METHODS OF PLANKTON RESEARCH. 531 



and plant life are present in snch small volumes; and, 

 secondly, that they are absurdly small for quantitative 

 estimations of a column of water or for finding the true 

 conditions in the area where the samples are taken. 

 This does not, "however, seem to be the case ; and with 

 regard to the first point, it is surprising what a mass of 

 material 15 c.cm. of water gives with the centrifuge, so 

 much that Lohmann had often to take less. The second 

 difficulty is also only apparent, because when the Hensen 

 plankton nets are used the sample of water taken for 

 counting bears an equally extremely small relation to 

 the quantity of water that has passed through the net. 

 The only real difference between the two former methods, 

 the net and pump and filter, and this method is, that in 

 the former the plankton is collected from relatively large 

 masses of water, and small quantities are taken out of this 

 as samples for counting, whilst by the centrifugal method 

 the small quantities are taken directly from the sea. In 

 stating this, however, we are again confronted by the 

 doubt as to the equal distribution of the plankton in the 

 sea, which will be mentioned later. Lohmann thinks 

 that the distribution is sufficiently like to allow of such 

 small samples being reliable guides to the quantitative 

 constitution of the plankton. 



In order finally to calculate the average number of 

 organisms present in a column of water, from which 

 various samples have been taken at various depths, the 

 following formula should be used. Assuming that the 

 samples A, B, C, D are taken at increasing depths, 

 separated by the distances a, b, c, then A is the average 

 for the column. 



Aa + B (a + b) + C (b + c) + Dc 

 A ~~ 2 (a + b + c) 



If one wishes to make a complete investigation of 



LL 



