THE ALGAE 



149 



water, being denser, sinks and displaces the warmer deep water with its 

 stores of accumulated food materials. The surface waters thus become 

 again rich in the materials of life, and the vernal outburst once more appears 

 with the increasing solar radiation in spring. 



m 



\ MUCILAGINOUS 

 SHEATH 



\ ABORTIVE 



NUCLEI 



GAMETES 



ZYGOTE VALVES OF 



_-.. OLD FRUSTULES . . 



Fig. 138. — Cymbella laticeolata. Sexual conjugation. A, Two naked protoplasts enclosed 

 in a mucilaginous envelope. B and C, Nuclear divisions. D, Division of the pro- 

 toplasts. E, Fusion of the protoplasts to form two zygotes. F and G, Enlargement 

 of the zygotes to form the protoplasts of two new cells. {After Geitler.) 



Manv oceanic Diatoms show peculiarities, such as fringes of long spines, 

 or marginal membranes, which are interpreted as flotation devices. One of 

 the commonest of these is the attachment of cells together into chains by 

 means of interlocking processes on the frustules. The dominant species of 

 plankton Diatoms var\' in different areas of the sea. Round the British Isles 

 species of a circular type, Coscinodisciis, are predominant. Diatom frustules 

 form the chief oceanic sediment in Antarctic latitudes, where the " Dia- 

 tomaceous ooze " covers many thousands of square miles of the ocean bed. 



