54 



occurrence (from about Mardi to May, cf. Fritsch and Rich, 

 1909, p. 47). Tins may well indicate a spécial degree of sensiti- 

 veness to the conditions of the environment(c/. also Klebs, 1896, 

 pp. 236, 237), and it is conceivable that the set of conditions 

 necessary for the appearance of S. varians are only rarely rea- 

 lised in Barton's pond. In this connexion attention raay again 

 be dr'awn to the very heavy rainfall of the last four months of 

 1906(13 , 11 in.), which was followed by a low rainfall in the 

 flrst three raonths of 1907 (3'98 in., as comparée! with 7*86 in. 

 in 1906 and 6*22 in. in 1908). The months October, November 

 and December of 1906 are described as having a « cloudy « or 

 » very cloudy »- sky, while Mardi of 1907 is stated to hâve had 

 an « unusually bright sky « (.1. Hopkinson, loc. cil.) (cf. also 

 the sunshine-table on p. 37). It is therefore quite obvious that 

 there were very exceptional meteorological conditions in the 

 months preceding the appearance of S. varians in 1907, and 

 thèse may hâve afforded the necessary combination of factors 

 for the occurrence of this species. 



Table shotoing relative dbundance of Zygnemaceae in 

 Barton's pond 1906-1908 (1) . 



(1) vc= very common; c ==> common ; rc = rather common; rr = ratlier 

 rare ; r = rare ; w = very rare ; i = isolated ; o ~ no sample ; — = absent. 

 An exclamation mark indicates reproduction. 



