PELAGIC PLANT LIFE 



3^3 



as 1 200 c.c. of sea-water. It made 700 to 800 revolutions per 

 minute, and after eight minutes the plants were all collected at 

 the bottom of the glasses. Our next proceeding was to pour 

 away the clear water, and after rinsing the deposit, to put it 



in a smaller glass with a tapering 

 bottom, where it was subjected to 

 the action of a small hand-centrifuge. 

 In this way we collected all the con- 

 tents of, say, 300 c.c. of sea-water in 

 one drop, which we examined in a 

 counting chamber beneath the micro- 

 scope, and noted carefully each single 

 organism. As a rule we had to 

 centrifuge the whole 300 c.c, but, if 

 the plankton was very abundant, 150 

 c.c. or even 100 c.c. might suffice. 

 Examination with the microscope is 

 always more difficult when the or- 

 ganisms in the counting chamber lie 

 close together. 



These investigations were carried Smallest 

 out all the way from the Canaries to Zo£m 

 Newfoundland, and thence to the in the open 

 Irish coast banks, and resulted in 

 our discovering that the smallest 

 organisms which pass right through 

 the silk nets are far more abundant 

 than the others in the open sea, 

 while the larger diatoms and peridineae 

 would appear to be so scanty that 

 the total of all their species together 

 only amounts to about ten per litre. 

 Despite this fact, however, we found 

 in the samples taken with our nets 

 that there were at least fifty species 

 Glasses of these larp-er forms at every station, 

 SO that as far as species go the flora 

 is exceedingly rich. 

 We were also able in this way to determine the occurrence Amount of 

 of algae at different depths. Samples from the surface, and §Jj-"Jejjf "^^ 

 from 20, 50, 75, and 100 metres were taken regularly, and depths. 

 we also examined samples now and then from still greater 

 depths. We found, invariably, however, that the plant life 



Fig. 25 



. — Centrifuge 

 AND Pipettes for use with 

 Lohmann's hand-centrifuge. 



