In the 18th Century, organisms were considered to belong to one of two kingdoms, Animalia or Plantae. As biologists gathered more information about the diverse forms of life on Earth, it became evident that the two-kingdom system did not accurately reflect relationships among different groups of organisms, and the number of kingdoms increased. In 1969, Robert Whittaker proposed a five-kingdom system consisting of monerans, protists, fungi, plants and animals. In the last few years, comparative studies of nucleotide sequences of genes coding for ribosomal RNA and other proteins have allowed biologists to recognize important distinctions between bacteria and archaebacteria. The graphic on this slide illustrates the phylogenetic relationships drawn from this information using a three-domain and a six-kingdom arrangement, compared to the traditional five kingdom system.

In our previous presentation about phylogenetic classification, we introduced classifying organisms under a broad three-domain system versus classifying organisms using a five, six, or more kingdom approach. For the purpose of this discussion, we will refer to the traditional five-kingdom system.  Organisms are divided into each of five kingdoms based on defining characteristics, such as: cell type; cell structures; whether the organism is unicellular, multicellular, or has both forms; and nutrition.  As new information is gathered, classifying approaches are constantly being refined.

In the five-kingdom classification system, Plantae refers to green plants, excluding the green algae.  The Kingdom Plantae includes the mosses, seed ferns, conifers, flowering plants and related groups.  Plants are multicellular organisms that develop from embryos. Plants have cellulose in their cell walls that gives strength and structural support, and use chlorophyll a and b to transfer energy from the sun to chemical energy, a process called photosynthesis. 

In the life cycle of plants, the two multicellular body forms, the gametophyte (1n) and sporophyte (2n), alternate. The predominant form and pattern of this cycle is a key characteristic of differing plant groups.

Plants are adapted primarily for life on land and have had to overcome problems of water loss and transport. Various groups of plants approach the problem of reproduction, support and transport with ingenious adaptations of vascular tissue, roots, stems, leaves, pollen, seeds, fruits, and flowers.

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.

Animals, fungi and some protists, and bacteria are dependent on plants for food and oxygen.  Humans have used plants for medicinal purposes since the emergence of the human mind, from about 1 million to 100,000 years ago.  Today, medicines from plants include heart medications, pain relievers, decongestants, stimulants, and drugs for treatment of cancer.

Plants provide raw materials for construction and many kinds of manufacturing.  Industrial uses for plants are numerous; for example, agriculture is a vital industry throughout the world, and wood is the next most valuable resource.  Just stop for a minute and think of all the products made from wood: paper, rayon, cabinets, guitars, toys, framework for houses, just to name a few.  Wood still is used as fuel for heating and cooking in many parts of the world.  Cotton is one of the world's most important fibers.

It is important to think about plant diversity and the problem of extinction.  Many plants are lost every day to exploding human populations and to the destruction of natural habitats to make room for human settlements.  According to E.O. Wilson, a leading voice for the preservation of biodiversity, ninety-nine percent of all species that ever lived are now extinct.  "An awful symmetry of another kind binds the rise of humanity to the fall of biodiversity: the richest nations preside over the smallest and least interesting biotas, while the poorest nations, burdened by exploding populations and little scientific knowledge, are stewards of the largest," he said. Wilson wrote that biological diversity is the key to the maintenance of the world as we know it.