419 
Alisma Plantago aquatica L. 
Oceurs sparsely at the margin ot the pond. 
Covers the bottom between the Typha stalks on the north and east sides 
of the pond. 
These three phancrogams occur in the pond. Near the margin of the 
pond oceur Bidens and Carex, whose principal relation to it is that they 
cause the deposition of much of the silt before it reaches the pond. 
PLANKTON. 
The accompanying table records the observations on the more abundant 
and more strictly plankton erganisms 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 Ditf- 
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 smaller than C. bicuspidatus usually is. 
ECOLOGICAL RELATIONS. 
In the ecology of any association of organisms, two complicated prob- 
lems or sets of problems present themselves. ‘These are (1) how was each 
of these organisms introduced, (2) what factors condition their continu- 
