ALGAE. 491 



sometimes diagnostic or semi-diagnostic, on some of the more characteristic 

 species. 



Among the algae as a whole, as the term is commonly restricted by 

 modern writers, three great sub-classes are recognized, known as the Chloro- 

 phyceae or the Green Algae, the Phaeophyceae or Brown Algae, and the 

 Rhodophyceae or Red Algae. The algae in general possess more or less 

 chlorophyl, the green pigment characteristic of most of the higher plants, 

 and in the sub-class Chlorophyceae, the chlorophyl is as a rule comparatively 

 free from admixture with other coloring matters, so that the plants impress 

 one as being more or less grass-green in color. In the sub-class Phaeophyceae, 

 the chlorophyl is accompanied by one or more brownish or yellowish pig- 

 ments and the resulting color is commonly a brownish green or an olive- 

 green. In the sub-class Rhodophyceae, the chlorophyl is ordinarily masked or 

 obscured by a red pigment, so that the plants usually exhibit some shade of 

 red, pink, violet, or purple, though in certain kinds the shade is so dark 

 as to be almost black. Associated with these color differences as exhibited by 

 these three sub-classes, and perhaps of more fundamental import, are cer- 

 tain differences in structure and in modes of reproduction. But the deter- 

 mination of these characters involves, as a rule, the use of the higher 

 powers of the microscope and they have been referred to only occasionally 

 in the discussion that follows. 



Sub-class CYANOPHYCEAE. 



In addition to the three classes of plants mentioned in the preceding 

 paragraph, the term algae is very often extended to include also another, 

 somewhat simpler class known as the Cyanophyceae or Myxophyceae, com- 

 monly referred to as the Blue-green Algae, a group that exhibits points 

 of contact, on the one hand, with the Bacteria and, on the other hand, with 

 the simplest Red Algae. In this group, the chlorophyl is associated with 

 another pigment which commonly gives the cell contents a bluish-green 

 shade, though in mass, to the naked eye, the plants very commonly appear 

 nearly black. They are usually plants of small size but when associated 

 in colonies as is their ordinary habit, they form masses that may readily 

 attract the eye. In many of them reproductive processes are so little 

 differentiated from those of ordinary vegetative growth that it is difficult 

 to say just what should be considered an individual plant and what an 

 aggregation of individuals. In some of them the single microscopic cell is 

 more or less obviously the individual; in certain others it is a filament, 

 made up usually of a single row of cells. The Cyanophyceae are wholly 

 non-sexual in their modes of multiplication. In the simpler forms the 

 multiplication of individuals occurs through simple division or fission ; in 

 the higher, certain cells, known as spores or resting cells, differentiated 

 from the ordinary vegetative cells in size and other characters, take upon 

 themselves the function of originating new individuals. The Cyanophyceae 

 may form gelatinous or slimy dark scums on rather stagnant water or 

 somewhat similar films or crusts on rocks or on the larger aquatic plants, 



