— 49 — 



were scantilj représentée!. Several other cases of the same kind 

 were observed. A decrease in the amount of water does not 

 take place so suddenly and opérâtes more slowly, but its elimi- 

 nating effed on many forms is apparent whilst the water-level 

 sinks more or less steadily during warm weather. 



Table to show periodicity of more Important Algae from 

 April to kugust of 1906-1907 (1 >. 



It remains to mention one other pronounced disturbance in 

 the annual cycle. In April of 1909 ducks were placée! in the 

 pond, and the conséquences of their présence appear to hâve 

 been rather far-reaching. They remainecl throughout the sum- 

 mer, but were removed in the following winter. During the 

 whole period of their présence in the pond the algal flora was 

 very meagre. The flora had however been poorly developed dur- 

 ing the early months of 1909 (perhaps owing to rather unusual 

 meteorological conditions, cf. the tables on p. 36-38), so tliat it 

 was not well equipped to meet the deleterious influence of the 



(1) Regarding the symbole, cf. footnote 1 on p. 54. 



