187 



reaches tlie bottom v. hen ;t is released. The process is reversed in the 

 ease of the beetles. They clasi) a bit of sunken plant stem in order to 

 keep them at the bottom. A part of this often separates and is carried 

 toward the surface. It is by the innumerable repetition of these processes 

 that the mass of finely comminuted particles at the bottom of the pond is 

 foi'med. 



COMPAKISON WITH LAKES. 



The fundamental difference between this pond and a lake is that of 

 dimension. It has a smaller area and is not so deep. A pond has no 

 abysmal region but has some of the characters of the littoral and pelagic 

 regions of lakes. From this fundaioental difference, secondary differences 

 arise. The changes in level affect the relative depth much more in ponds 

 than in lakes. The lowering of the level of a lake one-half meter would 

 not affect its fauna to any marked degree, while the same diiference in 

 level occurring in a pond whose depth was a meter or less would pro- 

 foundly influence the orgajiisms inhabiting it. 



The temperature in all parts of the pond is near that of the atmosphere 

 above it. In this it resembles closely the shallow littoral region of some 

 lakes, e. g., "barren shoals" of Walnut Lake (Hankinson, '07). So far as 

 observed the diflierence in temperature in difCerent parts of the pond at 

 any given time has not exceeded 2° C. It is practically holothermous, the 

 thermocline and associated phenomena are absent. 



Forel ('04) has shown m the case of Lake Geneva, that the littoral 

 region is one of variety. This fact is perfectly familiar to all students of 

 lakes. In the same lake, one part of this region may be covered with 

 rushes (Scirpus), another by water lilies (N iimphea) , another by Potaino- 

 geton, while another may be barren sand or rock or an equally barren 

 marl bed. 



An individual pond lacks this variety. It is in this particular that this 

 pond and others like it differ most from the littoral region of lakes. (It 

 is more nearly comparable to a limited section of a lake shore.) If Typha 

 is introduced, it soon spreads over the whole area, limiting the light, ex- 

 cluding other phanerogams, and developing very uniform conditions over 

 the entire pond. If a pond is developed on the side of a hill so that silt 

 is carried into it, a muddy, barren condition exists over the whole area. 

 Ponds in the woods rarely de\'elop aquatic seed plants ; the leaves from the 



