From tiny blades of grass to giant sequoia trees, the thousands of plants in the M world enrich our lives. They not only provide us with food and oxygen, but also bring us beauty, shelter, warmth, and clothing. So what makes a plant a plant? Sometimes it’s obvious and sometimes it’s obscure—and often there’s a lot more there than meets the eye. Getting to the bottom of what defines a plant as a plant is like investigating a mystery.

In the five-kingdom system
of classification, a tickseed flower (Coreopsis tinctoria), (insert), is a plant but diatoms, single-celled algae, are not.

Aristotle divided all living things into two kingdoms—plants and animals. His system of classification—the simplest and the longest-used—is the one used in this encyclopedia. As discussed in the preceding article, this classification was used by Linnaeus, who based his classification on visible structures. Even in this system, however, classifying an organism such as euglena, for example, was confusing because the euglena displays traits characteristic of both kingdoms. Since the early 1900’s, and increasingly in recent years, taxonomists and systematists have faced the challenge of developing new systems of classification. Taxonomists are scientists who identify, name, and classify organisms, and systematists are scientists who classify organisms or groups of organisms on the basis of their evolutionary relationship.

In the 1950’s, formal views of classification slowly began to change. Advances in the study of embryology, molecular biology, and biochemistry presented new and different ways of looking at plants and animals. However, proposals of new kingdoms met with resistance until sophisticated technological advancements in microscopy allowed scientists to study the internal structures of both small and large living things. For the first time, the differences between bacteria and fungi were seen as greater than the differences between plants and animals.

New classification systems

In the following years, many new multikingdom systems were proposed, including a 13-kingdom system. The most widely supported of these proposals is the 5-kingdom system presented by ecologist Robert H. Whittaker of Cornell University in 1959.

The five kingdoms of Whittaker’s system— Monera, Protista, Fungi, Plantae, and Animalia— are based on microscopic structural and bio-chemical characteristics as well as on evolutionary relationships. In the five-kingdom system, bacteria were distinguished from plants by being prokaryotic organisms (organisms whose cells lack a membrane around the nucleus) and were moved from the plant kingdom into their own kingdom—Monera. Algae (except for blue-qreen algae, which are Monerans) were moved from the plant kingdom into the kingdom Protista, which they share with protozoa, former members of the animal kingdom. Protista are recognized by what they don’t do and what they are not. For example, unlike the organisms of the plant kingdom, algae do not develop from an embryo contained in a seed. Fungi also have trieir own kingdom, and differ from other large eukaryotic organisms (plants and animals) in that they develop without an embryo.

What is a plant?

After removing bacteria, algae, and fungi, what is left in the kingdom Plantae? The plant kingdom has thousands of species, including mosses and liverworts, club mosses and horsetails, ferns, cone-bearing plants, and flowering plants. Most plants produce their own food through photosynthesis. But technically, plants differ from the other kingdoms based on their life cycle—and not on photosynthesis. All members of the plant kingdom have a life cycle called “alternation of generations.” In this type of life cycle, a haploid stage with only one set of chromosomes alternates with a diploid stage that has two sets of chromosomes. Haploid plants are called game-tophytes; diploid plants are called sporophytes. In the flowering and cone-bearing plants, only the sporophyte can be seen with the unaided eye. In other plants, such as some mosses, the familiar plant is the gametophyte.

A plant’s life cycle is the most basic qualification for membership in the plant kingdom, but there are other criteria. Almost all plants are land dwellers. In addition, unlike fungi, which absorb nutrition, and unlike animals, which eat food, most plants produce their own food through photosynthesis. Kingdom Plantae members are multicellular with specialized cells and tissues and have plastids, chlorophyll, and cellulosic cell walls. Some species of algae, such as the seaweeds, resemble plants and share some plant characteristics. Some of these species alternate generations, have plastids, and have cellulose in their cell walls. They also use chlorophyll to pho-tosynthesize and produce their own food. But they are not considered truly multicellular. Their specialized cells and tissues are much simpler than the complex structures in the true plants, so they are no longer considered plants under the five-kingdom system.

All systems of classification—new and old alike—are created by humans for human use and subject to judgment calls by people whose viewpoints and opinions frequently differ.

Further advances in technology, along with the accumulation of knowledge, are certain to bring changes in the classification of organisms. For example, DNA and RNA sequencing have begun to clarify the evolutionary history of bacteria, and may someday lead to reorganization of Moneran classification. But, in the study of plants, it is important to remember always that “a rose by any other name would smell as sweet.”

Small red gill mushrooms, are a member of the Fungi kingdom, in the five-kingdom system.