POND ASSOCIATIONS 339 



climatic conditions, such as rainfall and insolation, but also upon 

 what might be termed irregular microclimatic factors, e.g. 

 aeration in the body of water itself. In the case of many of the 

 species there is a profound relationship between the meteoro- 

 logical data and the frequency of the flora, e.g. Microspora and the 

 Protococcales with temperature, Oedogonium and Hormidium with 

 sunshine. The factors influencing the growth of aquatic algae are 

 (i) seasonal, (2) irregular, (3) correlated. The first group, which are 

 very obvious and need not be detailed, are principally of importance 

 for large bodies of water, but they tend to be masked by the other 

 two groups in small bodies of water: 



(2) Irregular factors. 



(i) Abnormal rainfall: 



{a) Species favoured by excessive rainfall. 

 {h) Species favoured by drought. 



(2) Abnormal sunshine : 



[a) Species favoured by excessive sunshine. 



{h) Species adversely affected by excessive sunshine. 



(3) Abnormal temperature : 



{a) Species favoured by low temperatures. 



{h) Species favoured by relatively low temperatures. 



{c) Species favoured by high temperatures. 



(3) Correlated factors. 



(i) Species depending on the enrichment of the water by decay 

 of other members of the flora. 



(2) Forms influenced in their development by competition with 

 others. 



(3) Forms influenced in their development by the presence of a 

 suitable host, e.g. epiphytic forms. 



A very definite correlation can frequently be established between 

 the amount of sunshine and the phenomenon of reproduction, the 

 latter process being most frequent when there is most sunshine. 

 This is in accordance with experimental work by Klebs (1896) who 

 showed that reproduction was initiated by the presence of bright 

 light. An unusual concentration of the salts in the water during a 



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