Classification of Plants

Can you imagine how confusing life would be if we had no names and every time you wanted to refer to a person, place, or thing you had to describe it and its relationship to all others of its kind? We are fortunate that our predecessors came up with the idea of names and that knowledge about plants has been successfully passed down from generation to generation.

Classification is a term used to describe the process of grouping related plants together and naming the groups. These groups have been historically based on morphological and anatomical features and, more recently, on genetics. Members of a species have characteristics that set them apart from all other populations of plants, and they naturally breed with each other. Closely related species with similar characteristics are grouped into a genus. The genera are grouped into families and the families are grouped into classes. All the land plant classes collectively form two divisions, the bryophytes and the tracheophytes.

Bryophytes are called nonvascular plants because they have a very poorly developed vascular system. Since the vascular system is involved in the transport of water throughout the plant, bryophytes are generally—but not always—found in moist places and go dormant when water is scarce. There are approximately 24,000 species of bryophytes, most of which are mosses. These plants are not widely cultivated commercially but may be of great interest to botanists and naturalists. Mosses may sometimes be found growing on moist surfaces in greenhouses.

Tracheophytes have a well-developed vascular system that enables them to grow taller than bryophytes and to survive temporary shortages of water. They are divided into seed plants and seedless plants. The seedless plants reproduce by spores and many are found in wet habitats. Ferns are seedless tracheophytes that are sometimes grown in gardens or as houseplants.

Tracheophytes that produce seeds are divided into two groups, the gymnosperms and the angiosperms. Gymnosperms are plants that produce seeds that are not enclosed in an ovary, such as the cycads, gingko, conifers, and gnetinae. The cycads have palm or fern-like leaves and produce cones. Gingko trees have broad, deciduous leaves and stinky, fleshy fruits. Conifers such as the Pinus spp. (pines) and Picea spp. (spruce) have needle-like evergreen leaves and produce seeds in cones, whereas Juniperus spp. (junipers) may have scale-like leaves and produce their seeds in berries. The gnetinae, which include Ephedra spp., have xylem vessels similar to those found in mosses, horsetails, and ferns. Of all the gymnosperms, the conifers are the most widely cultivated. Gingko trees are also often used for landscaping, especially along roads and in public places.

Angiosperms are seed plants that produce flowers. The seeds are enclosed in a mature ovary, which forms a fruit. There are more than 250,000 species of angiosperms; this group is the most widely cultivated group of plants on Earth. Angiosperms are divided into two classes, the monocots and the dicots, based on morphology and anatomy. Monocots have seeds with a single cotyledon; dicot seeds have two cotyledons. These traits are the basis for these groups’ names: monocot is short for “monocotyledon” (mono- means “single”) and dicot is short for “dicotyledon” (di- means “two”). The parts of monocot flowers (sepals, petals, stamens, ovary) usually occur in multiples of three, whereas the parts of dicot flowers are found in multiples of four or five. Additionally, monocot leaves have parallel venation. Dicot leaves have net venation because of the arrangement of their vascular system. Figures 2.1 and 2.2 show some of the differences between monocot and dicot plant parts.

The vascular system is a transportation network of connected cells that form tunnels in the plant that extend from the roots through the stem to the leaves, flowers, and fruits. Xylem transports water and minerals upwards from the roots and distributes it throughout the plant. Phloem transports the sugars created by photosynthesis from the leaves to other parts of the plant. These tunnels are bundled together and can be seen as the veins on a leaf.

Figure 2.1 (TOP) The lily is an example of a monocot with flower parts typically found in multiples of three. There are three outer sepals, which in this example look identical to the three inner petals. The stigma has three lobes and there are six stamens. Florists will often remove the anthers from the stamens because they shed large amounts of pollen. (BOTTOM) The veins of leaves of monocots, such as this lily leaf, run parallel to each other.

Figure 2.2 (TOP) The hibiscus flower is a dicot with five petals, five small green sepals, a five-lobed stigma, and many yellow stamens fused to the style. (BOTTOM) This hybrid camellia is a dicot with a rose-like flower. The veins of leaves of dicots form a net-like pattern, known as net venation.

The monocot and dicot classes are subdivided further into families. Members of botanical families share similar characteristics. For example, plants in the Lamiaceae family have characteristically square stems, and many have trichome hairs. These hairs can make the leaves or stems feel rough, and they help reduce airflow across the surface of the leaf. Glandular trichome hair secretes essential oils and resins, which can make these surfaces sticky or fragrant. Many medicinal and culinary herbs, such as mint, sage, lavender, and thyme, are grown for their ability to produce essential oils and resins.

Plants in the family Solanaceae produce mildly to highly poisonous alkaloids and have similar flowers. The introduction of tomato and potato crops from the Americas to Europe took some time because of the reputation of their highly toxic relatives, such as the deadly nightshade.

Plants in the family Leguminosae also have similar flowers and produce fruits called legumes. The pea flower and pea pods are examples of Leguminosae flowers and fruits. Many of these plants also have important relationships with soil bacteria that are capable of nitrogen fixation. The bacteria form nodules in the roots of the plants.

Seed catalogs, plant encyclopedias, and garden centers often group plants in ways other than by family—for example, by life cycle, edibility, cultural requirements, or geography. Life cycle can be annual, biennial, or perennial. Annual plants flower and set seed in the first year and then die. They generally bloom all summer but need to be replanted every year; sometimes they will self-sow if the flowers have been allowed to go to seed. Biennials flower and produce seed in the second year and then die. Perennials take two or more years to flower and set seed, and they live for many years. They generally bloom every year after the first flower. Perennials usually bloom for a short period each season and then die back until the following spring. They can be shortlived (four to five years) or long-lived (many thousands of years) like the bristlecone and Sequoia pine trees.

Deciduous plants lose their leaves in the fall, whereas evergreen plants retain their leaves year round. Some deciduous plants have brilliant displays of colorful foliage in the fall prior to leaf drop. This is due to the carotinoid pigments, which are colored in shades of red and yellow. The green chlorophyll pigments are more numerous when the plant is growing, so during this time carotinoid pigments are masked by the green. When the plant gets ready to go dormant, it stops producing chlorophyll and the red and yellow pigments become visible.

Cultural requirements include the amount of sun and water required by plants, the preferred temperature and soil type, and nutrient needs. Xeric plants survive periods of drought; they are adapted to hot, dry summers and cool, moist winters. Shade plants, as you may have guessed, like to be shaded from the summer sun. Vegetables are often separated into warm-season and cool-season crops based on the temperatures at which their seeds prefer to germinate. Grasses are also classified as warm or cool season. Annual flowers are sometimes labeled as frost tolerant or frost sensitive.

Native plants are those that are believed to have originated in the region where they are to be cultivated. Generally, the cultivation of native plants requires less work because they are already adapted to the soil and climate. Plants may also be grouped according to the ecosystem where they are found growing wild, such as alpine, tropical, desert, or riparian. Alpine plants are found at high altitudes, tropical plants in rain forests, and riparian plants by rivers and streams.