200 ALG^E. [CHAP. 



each, and consequently have neither stem, leaf, nor 

 root. 



We have elsewhere seen that Protococcus absorbs 

 carbon, and sets free oxygen. In this respect all the 

 Algae agree, and to this power the 

 (S& a '^^ Denizens of the deep, in ail their 



(g^ marvellous variety and infinite 



/^ ^---^3) number, owe their existence. 



vi^ & & It is frequently stated that the 



(Ojl^ ocean owes its power of sustain- 



ing life to its ceaseless motion 



Fir, 148. 



keeping it constantly oxygen- 

 ated. But the animal population of the deep is so 

 enormous, and the consequent consumption of oxygen 

 so vast, that this process alone would be insufficient 

 to keep such an immense body of water pure. The 

 submarine forests, which consist entirely of Algcz, 

 give off an immense volume of oxygen, besides sup- 

 plying the finny tribes, the crustaceans, and the mol- 

 luscs, with the staple of their food. 



There are several methods by which A /gas are 

 reproduced. In the lower forms reproduction takes 

 place by the repeated subdivision of 

 an individual plant. In others it is 

 attained by two individuals throwing 

 out little processes, which come in 

 contact, unite, and form a spore. In 

 the higher forms the reproductive 

 P r ocess is correspondingly more com- 

 plicated. In these two kinds of bodies 

 are formed the Sporangia and Antheridia. Fig. 

 149 shows a cross section of the stem of a species of 



