— 47 — 



Excepl for the character of the summer-phase there is a 

 considérable degree of gênerai correspondence between the 

 périodicité of the algal flora of Barton's pond and Abbot's pool. 

 In both cases we hâve a winter-phase characterised bv the 

 présence of a hardy filamentous form {Microspora in Barton's 

 pond. CladopJiora in Abbot's pool) and Diatoms, a spring- 

 phase distinguished by the important part played by Zygne- 

 maceae, and a heterogeneous autumn-phase, whose most marked 

 peculiarity lies in the présence of Oscillarieae. Il is probable 

 thatthe periodicity will be found to be more or less on thèse 

 Unes in ail British ponds having a Diatom-Conjugate végétation. 

 The pronounced différence between the summer-phase in the 

 two cases is no doubt due to the customary shrinkage of the 

 water in summer in Barton's pond. 



There is also resemblance between the algal periodicity of 

 the two ponds inasmuch as the four main phases are distin- 

 guished from one another not only by the quality, but also by the 

 quanti ty of algal growth présent (cf. Fritsch and Rich, 1900, 

 p. 35). Thus, there is a great increase in the amount of Algae 

 in Barton's pond with the approach of the spring-phase, and 

 tins is truc both of the number of individuals and of the number 

 of species. On the advent of summer in a normal year there is 

 again a decrease, often sudden and very marked, and the num- 

 ber of species may for a time be restricted. In the autumn- 

 phase we again hâve a revival, however, both in numbers of 

 species and individuals. There is often no very marked 

 decrease in the amount of Algae with the arrivai of the winter- 

 phase, but lins statement does not apply to the number of 

 species, which generaliy shows an appréciable decrease as 

 compared with the autumn-phase a> . 



Il can hardly be doubted lhat the variations in numbers of 

 species in the différent seasons of the year dépend on changes of 

 température and light-intensity, on changes in the bulk of wa- 

 ter and conséquent altérations of concentration, and on changes 

 in the air-content of the water, which take place more or less 

 regularly at thèse times (cf. Fritsch, 1906; Fritsch and Rich, 



(1) It may be mentioned at this point that at cerlain times of the year the 

 samples exhibited a \ello\v ochre or deep red colonrowing to the présence oi 

 aliundant iron bacteria: this was observed in March 1906, March 1907 

 August 1907 and Febraary 1908. 



