— 34 — 



The periodicity of fresliwater algal végétation was a well- 

 known fact, long before any précise observations on tins subject 

 were made. In our studies we were at flrst only interested in 

 determining the gênerai Unes of this periodicity, but it soon 

 became apparent that much as to the conditions governing the 

 appearance, extent of development and reproduction of fresli- 

 water Algae might be learnt from such investigations, especially 

 if considcred in conj miction with meteorological data. Weliave 

 been exceptionally fortunate in obtaining the aid of Mr. E. J. 

 Salisbury, B. Se, F. L. S., who for four years (from Mardi 

 1906 to May 1910) collected fortnightly samples for us from 

 Barton's pond, a small pond near Harpenden. Mr. Salisbury 

 lias ail thetime remained in sympathetic touch with the work, 

 and much of the value of tins instructive séries is due to lus 

 extrême care and efficiency as a collector. It gives us great 

 pleasure to thank him most cordially for his valuable lielp. 



Barton's pond is particularly well suited for an investigation 

 of tins type for two reasons; firstly because of its veiy rich and 

 varîed algal flora, secondly because an exceptionally complète 

 and reliable séries of meteorological data are available in the 

 publications of Mr. J. Hopkinson, F. L. S. on the weather of 

 successive years in Hertfordshire (see Trans. Hertfords. Nat. 

 Hist. Soc. and Field Club.) 



In view of the fact that the bulk of the samples from Bar- 

 ton's pond were collected at almost regular fortnightly inter- 

 vais, the monthly occurrence of the différent gênera and species 

 eau be stated with much greater précision than was the case 

 with respect to Abbot's Pool(Fritsch and Rich, 1909). In fact 

 we think it unlikely that there arc niany errors in our conclu- 

 sions as to relative frequency of gênera and species, since the 

 sample for one liait" of the month nerved as a check for the 

 other. The samples (nearly one hundred in number) bave been 

 worked through several times,both by each of us independently. 

 and also in collaboration. 



Finally, we think it is worth drawing attention to the fact, 

 that the labiés of occurrence and frequency of the différent 

 gênera and species were in most cases drawn up before any 

 coniparison with the available meteorological data was under- 

 taken. In tins wayall bias bas been avoided. 



