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Introduction to Organisms

Author(s): Deanne Erdmann, MS

Major Groups of Plants

The Plant Kingdom is often separated into bryophytes (mosses and liverworts), pteridophytes (ferns), and seed plants. Seed plants are divided further into two groupings, gymnosperms and angiosperms. Gymnosperms are the cone-bearing plants such as pines and conifers. Angiosperms are the flowering plants, which are traditionally divided into monocots or dicots. 

Bryophytes are plants that lack vascular tissue, true roots, stems, and leaves. They reproduce by dispersing large amounts of spores. These plants are small because of water transport problems, and they depend on diffusion and osmosis for movement of materials throughout the plant. The gametophyte (1n) generation is predominant. Examples of bryophytes are mosses, hornworts, and liverworts.

Pteridophytes have conducting tissue for nutrients, water and the products of photosynthesis. In this group, the sporophyte (2n) generation is dominant. The spores of these plants are resistant to drying. Examples include ferns, club mosses, and horsetails.

Evolution of the seed allowed plants to move further away from water, and to tolerate harsher climates. The seed offered new survival advantages for the embryo, such as protection, nourishment, dispersal, delayed growth. 

Gymnosperms include pines, junipers, cycads, and gingkoes. The gymnosperm seed, often described as "naked," is not enclosed in a fruit. Wind dispersal of pollen means that large amounts of seeds are needed to insure fertilization. In the gymnosperms, the sporophyte (2n) generation is predominant.

Angiosperms are flowering plants that produce seeds surrounded by a fruit barrier. What we think of as fruit is actually a mature ovary. Fruits are classified as simple (like an apple), aggregate (like a strawberry), or multiple (like a pineapple). The most recent group to evolve, angiosperms produce pollen and seeds. Angiosperms are traditionally divided into two groups, monocots and dicots, but scientists are now considering adding a third group, the eudicots. Some examples of monocots are lilies, orchids, yuccas, grasses, and grain crops. Examples of dicots are oaks, maples and sycamores. Eudicots are sometimes referred to as euangiosperms and are classified based mainly on their pollen structures. Recently, phylogenetic analyses, based on both structural data and molecular sequences, have begun to unravel higher-level phylogenetic relationships within the eudicot group. Eudicots make up approximately 75% of all existing angiosperms.