Higher Land Plants 



To most people the word "plant" means vascular 

 plant. Of the less than 250,000 known plant species 

 about 200,000 are tracheophytes. Almost always, 

 tracheophytes are man's source of plant food, are the 

 distinctive organisms that give the appearance of a 

 community, and are the plants in gardens. Moreover, 

 they are the primary source of food for almost all 

 strictly terrestrial animals. 



We might compare the position of tracheophytes 

 in the plant kingdom to that of arthropods in the 

 animal kingdom. In sheer numbers and bulk of 

 species, perhaps even of individuals, vascular plants 

 are not equaled by any other group of plants. 



Tracheophyte evolution probably started from the 

 green algae. The hypothesis accepted here is as 

 follows: Upon origin of archaic tracheophytes from 

 green algae, early evolution produced three persist- 

 ent, separate, somewhat equivalent major taxa, the 

 subphyla Lycopsida, Sphenopsida, and Pteropsida. 

 However, complication was an early feature in the 

 evolution of all three subphyla. Although each sub- 

 phylum first gained some complexity in structure and 

 function, each then evolved in two directions. One 

 direction was toward a coastal marsh existence and 

 involved simplification of previously gained com- 

 plexity; the other was toward an inland or truly 

 continental existence and involved further complexity 

 of previous features. The coastal marsh inhabitants 

 are first found in Silurian deposits, but continue in 



good numbers up through the Devonian. This 

 Silurian through Devonian evolution featured greater 

 simplification, presumably closer adaptation to the 

 coastal marsh habitats. After the Devonian most 

 became extinct; only three species live today. 



The coastal plants are called psilopsids and often 

 are recognized as a major taxon (here Subphylum 

 Psilopsida). However, if psilopsids are what recent 

 evidence indicates, that is, members of three sub- 

 phyla, they are not a natural group. For example, 

 the living members most likely are ferns. In spite of 

 all this, living psilopsids are treated here as a sub- 

 phylum — their fern alliance probably would be 

 shown best if they were considered a subclass under 

 the Class Filicineae (other living ferns would form a 

 second subclass). The justification for considering 

 them a subphylum of vascular plants and not a sub- 

 class of ferns is to emphasize a distinct trend in early 

 land plant evolution, an unusual trend for simplifica- 

 tion, and to follow the usual taxonomic assignment of 

 psilopsids. Apparently, living psilopsids have never 

 been formally assigned to the ferns. 



How, then, are we to explain the present subphyla? 

 The living Lycopsida and Sphenopsida each could be 

 descendants of a continental line, the coastal marsh 

 line of both subphyla now being extinct. Pteropsida 

 is more difficult to explain. Its coastal marsh line 

 now apparently has two representatives, one of 

 Devonian simplicity in the psilopsids and the other 



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