- 53 — 



A very heavy rainfall in the preceding months (1) therefore 

 Leads fco a Later appearance ordominance of the Zygnemaceae in 

 the subséquent spring. This is the same conclusion as we 

 arrived at with référence to the occurrence of Spirogyra in 

 Abbot's pool® (Fritsch and Rich, L909, p. 38), and in the case 

 of Barton's pond we are able to extend the same observai ion to 

 Zygnema and Mougeotia. We hâve already in our earlier 

 paper (toc. cit.) fully considered the possible modus operandi 

 of sucli meteorological conditions. In Abbot's pool thèse con- 

 ditions led to a practically complète suppression of the ensuing 

 Spirogyra-phase (in two out of the flve years of observation), 

 while in Barton's pond in 1907 and 1 ( .»08 they only led to a 

 later development. This may be due to the fact that in Barton's 

 pond a normal water-level {i. e. the requisite degree of concen- 

 tration) is attained early enough in spring (light- and tempéra- 

 ture- conditions still being sui table) to admit of germination of 

 the zygospores, while in Abbot's pool with its larger body of 

 water this was not the case. The matter is however reniark- 

 ably complicated and no clear elucidation can at présent be 

 arrived at (3) . 



Of the four species of Spirogyra occurring in the pond, 

 S. calaeniformis and S. tenuissima are the hardiest, appear- 

 ing earlier and generally persisting longer than S. quadrata. 

 Such stray filaments of Spirogyra as were found during the 

 late autumn and winter appeared also invariably to belong to 

 the two former species. A peculiar feature is found in the very 

 isola ted occurrence of S. varians, viz. only in May of 1907: at 

 this time it was présent in great abundance and formed numer- 

 ous zygospores. A careful search lias failed to reveal it in any 

 other sample. In Abbot's pool this species was a great feature, 

 but in comparison with the other species of Spirogyra présent 

 in that pond, it was characterised by its very limited period of 



(1) No direct relation to amounts oJ previous sunshine is apparent. 



(2) In the case of Abbot's pool we took the total rainfall of the last four months 

 ofthe previous year; if this were done in the case of Barton's pond, the resuit 

 would be exactly the same. The method of calculation adopted ahove however 

 seenis to be more logical, and it may be added that, il" applied to the case oi 

 Abbot's pool, it makes no altération in our conclusions necessary. 



(3) It is possible that the extremely low rainfall preceding Mardi 1909 led to 

 too great a concentration of the water and thus eut out the Zygnemaceae, 

 which may be adapted to a detinite concentration. 



