PLANT LIFE 147 



naked spores that are shed from the original 

 cell, and gradually develop new cell-walls 

 of their own. In Ceratium the reproduction 

 is by division, as in the diatoms. The cells 

 very often hang together in chains, and it 

 can be seen that the horns of the cell vary 

 considerably in form from one generation 

 to another. Ceratium tripos may give rise 

 to an intermediate generation of quite a 

 different type — much smaller, with short 

 straight horns. In the same species gem- 

 mation may take place instead of normal 

 cell division. The full meaning of these 

 variations has not yet been made out. 



Pyrocystis, discovered during the " Chal- 

 lenger " Expedition, belongs to the Peridineae, 

 and is very abundant in all tropical and sub- 

 tropical waters where the temperature exceeds 

 68° F. and where the salinity is not lowered 

 by the presence of coast waters. Pyrocystis 

 noctiluca is spherical in form and 0-6 to 0-8 mm. 

 in diameter, with brown pigment granules. It 

 is about the same size as the animal-flagellate 

 Noctiluca, which never has brown granules, 

 and flourishes in coast waters, while Pyrocystis 

 occupies the open water far from land chiefly 

 in intertropical regions. Both Pyrocystis and 

 Noctiluca are brilliantly phosphorescent. 



(3) Coccolithophoridce are brown globular 

 flagellates, which secrete calcareous button- 



