- r>i - 



M. parvula, Ilass, generaUy accompanies the wider form and 

 may become slightly more abundant in July and August, 

 although found in small quantities at most other times of the 

 year. The Mougeotia found in Abbot's pool (likewise a broad 

 species) was also abundant during the colder part of the year, 

 ils more marked persistence during the winter-months in tins 

 case being probably due to the less exposed habitat. 



Zygnema, while appearing rallier later than Mougeotia, 

 may also occasionally persist right into the winter (1907). 

 Possibly the high températures, low rainfall, and strong insola- 

 tion of July and August 190(3 were responsible for its early 

 disappearance in that year, as compared vvith 1907 and 1908. 



Spirogyra, on the other hand, always disappears l'rom 

 October-April in Barton's pond, only isolated filaments having 

 been found in thèse months. In this respect Barton's pond 

 differs very appreciably l'rom Abbot's pool and some other ponds 

 in which \ve hâve studied tlie occurrence of Spirogyra (Fritsch 

 and Rich, 1907, p. 436). There is consequently no autumnal 

 Spirogyra -phase in Barton's pond, and \ve do not think the 

 reason is very hard to find. In Abbot's pool we had a pond, 

 which never dried up and in which in factthe water-level was 

 stated to remain fairly constant. The persistence of a few 

 Spirogyra-filïïai&nts, afterthe spring-phase was over, was there- 

 fore sufficient to account for the development of an autumnal 

 phase by a mère process of végétative reproduction. In Bar- 

 ton's pond, however, in which drying up is a regular occurrence 

 at some period of the summer (generally in August, but not 

 until September in 1907, cf. p. 35), such a persistence oîSpiro- 

 gyra-ûlsments is out of the question; even if the ponddoes not 

 dry up completely, the great shrinkage that invariably takes 

 place must lead to such a concentration of the water as to affect 

 very adversely so sensitive a form as Spirogyra (1 >. We be- 

 lieve therefore that the absence of an autumnal Spirogyra- 

 phase in Barton's pond and similar exposed pièces of water 

 tending to dry up in summer is due to the complète élimination 

 of ail the filaments of the Alga during the period of desiccation. 

 In fact the data supplied by Barton's pond furnish to oui 1 think- 



(1) If collections of Algae, includin^- Spirogyra and Mougeotia are placed in 

 a vessel in the lal>oratory, Spirogyra very soon shows symptôme of ill-health, 

 whereas Mougeotia remains healthy forquite a lon^ time. 



