CHAPTER 3 



The plants of the phviktoii — the 'phytophiiiktoii 



There is an old, well-worn adage that 'all flesh is grass', and this is just as 

 true in the sea as on the land, but here the 'grass' is the phytoplankton, the 

 floating plants that are drifted by the water movements from place to place. 



Green plants containing chlorophyll are able to utilize dissolved carbon 

 dioxide, nutrient salts and the sun's energy to produce carbohydrates, 

 proteins and oils, the basic food materials of the animals. The main differ- 

 ence between the terrestrial and aquatic environments is that on the land 

 the water is in dark interstitial spaces between fairly stable soil particles 

 and the light and carbon dioxide are in the air above; in the sea there is no 

 such division and no stability. Light can penetrate sea water sufliciently for 

 photosynthesis to be effective at the bottom in only a very small fraction of 

 the total area of the sea, at depths less than 20 or 30 fathoms in the latitude 

 of the British Isles, too small a fraction to be worth considering in the general 

 chain of food production in the sea, and we can thus ignore the familiar sea- 

 weeds attached round our sea coasts. There can be no holdfast in the open 

 sea and the plants must float in the light zone, and floating, be carried by 

 the water movements from place to place, both horizontally in the currents 

 and vertically in areas of turbulence. 



If one looks at a sunbeam in a room one can see the dust particles floating 

 about, and the smaller the particle the easier it floats. So it is with the plants 

 of the open sea and they, too, are small, usually between 1/10,000 inch and 

 1/50 inch (Frontispiece, Plate VII and Fig. 7). Many have spines which increase 

 their surface area and this helps flotation, others produce oil globules which 

 reduce their specific gravity. Because of their small size, only the largest can 

 be seen without a microscope, and just as a mere speck, and they cannot 

 therefore be studied by the reader with only a general interest. Nevertheless, 

 skipping some of the details if he so wishes, he should be able to get an idea 

 of the types of plants that exist in the sea and so appreciate to advantage their 

 importance, as discussed later in Chapters 9 and 10. 



In brief, there are three main kinds of plants in the plankton, the nano- 

 plankton, the diatoms and the dinoflagcllates. 



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