— 55 — 



Mougeotia and Zygnema, aithough frequently persisting 

 till late autumn or winter in Barton's pond, can scareely be said 

 to show a decided autumnal phase. Tlio two gênera siniply 

 remain after their period of dominance in spring and early sum- 

 mer. gradually diminishing in abundance, until in the winter 

 they die out more or less completely. In 1907 alone we observed 

 a slight increase on the part of Zygnema in the earlj autumn, 

 so that a brief autumnal phase appeared ; tins may hâve been due 

 to the low rainfall in September of that year (involving un 

 almost complète drying up of the pond, cf. p. 35) (1) combined 

 with somewhat increased sunsliine as compared with previous 

 months and a relatively high température. 



B. — The Desmids. 



Desmids are to be found in Barton's pond nearly ail the year 

 round, the most barren months being January-March (specially 

 February), but during the whole of the autumn and winter the 

 représentation is very scanty. Ail three gênera tend to attain 

 their maximum at about the same time & \ ri:-, in late May or 

 June, and there is no very noticeable différence in the seasonal 

 distribution of the différent gênera. The Desmids therefore, as 

 already above indicated (p. 46), are at their maximum at a time 

 when the Zygnemaceae are generally markedly decreasing. In 

 comparison with the latter, however, they tend to persist much 

 more effectively into the latterpart of the year, appearing better 

 able to withstand the customary drying up of the pond during 

 the summer. 



The years 1907 and 1908 afforded a much richer Desmid-flora 

 (specially with référence to Cosmarium and Closterium) than 

 did 1906 or 1909. In the case of 1909 we may think of a possible 

 influenceof the ducks {cf. p. 49j,but this does not apply to 1906. 

 We may again seek for an explanation in the meteorological 



(1) The increase of Zygnema occurred in the first half of September and 

 before the drying of the pond, i. e. during the period of shrinkage. 



(2) It should be noted however that Closterium is generally a little later in 

 attaining its maximum than Cosmarium: 



