PELAGIC PLANT LIFE 335 



resting - cells. Halosphcsra occurs over the whole Atlantic 

 Ocean, and follows the Gulf Stream to its farthest ramifica- 

 tions in the north near the coasts of Norway and Spitzbergen. 

 In the North Sea there are quantities, especially in the winter, 

 and they form their zoospores in May, and thereby commence 

 their new generation. 



Just as HalosphcEra differs from all the rest of the pelagic 

 algae in having a pure green colour, so, too, it has its own special 

 mode of reproduction. The other forms, whose development- 

 history we know, are reproduced by division, and this goes on 

 incessantly, the rate of increase depending upon different 

 conditions of existence. Halosphcera does not undergo division, 

 but continues to grow for a comparatively lengthy period, and 

 then finally transforms all its contents, 

 as has just been stated, into a great 

 number of zoospores. 



In addition to Halosphcera viridis 

 there are one or two similar species 

 that have been described, but they do 

 not call for any particular discussion. 



In the foregoing I have sketched 

 the most important types of pelagic 

 algae and their biology, but the picture 

 Fig. 2ifi.—HALospHMRA VIRIDIS, would not be complete if I omitted to 



™a1m?T''''^"'°''''''^''''" describe the drifting species of sea- Floating sea- 

 weed. These do not really belong '''^^^^• 

 to the open sea. They grow along the coasts in the littoral 

 zone, and their gas - filled bladders assist them in main- 

 taining their position whatever be the state of the tide. 

 The violence of the waves finally tears them loose, and then 

 these same gas-bladders keep them for a long time floating 

 on the surface. These patches of-- seaweed are to be met 

 with in every coastal sea, the chief kinds along the coasts 

 of North Europe being Fuais vesicidosus and Ascophyllum 

 nodosum, and in the Mediterranean species of Cystosira. 

 They may also drift right out into oceanic waters, and in 

 the Sargasso Sea we have an immense eddy where the 

 patches of weed often collect in enormous quantities. The 

 prevailing weed is Sargassuni baccifertivi, but one fre- 

 quently gets patches of AscophyllMm nodosimi as well, the 

 whole being derived from the coasts of Central America. 

 The Sargasso weed is easily recognisable, owing to its 



