414 



SHARITZ AND LUVALL 



o 



75 



25 

 20 

 15 

 10 



5 







120 

 115 

 110. 



80 

 70 



*=74 



25 



11 



23 



25 



18 



en 

 Q 



O 



tr 



O 60 



(X. 



50 — 



40 — 



CO 



< 



LU 



ir 30 



20 — 



10 



T 



m^ 



T 



12 3 4 

 Population 1 



12 3 4 

 Population 2 



I 



12 3 4 



Population 3 



WEEKS 



12 3 4 



Population 4 



12 3 4 

 Population 5 



Fig. 1 Rate of frond increase in five populations of Spirodela 

 oligorrhiza under five temperature regimes: constant exposures to 

 10°C (A— A), 17.5°C (C^O), and 25°C (•-•); cyclic exposure at 10 

 to 25°C (n— n); and acyclic exposure at 10 to 25°C (A— A) over a 

 4-week period. Differences between temperature treatments greater 

 than the bars at the top are significant (a = 0.05) (multiple 

 significant difference; Morrison, 1967). N = 24. Populations are 

 described in Table 1. 



constant 17.5°C regimes during the early weeks disappeared in the 

 later part of the study as growth rates were reduced. 



Growth differences among the local populations of S. oligorrhiza 

 were seen under the high-temperature treatments. Plants from the 

 sewage-effluent pond grew less under all regimes, and those from the 

 roadside ditch generally showed the highest rate of population 



