— 50 — 



new arrivais in April. Spirogyra and Zygnema were com- 

 pletely absent during the whole of tins year, while Mougeotia 

 only appeared in very small quanti ty in Mardi ; Microspora, 

 Oedogonium and Conferva developed in the normal way, but 

 were very scantily représentée! ; Euglena, after attaining a 

 slight maximum in April, was very rare during subséquent 

 months. On the other hand the bulk of the unicellular and colo- 

 nial forms (Ophiocytium, Sphaerocystis, Cosmarkim, Closte- 

 rium, Sêaurastrum, Trachelomonas), as well as Chaeto- 

 phora do not appear to hâve been very appreciably affected at 

 first. Thèse forms however also ultimately suffered and from 

 August onward they were far less abundantly represented than 

 usual. It is a little difficult to say how far ail thèse diffé- 

 rences in the algal flora for 1909 are to be ascribed to the agency 

 of the ducks, since the summer of that year was cold and showed 

 a rather exceptional rainfall, so that during the whole of the 

 period the pond was Ml of water to above the normal level. 

 It is however significant that in the following spring (1910), 

 when the ducks had been removed, an abundant filamentous 

 flora of the normal kind again appeared, although the paucity of 

 unicellular and colonial forms still prevailed. It appears that 

 ducks are known to feed upon water-plants of certain kinds (1) , 

 so that they may hâve been directly responsible for the extrême 

 paucity of the algal flora; on the other hand tins may also 

 hâve been due to their stirringup the mud at the bottom of the 

 pond or to the fouling of the pond by their fseces. 



1). Considération ofthe periodiciiy of certain forms. 



A. — The Zygnemaceae. 



Of the three gênera of filamentous Conjugatae foUnd in Bar- 

 ton's pond, Mougeotia is certainly the most hardy {cf. also 

 Fritsch and Rich, 1909, pp. 36 37); it nearly always appears 

 first in the spring, it generally persists longer and may occa- 

 sionally (1907-1908) last during part of the winter. Tins applies 

 specially to the wide stérile speciesof Mougeotia foundin the 

 pond, which appears to be a cold-water form ordinarily attain- 

 ing its maximum during the spring-months; the mucli narrower 



(1) See Cambridge Natural History, vol. 9, Birds (by A. II. Evans), 

 p. 114. 



