— 81 — 



sàme in the différent cases ; thus while dilution of the water 

 apparently Led to the absence of Eudorina in 1908, it did not 

 seriouslv affect Gloeocystis, and whereas Pandorina was 

 almost completely élimina ted by the concentration of the water 

 in April of 1906, Sphaerocystis was not at once so seriously 

 affected. Finally, it may be well to mention that \ve liave 

 been unable to recognize anyobvious relation betWeen the sun- 

 shine-data and the frequency of the Protoccales, altliough 

 bright sunshine evidently agrées wellwith thèse l'orms. 



With référence to the other Protococcales mentioned on 

 pp. 42 and 43 ail except Ankistrodesmus falcatus and Chara- 

 cium Pringsheimii are very decided spring-forms, show- 

 ing such maximum as they possess in April, May or early 

 «Tune. We hâve scarcelv obtained snfficient data regarding the 

 two exceptions mentioned to say much about their periodicity, 

 but Ankistt^odesnms at anv rate also appears to hâve its maxi- 

 mum in spring. 



H. — The Cyanophyceae. 



Except for Anabaena catenula and Lyngbya versicolor 

 blue-green Algae are very poorly represented in the pond. The 

 Anabaena is a very deflnite summer-form, being generally 

 completely absent from November till June and therefore seems 

 to favour warmer températures. Apart from this however there 

 does not seem to be anyobvious relation to the temperature-data, 

 but some dependence on the amount of sunshine (1) appears to be 

 recognisable. In 1906 and 1909 the représentation of Ana- 

 baena was very scanty as compared with 1907 and 1908. In 

 the case of 1909, this may be due tothe poor development of the 

 Phanerogamic végétation under the influence of the ducks ; the 

 shadc thus norinally obtained by the Anabaena during its 

 period of abundance would be lacking. In the case of 1906 

 however this explanation does not apply. The numberof hours 

 of bright sunshine for the period from June to September in the 

 \ cars 1906, 1907 and 1908 is as l'ollows : 



1906 1907 1908 



971 hrs. 691 hrs. 816 hrs. 



(1) It may be noted hère that experiments conducted by one ofus hâve shown 

 that An&baena tends to grow lietter when ehaded than when exposed to a 

 BtroDg light. 



G 



