144 THE OCEAN 



to keep them in a horizontal position in the 

 water, and so prevent them from sinking. 

 Rhizosolenia offers an example of this type. 



{d) The branching type with the surface of 

 the cell enlarged by various kinds of hair- 

 shaped outgrowths. Chcetoceras, usually asso- 

 ciated in chains, is a well-known example of 

 this type. 



Many of these require to vary their form in 

 order to adapt their floating power to the vary- 

 ing conditions of viscosity in ocean water. 

 Their tendency to sink increases as the vis- 

 cosity decreases with a rise of temperature, 

 and they then develop special suspension- 

 organs to keep them near the surface. In 

 temperate regions some species may have 

 distinct summer and winter forms so unlike 

 that they have been regarded as different 

 species. In the tropics there are species 

 corresponding to the summer forms, and in 

 polar waters species corresponding to the 

 winter forms. The summer forms have 

 usually thinner cell-walls and a more slender 

 structure. 



In coastal waters, where the physical condi- 

 tions vary far more than in the open sea, 

 most diatoms have a special adaptation 

 (unknown in true oceanic species), called 

 resting spores, to enable them to pass through 

 unfavourable seasons of the year. The cell- 



