64 EVOLUTION OF PLANTS 
kelps being plants of gigantic size. Both groups differ 
in many respects from the green alge, 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. 
Tur Brown ALG& (Phawophycew) 
Before examining the more highly organized plants 
to which the term Phzophycex 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- 
vocinez, 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 
Phaophycex, and possibly may bear the same relation to 
this class that the Volvocinez do to the green alge. 
