200 ALG. [CHAP. 
each, and consequently have neither stem, leaf, nor 
root. 
We have élsewhere seen that Pvrotococcus absorbs 
carbon, and sets free oxygen. In this respect all the 
Algz agree, and to this powerthe 
a denizens of the deep, in all their 
ee marvellous variety and infinite 
ri S number, owe their existence. 
(-\>) 
eae It is frequently stated that the 
001 ocean owes its power of sustain- 
ing life to its ceaseless motion 
keeping it constantly oxygen- 
ated. But the animal popujation 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 Alga, 
sive 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 Alg@ 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 forma spore. In 
the higher forms the reproductive 
Fic. x49-—Section of process 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 
Fic. 148. 
