— 86 — 



or June and attaining its çhief development during the last four 

 monthsof theyear. T.volvocina differsin frequentlyalso reach- 

 ing a maximum in February and Mardi (cf. Brunnthaler, 

 1907, p. 195). The maxima of the species of Trachelomonas 

 do not coincide except in the autumn months with any particu- 

 larly abundant development of Euglena, and it does not seem 

 likely that the frequency of Trachelomonas is governed by the 

 same conditions which control the abundance of Euglena. In 

 fact there is evensome slight évidence of the addition of organic 

 substance to the water by decay of the fi lamentons fiora 

 being unfavourable to the development of the species of Trache- 

 lomonas, as testified to by the greater abundance of the genus 

 in August of 1907 and 1909 as compared with 1906 and 1908 

 (compare with table on p. 84). But it is quite évident that otlier 

 factors are concerned, even if theone just suggested really 

 applies, and we hâve found it impossible to obtain an insight 

 into thèse factors. Nor can we otter any explanation for the 

 différent periodicity of the two species of the genus. 



J". — Peridinium tabulatum, Ehrenb. 



Tins organism is a rather constant, though rare, constituent 

 of the flora of Barton's pond, and a brief considération of its 

 periodicity is therefore advisable. It was best represented in 

 1907 and 1908, appearing very constantly in themonth of April 

 and lasting generally until December with an ill-defined maxi- 

 mum during the warmer months. West (West, 1909a ; see 

 also West and West 1912, p. 427) lias recently investigated the 

 periodicity of certain Peridineae occurring in pools in Sutton 

 Park, Warwickshire and demonstrated a marked connection 

 between their frequency and the température, some of them 

 being cold-water, others warm-water forms Although the 

 member of Peridineae présent in Barton's pond never attains 

 to great abundance, its relation to warmer températures is quite 

 obvious. This is shown in the first place by the attainment of 

 the maximum during the warm summer-months; further the 

 persistence of Peridinium till January of 1909 may be related 

 to the exceptional warmth of the preceding October and Novem- 

 ber; its persistence in greater quanti ty than usual into Decem- 

 ber 1907 corresponds to an exceptionally warm month with a 

 rather warm November preceding it. On the other hantl itsalmost 

 complète absence after April 1906 until October of that year indi- 



