PHYTOPLANKTON 



refractive pyrenoid which may be a food reserve. There is a single flagellum. 

 Its movements arc dehghtful to watch and it often spins rapidly on its 

 own axis, hideed this dancing effect prompted Butcher to call one of his 

 most graceful fmds Pavlova gyraus; this has two flagella of unequal length 



(Fig- 7; 2). 



These organisms reproduce by simply dividing into two halves (Fig. 7; 

 9(7 and h). When conditions are bad, they can form 'resting spores' which 

 can withstand very severe drying, or cold, and grow again into the active 

 form when things become normal again. 



(/)) The Diatoms 



These plants rarely exceed 1/50 inch in size. They cannot swim in the 

 accepted sense and have thick shells of silica. 



Until the importance of the nanoplankton was realized diatoms were 

 considered to be the main source of the productivity of the sea and the 

 fundamental food supply of the animals including the fish themselves. Even 

 considering the nanoplankton, their importance is immense. They are big 

 enough to be caught by the finest grade of plankton nets and can be studied 

 with a good microscope. Their hard shells or frustules of silica can be 

 chemically cleaned with acid, and mounted for microscopical examination 

 revealing a vast range of minute detail of form which has been a valuable 

 aid to classification. 



They belong to the class of Algae called the Bacillariophyceae and each 

 consists of a single cell, although these may remain attached to each other 

 after reproductive division, so that chains of cells are formed (Plates I and 

 VII). Each cell has a skeleton composed of two valves that fit into each other 

 like a piU-box and its lid (Fig. 8). When seen from the surface or bottom of 



Fig. 8. Diagrams showing the simple division of a pillbox-hi^e diatom: (a) the normal cell, 

 {b), (c) and (d) sections through a dividing cell. Note that the new lower cell in (J) is the same 

 size as the original but the upper cell is smaller. (From Professor Hardy's book, by kind 



permission.) 



the box we get a 'valve view' which is primarily circular (e.g. Plate I; 2a) but 

 may be a variety of shapes. When seen from the side we get a 'girdle view', 

 which is primarily rectangular (Plate I; 2/)) but also varies. It may be a 'flat' 

 pill-box or a 'tall' one, even needle shaped, and there are indeed many 



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