l6] VEGETATIONAL TYPES OF SEAS 517 



tion of many northern waters at certain times of the year is due 

 largely to Diatoms, while the Red Sea owes its name to the red 

 accessory pigment of the planktonic cyanophycean Trichodesmiutn 

 erythraetnn. Diatoms are present in all seas and indeed almost 

 everywhere, being usually numerous as regards both forms and 

 individuals ; Peridinians are also extremely widespread, but tend 

 to be numerous chiefly in terms of individuals in cold seas and of 

 different forms in warm ones. Green flagellates and Bacteria may 

 also be very abundant in marine plankton — the latter especially near 

 coasts, though there is some doubt as to whether they should be 

 considered truly planktonic {see p. 521). Various other greenish 

 brownish, or yellowish types are also prone to occur — such as, respec- 

 tively, Halosphaera viridis, Phaeocystis, and certain Silicoflagellates. 



Marine plankton has to remain suspended in water. Consequently 

 the component organisms either swim or are very minute or have 

 some appropriate ' form-resistance ', meanwhile employing the 

 seemingly least and lightest possible structural material. This is 

 towards maintaining the specific gravity near the density of the 

 surrounding medium — much as in freshwater types. Such needed 

 buoyancy may be increased by gas-bubbles or, as in the case of 

 Diatoms, by globules of oil. Adhesion of cells together may also 

 increase their tendency to float, though, in general, reduction to 

 small size is highly advantageous. For the rate of sinking of a body 

 heavier than water, as most phytoplankters (phytoplanktonic 

 individuals) are, depends upon the ratio of surplus weight to friction, 

 which is determined mainly by surface area, and the simplest way 

 to obtain a relatively large surface-to-volume ratio is to reduce the 

 absolute size. Thus the smaller the body, the less tendency will 

 it show in general to sink. 



Otherwise, if no means of active locomotion or actual flotation 

 is available, reliance has to be placed on an increase in surface area 

 and complexity through structural adaptations involving the external 

 form. This may be as of a bladder, in which much of the relatively 

 large cell is occupied by light fluid, or of a disk, which sinks in zig- 

 zag fashion and so covers a greatly increased distance, or of a needle, 

 which sinks slowly when the long axis lies horizontally — as the 

 mechanics of sedimentation make it tend to do. Or the elongated 

 body may be curved or provided with bevelled ends in such a manner 

 that, if displaced, it is soon brought back to the horizontal position, 

 sinking being accomplished in wide circles. Or the cells may be 

 attached in ribbons or chains, or they may be branched or possessed 



