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 

 numbers. However, geologically speaking, the group 

 is not an extremely old one, but appears somewhat 

 suddenly in the later secondary, and early tertiary rocks. 



The PHiEOPHTCE^ 



The true Phseophycese are almost exclusively marine 

 and form a clearly defined class with no certain affinity 



