What Is a Plant?

Although it may seem unnecessary to begin by defining “plant,” in fact hundreds of researchers—including several Nobel Prize winners—in laboratories all over the world are discovering previously unknown relationships among living things by examining for the first time the genetic codes that direct the very essence of being. In the process, our ideas on what constitutes a plant are changing. While trees are still obviously plants, and cats and dogs are still animals, the newly forming classifications separate algae and fungi (mushrooms) from the plant kingdom and give super‐kingdom status to the bacteria. Thus restricted, the plant kingdom now includes, in general: mosses, liverworts, hornworts, ferns, fern allies, gymnosperms, and flowering plants. Instructors in most plant biology courses continue to discuss many of the removed “non‐plants” because of the significance of these organisms to the origin and development of the acknowledged plants.

Characteristics of organisms

All living things, despite differences in appearance and size, share basic characteristics. Organisms:

  • Are composed of cells, the smallest units able to conduct the functions of living.

  • Have genes, sequences of deoxyribonucleic acid ( DNA) that carry the instructions for the organization and functioning of the organism.

  • Are made principally of four elements— carbon, hydrogen, oxygen, nitrogen—which were most abundant when the first life appeared eons ago on an early Earth. They combine to form the familiar compounds associated with life, such as—water (H 2O), carbon dioxide (CO 2), methane (CH 4), ammonia (NH 3) and a host of others.

  • Need energy to conduct their metabolism (all of the chemical processes occurring within their bodies).

  • Require materials from the environment to both build and maintain their bodies.

  • Are structurally organized. Multicellular organisms build tissues (groups of similar cells that perform certain functions) and organs (structures formed of different tissues that act as a group to perform specialized functions).

  • React to stimuli and respond, thereby adapting to their environment.

  • Grow (increase in size or weight).

  • Reproduce, producing offspring that insure the continuity of the genetic code from generation to generation.

  • Evolve (change over time).

Special characteristics of plants

  • A plant has all the features of organisms listed above and, in addition most plants have the following special plant characteristics:
  • Plants can photosynthesize (capture light energy and make organic compounds from inorganic materials), which makes them different but not unique—a few other organisms also are photosynthetic, such as some algae and bacteria.

  • In the life cycle of plants there is an alternation of generations in which two genetically different plant bodies alternate: a haploid gametophyte alternates with a diploid sporophyte.

  • Plants develop from embryos, immature sporophytes formed by a fusion of egg and sperm cells, supported by nonreproductive gametophytic tissue.

  • Plants have indeterminate growth. While animals reach a certain size and stop growing, plant cells in their meristematic tissues retain the ability to divide and grow throughout the life of the plant.

  • Plants are sedentary, unlike most animals, but have evolved myriad ways to obtain the materials they need for their metabolism and efficient ways to reproduce and distribute their genes while anchored in one place.

  • Although lacking the nervous systems of animals, plants react and adapt to environmental stimuli (with dramatic and surprising speed in some instances); they also produce secondary metabolites, chemical compounds not directly needed for survival, which deter other plants, fungi, and animals from attacking or consuming the plants.

  • The terrestrial plants of today have evolved with a dependence on water (inherited from their aquatic ancestors); they have developed an elaborate system for obtaining, moving, using, and retaining water for all their metabolic processes and reproductive needs.

A phylogenetic tree of life

Biologists are interested in how organisms are related to one another and have as a general goal the construction of a tree of life in which the evolutionary relationships of all organisms are traced through time much as genealogists trace human family histories. In biology the study of developmental history and evolutionary relationships is called phylogeny. It is possible to trace phylogenies because

  • Organisms have heredity; parents transmit genetic information to their offspring through the renowned molecule, DNA ( deoxyribonucleic acid).

  • Organisms change over time; they evolve to meet changing environmental conditions. The changes gradually become encoded in the DNA and separate lineages of organisms appear.

The sequence of base pairs in individual molecules of DNA and RNA are used to track relationships. Organisms with similar sequences are assumed to have had a common ancestor, and the more alike the strings of base pairs, the closer is the relationship. By combining molecular data with the information already known about organisms, biologists construct phylogenetic trees like that shown in Figure . Most, but not all, biologists agree that at present this is the best way to arrange the branches, but new data undoubtedly will require new interpretations and, perhaps, different phylogenetic trees.