The Plant Kingdom
The systems of classification of the plant kingdom vary in naming and placing the larger categories (even the divisions) because there is little reliable fossil evidence, as there is in the case of animals, to establish the true evolutionary relationships of and distances between these groups. However, comparisons of nucleic acid sequences in plants are now serving to clarify such relationships among plants as well as other organisms.
A widely held view of plant evolution is that the ancestors of land plants were primitive algae that made their way from the ocean to freshwater, where they inhabited alternately wet-and-dry shoreline environments, eventually giving rise to such later forms as the mosses and ferns. From some remote fern ancestor, in turn, arose the seed plants.
The plant kingdom traditionally was divided into two large groups, or subkingdoms, based chiefly on reproductive structure. These are the thallophytes (subkingdom Thallobionta), which do not form embryos, and the embryophytes (subkingdom Embryobionta), which do.
All embryophytes and most thallophytes have a life cycle in which there are two alternating generations (see reproduction). The plant form of the thallophytes is an undifferentiated thallus lacking true roots, stems, and leaves. The subkingdom Thallobionta is composed of more than 10 divisions of algae and fungi (once considered plants). The subkingdom Embryobionta is composed of two groups: the bryophytes (liverwort and moss), division Bryophyta, which have no vascular tissues, and a group consisting of seven divisions of plants that do have vascular tissues. The Bryophyta, like other nonvascular plants, are simple in structure and lack true roots, stems, and leaves; they therefore usually live in moist places or in water.
The vascular plants have true roots, stems, and leaves and a well-developed vascular system composed of xylem and phloem for transporting water and food...