The plant kingdom, as here restricted, is fairly easy 

 to define if rare exceptions to the rule are ignored. 

 Plants are organisms with cellular, tissue, or organ 

 organization in which the individual cells contain a 

 cell wall, chlorophyll in plastids, and a central vacu- 

 ole. Among the Protista certain flagellates have the 

 latter structures in their single, cell-like bodies. In the 

 classification followed here, the Kingdom Plantae is 

 divided into two subkingdoms, Algae and Embryo- 

 phyta. 



In the rare exceptions to an absolute definition of 

 the plants all criteria break down. However, one can 

 consider the exceptions in relation to level of organi- 

 zation alone. No algal phylum is without unicellular 

 members. (Some algae even have organs.) The uni- 

 cellular forms cannot be separated from protistans by 

 simple means. However, detailed study definitely 

 relates these problematical species to algae rather 

 than to protistans. 



The presence of exceptions to the level of organiza- 

 tion definitions of phyla can be perplexing as well as 

 frustrating. Previous exceptions were noted (e.g., 

 tissue-like fungi) and further exceptions will occur. 

 Rather than being frustrating to the biologist, these 

 exceptions are welcomed. They serve as another item 

 supporting the principle of organic evolution. Al- 

 though we might not logically expect transitional 

 forms, "missing links," their representation in living 



species is useful in ascertaining phylogenies or "trees 

 of life." 



Unfortunately, exceptions in the form of "missing 

 links" are all too few. This is indicated by lack of 

 agreement on the phylogenetic position of the Algae. 



ALGAL CLASSIFICATION AND 

 PHYLOGENY 



Some biologists prefer to include Algae as a taxon 

 in the Subkingdom Thallophyta (Gr. ihallos, a young 

 shoot + phylon, a plant), which also includes the taxa 

 Monera, chlorophyll-bearing Flagellata, and Fungi. 

 However, since it is now believed unlikely that the 

 Thallophyta constitute a natural grouping of life (that 

 is, thallophytes probably were not derived from a 

 common ancestor), that grouping will not be followed 

 here. 



In spite of the doubtful evolutionary significance of 

 the Thallophyta, the terms thallophyle and thallus are 

 often used in describing the simple structure of cer- 

 tain organisms, specifically the Monera, Protista, and 

 Algae and the gametophyte generation of the Bryo- 

 phyta (mosses and liverworts). A thallus is a struc- 

 ture in which there is little tissue or organ difli'erentia- 

 tion. More specifically, a thallus has none of the fol- 

 lowing structures: true roots, stems, leaves, complex 



148 



