419 



Alisma Plantago aquatica L. 



Occurs sparsely at the margin of the pond. 

 Covers the hottoni 1)et\veen the Typha stalks on the north and east sides 

 of the pond. 



These three phanerogams occur in the pond. Near the margin of the 

 pond occur Bidens and Carex. whose principal relation to it is that they 

 cause the deposition of much of the silt hefore it reaches the pond. 



PLANKTON. 



The accompanying table records the observations on the more abundant 

 and more strictly plankton organisms in the pond from Jan. 25 to Aug. 

 12, 1910. The most apparent fact is the dearth of organisms in the open 

 water during the extremely low temperature of January and February, 

 Euglena virides Ehr., and Euglena acus Ehr. being the most abundant. 

 A few rotifers were observed during the winter, but no marked develop- 

 ment of this class was observed until the latter part of April. Polyarthra 

 reached its maximum on May 28, and Monostyla in August. Hydatina is 

 strictly a summer form. 



Wesenberg-Lund ('08, p. 255) states: "Rhizopoda are, so far as my 

 experience goes, of quite secondary importance in the pond plankton." This 

 pond certainly differs from those of Denmark, for the development of Dif- 

 flugia is constant and fairly regular from February to August, when 297,800 

 per cu. m. were present. Actinophrys was very common near the margins 

 during May. 



There are two pulses of cyclops. A very slight one in April and an 

 enormous one in August. It is possible that some of the cyclops were 

 able to avoid the intake of the collecting apparatus. This of course would 

 make the members in the table too low. In April the cyclops were quite 

 evident in the shallow water near the shore. However, it was difficult to 

 apply quantitative methods to this region. During the August pulse, none 

 was seen near the shore. This may have been due to the fact, noted else- 

 where, that they were snmller than C. hiciispiiUitiif; usually is. 



ECOnOGICAL RELATIONS. 



In the ecology of any association of organisms, two complicated prob- 

 lems or sets of problems present themselves. These are (1) bow was each 

 of these organisms introduced, (2) what factors condition their continu- 



