82 A TEXTBOOK OF OCEANOGRAPHY 



in the suspension organs of marine plants. The small marine 

 plants which are found floating practically everywhere in the 

 upper layers of the sea, at any rate to the depths to which light 

 penetrates, are provided with contrivances to prevent (or at 

 any rate to hinder) them from sinking below the light level, 

 where they would die. These contrivances are termed sus- 

 pension organs, and are of four main types : 



The Bladder type, in which the cells of the plant are very 

 large, the protoplasm forming a thin layer round the inside of 

 the cell wall, the centre of the cell being filled with a fluid of 

 about the same specific gravity as sea-water (e.g., Coscino- 

 discus). 



The Ribbon type, where the surface is enlarged owing to 

 the cell being flattened and even bent or twisted (Fragilaria). 



The Hair type, where the cell is much prolonged in one 

 direction (Rhizosolenia). 



The Branching type, in which the surface of the cell is 

 enlarged by various hair-shaped or lamelliform outgrowths 

 (species of Chaetoceras). 



Now, the curious thing is that some of these species of 

 marine plants of wide distribution develop an external form in 

 accordance with the viscosity of the water in which they find 

 themselves. Formerly these plants were thought to be distinct 

 species. A good example is Rhizosolenia hcbetata, found both 

 in Arctic and Atlantic waters. In the Arctic, where the water 

 is cold and viscosity high, this plant is thick-walled and gross ; 

 in the Atlantic, where the water is warmer and viscosity less, 

 this plant, in order to protect itself against the greater tendency 

 to sink, develops thinner walls and is proportionately longer, 

 being furnished with a long hair-like spine at each end. 

 Transitional forms are met with which are obtuse (hebete) at 

 one end and spiny at the other. 



Compressibility. 



Liquids, as a rule, are not compressible. Tait gives the 

 compressibility of pure water to that of sea-water from (he 



