— 52 — 



ing thebest confirmation of the view put forward in an earlier 

 paper (Fritsch and Rich, 1909, p. 39), that the autumnal Spiro- 

 gyra-ipha.se, when itoccurs, isdue to végétative propagation of 

 a t'ew persisting filaments and not to germination of anj of the 

 zjgospores produced in spring. For, if the latter were the case 

 there is no reason why Barton's pond should not show an 

 autumnal Spirogyra-iplrnse as pronounced as that in Abbot's 

 pool, for the conditions for ils development must certainly hâve 

 been favourable in one of the fouryears that we had the pond 

 under obsei'vation (1) . In tins connexion attention may be 

 drawn to the fact that the four species of Spirogyra présent in 

 Barton's pond ail formée! zjgospores regularly before they dis- 

 appeared. Tins is very différent from what was observed in 

 Abbot's pool, where two species (S. (//finis [Hass.], Petit and 

 S. rivularis, Rabh.) never reproduced by zygospores during 

 the whole period of observation (cf. Fritsch and Rich, 1909, 

 p. 47). Such species could probably not exist more than one 

 season in a pond like Barton's pond. 



The almost complète absence of Zygnemaceae &) during 1909- 

 1910 bas already been referred to above (p. 50). There is little 

 in the meteorological data in the autumn of 1908 or early months 

 of 1909 that would seem sufficient to account for tins, although 

 November, January and February had an exceptionally low 

 rainfall going hand in hand with a rather larger amount of 

 sunshine than the average. We cannot however draw any safe 

 conclusions as to the eftect of thèse conditions owing to the pro- 

 bable disturbing influence of the ducks, when introduced in 

 April 1909. 



On the other hand the considerably later dominance of ail 

 three gênera of Zygnemaceae in 1907 and 1908, as compared 

 with 1906 corresponds well with meteorological conditions. 

 Thus the total rainfall for the six months preceding April (i. e. 

 the time at which Zygnemaceae ordinarily become important in 

 the pond) in the différent years is as follows: 



October to 1905-6 1906-7 1907-8 1908-9 

 Mardi. 13" 68 in. 16 02 in 16*95 in 9*64 in. 



(1) Itshould however be noted that noue of the species showini; an autumnal 

 phase in Abbot's pool are found in Barton's poml. 



(2) Spirogyra was airain observed in the last sample (May 1910) collected, but 

 the other two forais had not yel reappeared. Ail were lbund on a subséquent 

 occasion. 



