high temperatures was significant and repre- 

 sented the production of a daughter frond by each adult every 4 

 days. There was a significant reduction in growth at 10°C constant 

 temperature in three of the S. oligorrhiza populations, and all 

 populations produced fewer fronds at the low temperature than 

 under the other temperature regimes. Only one new generation was 

 produced at this temperature. Cyclic and acyclic thermal regimes 

 generally affected the growth rate of S. oligorrhiza in the same way 

 as exposure to a constant temperature equal to the mean of the 

 variable temperatures. In all populations, growth under cyclic and 

 acyclic conditions was not statistically different (o: = 0.05) from that 

 at 17.5°C, and in only one population did the two fluctuating 

 thermal regimes produce growth responses that differed from each 

 other (Table 1). A new frond was produced every 5 to 6 days under 

 these three treatments. 



In all populations rate of growth was highest at 25° C during the 

 first 3 weeks (Fig. 1). Four populations showed a decline in the 

 growth rate at this temperature during the final week of the study. 

 Occasional differences in growth rates under the fluctuating and 



