364 THE ALGAE 



but they tend to be masked by the other two groups in small bodies 

 of water: 



(2) Irregular factors : 



(a) Abnormal rainfall : 



(i) Species favoured by excessive rainfall. 

 (ii) Species favoured by drought. 



(b) Abnormal sunshine : 



(i) Species favoured by excessive sunshine. 



(ii) Species adversely affected by excessive sunshine. 



(c) Abnormal temperature : 



(i) Species favoured by low temperatures. 

 (ii) Species favoured by relatively low temperatures, 

 (iii) Species favoured by high temperatures. 



(3) Correlated factors : 



(a) Species depending on the enrichment of the water by 

 decay of other members of the flora. 



(b) Forms influenced in their development by competition 

 with others. 



(c) 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 which has shown 

 that reproduction is initiated by the presence of bright Hght. An 

 unusual concentration of the salts in the water during a period of 

 drought may, however, counteract the influence of sunshine. 



A study of algal periodicity in some ponds near Sheffield, to- 

 gether with the results of fortnightly analyses, has suggested a cor- 

 relation with the nitrate factor for some species. The maximum 

 for this occurs in December whilst there is a minimum in 

 June, and it was observed that Volvox received a severe check when 

 the nitrate was high and only reproduced at times of low nitrate 

 value. Ulothrix reappeared yearly in these ponds, whilst Euglena 

 annually attained to a maximum between July and August soon 

 after the nitrate minimtim. 



A similar study by Brown (1908) of some pools near the Uni- 

 versity of Indiana, revealed the fact that the species tended to attain 

 their maximum abundance in autumn and spring. In one pond 

 (Fig. 200) the phases were as follows : 



