66 EVOLUTION OF PLANTS 
the ocean that they are of most importance. Here, es- 
pecially in the colder parts of the sea, they form the 
greater part of the floating Vegetation, and sometimes 
occur in such enormous masses as to discolor the water 
over wide areas. It is these masses of floating unicel- 
lular plants which are the primary source of food for 
all the hosts of animal life in the ocean, and it is to 
these minute organisms that the manufacture of organic 
substances is due, and they serve as food for innumera- 
ble smaller animals, and sometimes larger ones as well, 
which, in their turn, are devoured by higher forms. 
In the warmer waters, the diatoms are largely re- 
placed by the other unicellular plants already referred 
to, as well as others whose affinities are still obscure. 
As the larger sea-weeds, with few exceptions, are at- 
tached, they are of necessity confined to a narrow zone 
of shallow water skirting the shore, and in spite of their 
large size, are of slight importance as compared with 
the hosts of minute pelagic plants. 
The silicious shells of diatoms are almost indestruc- 
tible, and have been preserved in a fossil condition so 
that even the species are readily determined. These 
deposits are often of great thickness, showing that, for- 
merly, as at present, these plants occurred in immense 
uumbers. However, geologically speaking, the group 
is not an extremely old one, but appears somewhat 
suddenly in the later secondary, and early tertiary rocks. 
THE PHAHOPHYCEA 
The true Pheophycee are almost exclusively marine 
and form a clearly defined class with no certain affinity 
