The various adaptations to the terrestrial environment (e.g., waxy cuticles, stomata, vasculature, gametangia, seeds, and fruit) have evolved slowly during the 475 million-year history of plants. With these adaptations in mind, we will move on to a discussion of plant phylogeny and begin our review of the major characteristics of each of the plant lineages.
Before we begin to discuss each of the plant lineages, it is important to understand the phylogenetic relationships among them. According to this figure, are nonvascular plants "older" than nonflowering seed plants? The correct way to interpret a phylogenetic tree is to read which groups are more closely related to one another, and which groups are more primitive or more highly diverged. Living nonvascular plants are not "older" than nonflowering seed plants, but they possess a greater number of primitive character states than do nonflowering seed plants. Also, the origin of the nonflowering seed plant lineage occurred later in time than the origin of nonvascular plants, but this does not mean that currently living nonvascular plants are any older than currently living nonflowering seed plants. Flowering seed plants are the most derived lineage of plants.Now that you have a working knowledge of the major adaptations present throughout the plant kingdom and understand the evolutionary relationships among them, you will be introduced to the four lineages: (1) nonvascular plants, (2) seedless vascular plants (3) nonflowering seed plants, and (4) flowering seed plants. Always keep in mind how the adaptations found in each lineage of plants reflect the environmental conditions in which each lineage developed.
As the name of this group indicates, plants in this lineage do not have vascular tissue (or if present, it is very reduced). Because they lack substantial vasculature, plants in this lineage are generally small in size, lack significant structural support, grow close to the ground in moist areas, and lack significant water-conducting cells. Plants first evolved in environments that were transitional between the land and the sea, and although modern nonvascular plants are dependent on water to complete their life cycles, they are able to withstand long periods of desiccation. Nonvascular plants include mosses, liverworts, and hornworts.
Of all the plant lineages, nonvascular plants are the most basal. This means that these lineages diverged before that remaing groups of land plants diversified. Current information suggest that the liverworts diverged first, followed by the mosses then the hornworts. They are less derived than seedless vascular plants and seed plants, but the nonvascular plants are highly successful in the environments they inhabit.
The image below shows a phylogenetic tree showing the hypothesized relationships among the lineages of land plants. The three groups that comprise the nonvascular plants are not members of a monophyletic groups, with hornworts more closely related to vascular plants.
As you walk through a wooded area, you will likely find mosses growing on rocks, rotting wood, trees, the ground. Nonvascular plants are generally small and do not extend much more than a few inches above the surface they are growing on. Their appearance can best be described as a "carpet of green." The plant body that is most obvious is the gametophyte generation, which is haploid.Nonvascular plants typically grow in moist environments. Their lack of vascular tissue requires them to maintain close contact with water to prevent desiccation. They do not have true roots, true stems, or true leaves (which are distinguished by the presence of vascular tissue). Rhizoids are the root-like structures that function to anchor them to the surface they are growing on, however, they are not capable of water uptake. Water is absorbed throughout the "leafy" plant body of the gametophyte. They also require a moist environment for successful fertilization. They do not produce pollen grains and have retained the primitive condition of a flagellated sperm. The male gametes are motile in water and must be released into a moist environment so that the sperm can swim to the female gametangium (where the egg cells are located).