H4 THE PLANTS 



upon the bodies of other animals or plants. They are 

 able to manufacture their own food from the simple 

 gases and mineral substances in the water. The 

 external " power," by means of which their " food- 

 factory " is run, is in the light of the sun, which they 

 intercept by the help of various colouring matters. 

 The plants of the sea are thus able to increase and 

 grow independently of the other marine organisms, 

 and so ultimately they must be the source of the 

 food-supply of all the animals which can only live by 

 preying upon one another or upon plants. 



The colouring matter used to trap the light is on 

 land nearly always a green one ; in coastal plants it 

 may be green, red, or brown ; in the open sea most of 

 the members of the phytoplankton show a brown 

 pigment. Only a few are green. 



Collection. The phytoplankton is, of course, col- 

 lected in the same way as the animal plankton. For 

 the various types of nets, etc., reference must be made 

 to the section dealing with the Floating Animals. Owing 

 to the small size of most of the vegetable organisms, 

 the finest silk gauze, of 180 meshes to the linear inch, 

 should be used. Even then the meshes of such gauze 

 are far larger than many forms of the phytoplankton 

 e.g., the Coccospheres and Rhabdospheres, and the 

 smaller Diatoms so that quite 50 per cent, of the 

 possible catch may escape. The capture of all, or 

 nearly all, the phytoplankton in a sample of water 

 can only be achieved by slow and elaborate methods 

 (filtering through special filter paper or through a 

 porous filter of the Chamberland type), which are 

 not suitable for use on board ship or for any con- 

 siderable bulk of water. It is to be noted that nets 

 which have been in use a few times become less per- 

 meable ; this is due to the pores becoming reduced 





