— 93 — 



fourid relation between meteorological data and the frequency 

 of the algal flora of Barton's pond lias bocn shown to exist in 

 a considérable number of cases. It may be well at tins point to 

 emphasise once again that the tables of frequency were made up 

 before any référence to meteorological data was made, so that 

 no personal bias lias entered into our conclusions. It is probable 

 that further observations of a similar nature on sraall pièces of 

 water may tend to modify some of the conclusions to which we 

 hâve corne, but in certain cases the évidence seems so overwhelm- 

 ing that we do not anticipate such a resuit. It still remains to 

 be seen however how far the relations déterminée! in the case of 

 the algal représentatives of Barton's pondapply to the gênera as 

 a whole. It may well be for instance that other species of Oedo- 

 gonium, Microspora, Conferva, etc. may be influenced by 

 conditions différent from those which détermine the frequency of 

 the forms présent in Barton's pond. That such différences bet- 

 ween species exist is shown by the behaviour of the two species 

 of Trachelomonas and of Spirogyra varians in the pond, as 

 well as for instance by West's observations on Peridineae 

 (West, 1909a; cf. also Fritsch, 1906). 



For the détermination of the relation between frequency of 

 freshwater Algae and outside conditions small pièces of water 

 like Barton's pond are alone suitable. In larger pièces of water, 

 especially in lakes, the changes produced by meteorological 

 influences are brought about so slowly that greater annual uni- 

 formityisattained and effects could probably only be satisfact- 

 orily stuclied overalong period of years; moreover the difficulty 

 of obtaining a représentative sample is almost insuperable in 

 large pièces of water. Thèse remarks do not apply to the typical 

 Plankton, which, owing to its ubiquity in a pièce of water, is of 

 course very readily collected, and which in a given expanse of 

 water tends to show pronounced and apparentiy rather uniform 

 annual periodicity. In the case of the Plankton of large pièces 

 of water however more lias been learnt regarding the conditions 

 governing abundance of the différent forms by a comparative 

 study of différent lakes than from periodicity (see West and 

 West, 1909 and 1912; also Zacharias, 1899). 



In an earlier paper by one of us (Fritsch, 1900, pp. 159-101), 

 the factors influencing aquatic algal growth were classified into 

 three groups, — seasonal, irregular and correlated. Seasonul 

 factors, which are probably the chief déterminants in larger 



