64 EVOLUTION OF PLANTS 



kelps being plants of gigantic size. Both groups diiier 

 in many respects from the green algse, and it is an open 

 question whether they have been derived from the latter, 

 or whether they are to be traced back to unicellular 

 ancestors, in which their peculiar pigments were already 

 developed. These red and brown pigments are doubt- 

 less associated with the process of photo-synthesis, and 

 are probably the results of the peculiar environment of 

 these sea-weeds. 



The Brown Alg^ {Phceophyceoe) 



Before examining the more highly organized plants 

 to which the term Phseophycese is usually applied, it 

 may be well to consider a number of simple forms pos- 

 sibly allied to them, and, although minute in size, of 

 great importance in the economy of nature. Some of 

 these are inhabitants of fresh water, but the greater 

 number are free-swimming or pelagic organisms occur- 

 ring in the open ocean, and forming an important con- 

 stituent of the so-called " plankton " or floating life of 

 the ocean. 



The simplest of these (Fig. 15, A, B) are very minute 

 ciliated organisms recalling the green, fresh-water Vol- 

 vocinese, and possibly related to them. Like these they 

 show evident resemblances to the flagellate infusorians, 

 from which they differ mainly in the presence of chromat- 

 ophores, and the absence of an opening by which solid 

 food can be ingested. These plants have chromato- 

 phores which contain a pigment much like that of the true 

 Phseophycese, and possibly may bear the same relation to 

 this class that the Volvocinese do to the green algse. 



