PLANT LIFE 149 



which are as yet imperfectly known, though 

 they probably play a great role in the economy 

 of the ocean. Such are the xanthellae or 

 " yellow cells " of the radiolarians — a case of 

 symbiosis. This association of plant and 

 animal cells is evidently beneficial to both, 

 for the starch developed by the yellow cells 

 with the formation of oxygen may serve as 

 nutriment to the animal, while the carbonic 

 acid yielded by the animal is available 

 to the plant-cell. These yellow cells occur 

 also in foraminifera (Globigerina and Orbito- 

 lites), and in corals and other invertebrates, 

 as well as floating independently in the 

 ocean. 



In the legion of the Radiolaria called Phaeo- 

 daria, which inhabit the deep sea, the place 

 of the yellow cells in the other Radiolaria is 

 apparently taken by the phasodellae, dark- 

 coloured cells, which may possibly be a lower 

 form of algal life than the yellow cells and 

 capable of evolving oxygen under the influence 

 of the phosphorescence of deep-sea animals. 



Blue-Green Algce (Cyanophyceae) predomi- 

 nate over all other algae in fresh water, but in 

 the sea they are represented Jby only a few 

 species and genera. The well-known " blos- 

 soming " or " water-bloom " of lakes at 

 certain times of the year is due to the enormous 

 development of certain species of Oscilla- 



