Although this text is not intended to replace a more thorough study of botany, the fullest appreciation of plants results from a basic understanding of their place in the natural world. For centuries, beginning with a Greek named Theophrastus in about 300 b. c., scientists have attempted to classify the planet’s organisms into a tidy system that accommodates all current knowledge of the natural world and allows new knowledge to be added. As long as the new knowledge fit the criteria of the existing classification system, new knowledge was easily accepted. Thus, the organisms of the world were once classified into two kingdoms: plants and animals. If an organism moved, had a nervous system, and could not make its own food, it was classified as an animal. if it was anything else, it was classified as a plant. Early classification systems focused on those living organisms that were easily seen because of their size (macroscopic) causing the classification to be based more on physical appearance than on natural factors. The development of the microscope and the ongoing expansion of knowledge of our world have allowed the world’s organisms to be classified in a more natural way. There are now five known kingdoms, based on the structure of the organisms’ cells. Which kingdom includes which organisms is determined by criteria such as the organism’s size, complexity of development, method(s) of reproduction, and how it captures nutrient energy. If the organism is only one-cell in size (such as bacteria) it is termed a prokaryote. These simple organisms have been around for roughly two
billion years, reproducing asexually, and account for a large portion of earth’s past and current biomass. Prokaryotes are so primitive that they do not encase their genetic material (dNa) in a nucleus, instead containing it within an area termed the nucleoid region, which is enclosed by a membranous cell wall. Prokaryotes are able to move with the aid of whip-like appendages termed flagella (singular—flagellum). If an organism’s cell is more complex, greater in size and volume, and containing its DNA within a nucleus, then it is termed a eukaryote. While eukaryotes may include single-celled organisms, they also include the fungi, animals, and plants. Eukaryotes are more recently evolved than prokaryotes, having been around only about one and a half billion years. They reproduce sexually, thereby allowing more genetic combinations to occur. In turn, that allowed multicellular organisms to evolve. So to understand where plants fit into the grand scheme of nature, begin by knowing them to be eukaryotes.
Based on their cell structure, their method of reproduction, whether they do or do not capture their own nutrient energy, and other criteria, the organisms of the natural world are classified into five kingdoms:
1. Protista—Mostly one-celled prokaryotes: the bacteria and blue – green algae.
2. Protoctista—Aquatic eukaryotic microorganisms, exclusive of the fungi, animals, and plants.
3. Fungi—Nonmotile eukaryotes, possessing a cell wall. They reproduce by spores and derive their nutrition and energy from other living or nonliving hosts.
4. Animals—Motile eukaryotes, lacking cell walls. They derive their nutrition and energy from other living and nonliving hosts.
5. Plants—Nonmotile eukaryotes that have cell walls. They derive their energy through photosynthesis.
The Plant Kingdom
The study of plant classification is termed taxonomy. Plant scientists who engage in the study are known as taxonomists. While not all taxonomists agree completely on how to classify every plant in the world, they do agree that the classification of plants must be a natural system that is based on the genetic and evolutionary relationship among plants. In turn, that system must be sufficiently flexible and dynamic to accommodate the continuing and evolving knowledge of plant life. Currently the plant kingdom is divided into divisions, classes, orders, families, genera, and species. The system is arranged in a hierarchal manner. From any point in the classification, the groups of plants share increasingly more genetic and anatomical similarities as their placement progresses down the system from division toward species.
While eleven of the twelve divisions of the plant kingdom contain important plants such as the algae, diatoms, stone-worts, and mosses, it is the higher plants, including the ornamentals, which are of relevance to this text. They are classified in the division Tracheophyta. Plants in this division are collectively termed the vascular plants, meaning that they are able to move water, nutrients, and the products synthesized in the leaves through internal conducting tissues, the xylem and the phloem.
Some of the vascular plants, such as ferns, reproduce from spores (reproductive cells), while the majority of the higher plants reproduce from seeds. Coniferous plants, such as pines, are placed in the class Coniferopsida, signifying that their seeds are produced in either a cone (such as a pine cone) or in a berry-like formation (such as a yew). Conifers do not flower, and their leaves are needle-like, scale-like, or awl-like. The flowering plants are placed in the class Angiospermopsida. They are the most evolved members of the plant kingdom, producing their seeds inside a fruit that developed as a swollen ovary from a fertilized flower. Within the class are two orders, defined by the number of seed leaves (cotyledons) contained in their seeds. Monocots (in the order Monocotyledoneae) have a single seed leaf. Grains and grasses are classified as monocots. So are ornamentals such as palms, iris, lilies, and orchids. Dicots (in the subclass Dicotyledoneae) are angiosperms that have two seed leaves in their seeds. Most of the flowering plants, including the fruits, vegetables, and ornamentals, are dicots.
As the classification system progresses from broad generalities to more detailed specificity, the genetic makeup and physical attributes of the plants contained within each family, genus, and species become increasingly similar. The categories in increasing order of specificity are:
Eventually every plant in nature that has been identified and classified can be identified by its inclusion in these categories.