Maech 9, 1900.] 



SCIENCE. 



383 



ment, but because of the fact that there is 

 a greater variety of forms at this time than 

 at any other time of the year. This is un- 

 doubtedly because of the peculiarly favor- 

 ing conditions of temperature. It is this 

 time of the year that seems to be especially 

 favorable to the growth of the algse. The 

 ' water-bloom' may appear in June and re- 

 mains sometimes until into September, but 

 it is in July and August that it is present 

 in the greatest quantity. This summer, on 

 Lake Winnebago, it was most abundant 

 during the latter half of August. 



The minimum of plankton development 

 is in the winter months, especially in Janu- 

 ary and February. At no time of the year, 

 however, are either plant or animal forms 

 wanting, and collections made through the 

 ice will give a considerable variety of kinds, 

 as well as numbers of individuals. 



It will be noticed that the period of max- 

 imum plankton development corresponds 

 to the time of highest temperature of the 

 ■water, and that the period of minimum 

 development corresponds to the time of 

 lowest temperature, so that we can be cer- 

 tain that the one important controlling 

 factor in the growth of plankton is the tem- 

 perature. 



Hensen and his co-laborers who worked 

 out a very elaborate system of measuring 

 the plankton of the sea claimed that the dis- 

 tribution of the plankton over wide stretches 

 of the sea was nearly uniform. Granting 

 this to be true, it evidently is possible by 

 a series of collections and measurements to 

 compare different parts of the ocean in re- 

 gard to their productiveness in animal and 

 plant life. This conclusion, that the hori- 

 zontal plankton of the sea has a practically 

 uniform distribution, has been accepted by 

 the majority of scientists, although vigor- 

 ously combated by Hackel. 



Apstein has applied the same methods to 

 the examination of lakes, and has concluded 

 from his investigations that the plankton 



of lakes is uniformly distributed. It is 

 easily seen that there is very good reason 

 for assuming this to be the case from the 

 actual conditions under which the plank- 

 ton exists. The plankton consists of organ- 

 isms that do not move voluntarily from 

 place to place, and therefore do not change 

 their positions. They are dependent upon 

 heat and light for their development, so 

 that their growth is mainly within from 

 twenty to forty feet of the surface, so that 

 a depth exceeding this would not mean any 

 greater production of plankton — or in other 

 words, the amount of plankton depends not 

 upon depth, but upon surface. 



This is a most important conclusion, if 

 true, for it gives us an exact method by 

 which we can compare one lake with 

 another and determine their relative pro- 

 ductiveness, or from a series of collections, 

 determine the absolute annual productive- 

 ness of any body of water. Such deter- 

 minations would have an important com- 

 mercial value, for by them could be 

 estimated the possibilities of fish produc- 

 tion in a lake. The method was worked 

 out in detail and very elaborately by Hen- 

 sen. The collection was made by a con- 

 ical net of bolting silk drawn vertically 

 through the water, thus straining out the 

 organisms of a column of water of the size 

 of the opening of the net. Then the ma- 

 terial was counted under the compound 

 microscope by a very laborious process. In 

 this way exact numerical values can be 

 obtained not only for the plankton as a 

 whole, but for the individual constituents. 

 Most investigatoi's use Hensen's methods 

 with greater or less modifications, and they 

 have been productive of very fruitful re- 

 sults. But, unfortunately — I say unfortu- 

 nately, because we all like to claim exact 

 results, and are prone to think that nature 

 works according to certain inflexible laws — 

 much of the laborious detail of the work 

 is a waste of time. 



