THALLOPHYTA: ALGAE 99 



which these groups have been presented is merely one denoting an ever- 

 increasing complexity in vegetative and reproductive structures. It 

 does not denote descent of one group from the one preceding it in the 

 series, although in some cases this may be true. Each group merely 

 stands for a different degree of progress from what was originally a 

 primitive condition. 



The most important evidence concerning the interrelationships of 

 plant groups is derived from paleobotany. The dearth of fossils belong- 

 ing to groups below the pteridophyte level is so great, however, that 

 practically all conclusions regarding phylogeny must be based on the 

 comparative structure and development of existing plants. This means 

 that such conclusions, even though well substantiated, are largely 

 speculative. 



The Cyanophyceae are the most primitive group of autotrophic plants. 

 Except for the presence of chlorophyll, they are strikingly like the bac- 

 teria. Which of these groups appeared first on the earth is very uncer- 

 tain, but is unimportant in connection with the present discussion. Both 

 groups are at a very low level of structural organization. The classes 

 consisting mainl}^ of flagellates show a considerable advance over the 

 Cyanophyceae by their well-organized nuclei, definite plastids, and 

 ciliated cells. In the absence of transitional forms, any direct connection 

 between the Cyanophyceae and flagellates is difficult to visualize. It is 

 easier to think of the flagellates as having arisen directly from the bac- 

 teria. A direct relationship between the Cyanophyceae and any of the 

 higher algal classes is also unlikely, although there is some evidence of 

 this in the case of the Rhodophyceae. Ciliated cells are not present in 

 either group; some members of each have both phycocyanin and phyco- 

 erythrin; and a few primitive Rhodophyceae have a nuclear structure but 

 little advanced over that of the Cyanophyceae. It is primarily the 

 absence of ciliated cells that would seem to preclude the possibility of a 

 relationship between either group and the flagellates. 



That the Xanthophyceae and Chlorophyceae have arisen independ- 

 ently from a flagellate ancestry is strongly indicated by the presence of 

 naked, free-swimming reproductive cells in the life history and by the 

 occurrence of intermediate forms. The derivation of the Bacillario- 

 phyceae and Phaeophyceae directly from flagellate ancestors is less evi- 

 dent. The only connection between the diatoms and flagellates is the 

 presence of ciliated reproductive cells in a few diatoms. The origin of 

 the Phaeophyceae is obscure because the group is without unicellular 

 members. Yet their motile reproductive cells suggest that they may 

 have arisen from brown, laterally biciliate flagellates. There is also the 

 possibility of a direct connection between the Phaeophyceae and Chloro- 

 phyceae. The Charophyceae are an isolated group, yet seem to repre- 

 sent a specialized offshoot from the Chlorophyceae. 



