n6 THE PLANTS 



From greater depths Kofoid's water-bottle brings up 

 20 litres of water, a quantity sufficient to yield a fair 

 sample of the minutest forms. The instrument is, 

 however, rather expensive (about £20) . 



Preservation of Material. — If only the larger forms 

 are required, the catch can be treated in the same way 

 as the other plankton (p. 367). A solution of formalin 

 (1 part in 6 to 10 of water) is perhaps the best general 

 preservative, but will dissolve the lime in Coccospheres 

 and Rhabdospheres. 



Nature of Phytoplankton. — The representatives of 

 the marine phytoplankton fall into six main divisions — 

 Diatoms, Peridiniales, Pyrocystales, Coccosphaerales, 

 Green Algae, Blue-Green Algae (Cyanophyceae), and 

 Bacteria. By far the larger number of forms are 

 found in the two first groups. 



The Bacteria are a very special group of plants, 

 mostly colourless, which, owing to their excessively 

 minute size, are not secured by the ordinary methods 

 of capture, and so will not be dealt with here. 



Diatoms. — Organisms of this class are found in all 

 marine water, and sometimes in such enormous num- 

 bers (for example, in Scotch lochs at certain seasons) 

 that a net, which is towed only for a short time, is 

 found after draining to be partly rilled with a pasty 

 brown mass. 



The distinguishing feature of all the diatoms is the 

 possession of a translucent coat or shell, consisting 

 mainly of silica. The shell is made of two halves, 

 which, in the case of circular forms, fit into one another 

 like the top and bottom of a pill-box, the edge of one 

 portion overlapping that of the other. In some forms 

 the double nature of the shell is clearly visible, while 

 in others it is hardly distinguishable. Inside the 

 transparent shell is the living body of the plant, of 



