FLOATING PLANTS 115 



in size by fragments of Diatoms and other organisms, 

 which become entangled in the meshes ; the efficiency 

 of the net as a filter is thus reduced, and the total 

 catch becomes smaller in quantity but contains 

 organisms of smaller size than were captured when the 

 net was new. The decrease in permeability in such 

 nets is progressive with use, so that finally they will 

 hold water like a bucket. Such nets should be dis- 

 carded, or washed by rubbing gently in soap and 

 water, though after such treatment they are liable 

 to rupture in use. For the care of silk nets, see 

 p. 189. 



Owing to the small size of the plants, the phyto- 

 plankton suffers less damage than the other organisms 

 when collected from the water pumped through the 

 ship's hose. This method, however, is not to be 

 recommended if any other is available, as the or- 

 ganisms always suffer some damage from the rough 

 treatment to which they are subjected. The fact that 

 a certain amount of grease from the ship's pumps 

 always finds its way into the collecting net, and that 

 the water necessarily comes from one particular depth, 

 — that of the intake — are also further objections to 

 the method. 



For the capture and detention of the minutest 

 plants numerous special methods have been devised, 

 but so far as we know not even their authors are as 

 yet wholly satisfied with them. For collection at the 

 surface, nothing is better than to dip up a series of 

 buckets full of water, and to empty them into a filter 

 of closely woven undyed silk fabric (not a bolting- 

 cloth). Below the surface, but still at small depths, 

 Lohmann has used a hose lowered to a known depth, 

 and by a small hand-worked centrifugal pump deliver- 

 ing into a filter, has obtained very good results. 





