How Are Plants Classified?

by Jay Sharp

For centuries, biological scientists have worked to classify organisms in a way that would help clarify relationships among species through time and across different and constantly changing environments. In trying to delineate the order of the community of living things on earth, they have faced a mind-numbingly extensive and intricate endeavor. Some estimate that 5 to 40 million living species, with a combined dry, or water-free, weight of roughly two trillion tons, populate earth’s lands and waters. So far, the scientists have managed to classify and name some 1.6 million species, including approximately 300,000 plants, more than a million animals (mostly insects) and several hundred thousand other species. They still have somewhere between three or four million to up to 38 or 39 million species to classify and name.

Agave, with leaves that, unlike the yucca, have spiny margins.

Meanwhile, the scientists have found their work convoluted and disrupted by the accelerating rate of extinction – the disappearance of both known and unknown species – which is a direct result of humankind’s relentless destruction of habitat. In our Southwestern deserts, for instance, the scientists are finding a growing number of species imperiled as a direct result of overgrazing of grasslands, expansion of agriculture, the growth of urban communities, and wastage of water. Worldwide, most scientists believe, man has triggered the most rapid mass extinction of species in earth’s 4.5 billion year history, according to a news release issued by the American Museum of Natural History in April of 1998.

The biological scientists, in the variety of their disciplines, reflect the bewildering complexity of their field. The broader disciplines include, as a few examples, paleontologists, who study the history of life on earth; ecologists, who study systems of living organisms; botanists, who study communities of plants; and zoologists, who study communities of animals. More narrowly focused disciplines include, for example, mammalogists, who study mammals; ornithologists, who study birds; herpetologists, who study reptiles; ichthyologists, who study fish; entomologists, who study insects; arachnologists, who study spiders; parasitologists, who study parasites; and cryptozoologists, who study animals that are presumed (but not proven) to exist. It is the taxonomists who classify the organisms, putting plants and animals in their places.

Further, the biological scientists, in using a diversity of classification schemes, offer further insight into the complexity of life on earth. Depending on their academic backgrounds and mentors, the scientists use varying criteria for classifying organisms.

The Linnaeus System for Classification

Carl Linnaeus (1707 to 1778), a Swede who taught at the prestigious University of Uppsala, laid the foundation for a system that is used for classifying organisms on the basis of shared physical characteristics. He published the results of his work in his 12 editions of Systema Naturae, one of the milestone documents in the history of the biological sciences. His basic system, continually expanded and modified, is perhaps the one most commonly used today.

In classifying organisms in accordance with Linnaeus’ scheme, scientists rely on certain overriding shared physical characteristics to establish the very broadest categories of organisms. For instance, they categorize as “plants” those multicellular organisms that have no independent ability of movement and that manufacture their own food through photosynthesis. They categorize as “animals” those multicellular organisms that do have independent ability of movement and that acquire their food by eating other organisms. Within each of the broad categories, the scientists place organisms into a hierarchy of groupings.

Yucca in bloom

While we will be focusing on the plant and animal kingdoms, the most conspicuous in our deserts, the scientists actually have identified five major categories, or “kingdoms,” of living organisms:

Plantae (the plants)
Animalia (the animals)
Fungi (for instance, toadstools and mushrooms)
Monera (bacteria and blue-green algae)
Protista (for instance, microscopic organisms called protozoa)

Within each kingdom, the scientists have established six basic hierarchical groupings:

Phyla (or divisions in the plantae kingdom)
Classes
Orders
Families
Genera
Species

At each hierarchal level, from phyla to genera, the scientists group organisms with increasingly closely shared characteristics. At the final level, species, the organisms all have very similar characteristics. Conceptually, kingdoms comprise groups of phyla (or divisions); phyla (or divisions) include groups of classes; classes, groups of orders; orders, groups of families; families, groups of genera; and genera, groups of species.

In accordance with the practice established by Linnaeus, scientists call a species by the name of the genus (singular of genera), capitalized, and the species, uncapitalized. For instance, the Mojave yucca carries the scientific name Yucca schidigera; the Joshua tree yucca, Yucca brevifolia; the soaptree yucca, Yucca elata; and the Torrey yucca, Yucca torreyi. All these plants belong to the same genera, the yucca, and each has its own species name.

Classifying the Plants

Taxonomists, at least in the traditional Linnaeus method for classifying plants, separate the plantae kingdom into four major groups, or divisions, including: mosses and liverworts, which have no proper root systems; ferns, which have proper roots and produce spores (specialized reproductive cells rather than conventional seeds); coniferous trees, which have root systems and needle-shaped leaves and cones; and flowering plants, which have root systems and flowers that produce seeds. The dominant division in many biomes (that is, biotic communities, for instance, our deserts), with a quarter of a million species, is that of the flowering plants.

Closeup of yucca's showy creamy white bloom cluster

Depending on the method they follow, taxonomists may divide the flowering plants – the most recently evolved of the botanical divisions – into two broad groups, or classes.

According to Wikipedia, one class, the “monocot,” produces a single first, or “embryonic,” leaf from its seed. Typically, its leaves have parallel veins, and its stems have vascular bundles (the water-conducting vessels) that occur in a random pattern in cross section. It produces flowers with parts that occur in multiples of three—for instance, three, six or nine petals.

Acacia, a member of the pea family

The second class, the “dicot” produces two embryonic leaves from its seed. It produces flowers with petals that occur in multiples of four or five. Its leaves have network veins. Its stems have vascular bundles that occur in a concentric ring pattern in cross section. (It should be noted that the taxonomists are continually modifying and updating the designated classifications for the flowering plants, especially the dicots, as new technologies and knowledge emerge.)

Taxonomists have struggled in their efforts to divide the classes into orders that contain logically related groups of the flowering plants. By one method, they have established six superorders, which comprise family groups thought to have evolved, along different pathways, from common ancestors. Further, they have divided the superorders into various orders. They have, however, disagreed over the groupings and divisions. In fact, at this time, as Michael Knee, in his article “Orders and Classes,” on Ohio State University's Hort & Crop Science website, says, “orders are not much used as a taxonomic unit.” The taxonomists have had somewhat more success in classifying plants as families, genera and species.

They group plants with many common – and often easily recognizable – botanical features into families and perhaps subfamilies. A few of the better-known families in our deserts include the lily, cactus, pea and grass families. The plants of the lily family have leaves with parallel veins, showy flowers with six parts and root systems with rhizomes. (Rhizomes give rise to new plants.) The plants of the cactus family have two small leaves at seed germination and fleshy and jointed stems with a waxy green bark. Those of the pea family, which includes trees, shrubs, herbs and vines, bear five-petal blooms, bean pods and, typically, pinnate leaves (that is, leaves with a stem and leaflets). The plants of the grass family have segmented stems with each segment bearing a two-part leaf with parallel veins.

Within families and subfamilies, the taxonomists include genera that comprise groups of closely-related species. In the Southwest, a lily subfamily, the Agavaceae, for example, includes four genera, one consisting of the group of the various yucca species; another, of the agave species; another, of the sotol species; and another, of the nolinas species. The cactus family, in the Southwest, includes 11 genera, each made up of closely related species. The opuntia genus, for instance, includes the prickly pears and chollas; the cereus genus includes saguaro and organ pipe cacti; the echinocereus, the numerous hedgehog species; and the mammillaria, the densely-thorned pincushions and fishhooks.

Sotol, with a rosette of leaves, like its relatives the yuccas and agaves.

Putting a Plant in its Place

Consider, for example, the datil yucca, the common name for a plant widely distributed across the higher desert elevations across the Southwest and northern Mexico. On close examination of the datil yucca, we discover that it has 20- to 40-inch long, two-inch wide, sharply pointed, shallow-channeled leaves that issue in a rosette from a very short stem. The leaves have parallel veins with margins that have curled white fibers but no spines. Its stem has vascular bundles that occur in a random pattern in cross section. It has a long tap root, and it has radiating roots, or rhizomes. The plant bears a dense showy cluster of bell-shaped cream-colored flowers, each several inches long, at the top of a very short bloom stalk. Each bloom has parts that occur in multiples of three, for instance, three colored sepals (petal-like parts that protect the flower in bud), three colored petals, and six stamen (the “male” parts that produce the pollen necessary for propagation). Notably, the datil yucca flowers produce 12- to 14-inch long fruit.

Grass, with segmented stems and two-part leaves.

Since the datil yucca has roots and bears blooms that produce seeds, we know that it belongs in the flowering plants division. Since it has leaves with parallel veins, stems with scattered vascular bundles and flower parts in multiples of three, we know that the datil yucca falls in the monocot class. Since it has leaves with parallel veins, showy flowers with three or six parts, and a root system with rhizomes, we would suspect that it belongs to the large lily family. Since it has much in common with the agaves, sotols and nolinas – all, for instance, have rosette leaf forms – we would place the datil yucca in the same subfamily with those plants. Since, however, it also has differences, for example, leaves with white fibers rather than spines along the margins, we can see that it belongs in its own genus, the yucca, along with the Mojave yucca, the Joshua tree yucca, the soaptree yucca, the Torrey yucca and others. Since the datil yucca differs from the other yuccas, most conspicuously in its elongated seed pod, we know that it stands as a separate species. In accordance with tradition of using the genus and a species name, taxonomists have given the datil yucca the scientific name of “Yucca baccata,” with the “baccata” being the Latin word for “berry bearing,” an allusion to the plant’s distinctive seed pod.

Barrel cactus in bloom, a member of the cactus family.

In sum, from a close examination of the plant, we can see that the Yucca baccata, a representative of the plantae kingdom, belongs in the flowering plants division, the monocot class, the lily family, the agave subfamily and the yucca genus. Knowing how the datil yucca fits into a classification system, we can more clearly understand its biological place among the living things of our desert.

Now Come the Animals

Although plants account for more than 90 percent of the earth’s two trillion tons of biomass, they represent only some 20 percent of earth’s classified and named organisms. In contrast, while the animals make up only a small fraction of the biomass, they represent more than 60 percent of the classified and named organisms—our supreme example of biodiversity. The animals’ place in the classification of living things will be the subject of the second and concluding part of this article. Click here for animal classification.

How Are Animals Classified?
Desert Food Chain
Desert Animals & Wildlife
Desert Plant and Wildflowers