Classification of Plants

Gardeners should have some knowledge of how plants are classified. The plant kingdom includes many kinds of plants ranging in size from microscopic bacteria to giant redwoods. The science of taxonomy deals with the orderly classification of these plants.

The taxonomist divides plants into three major groups: the Thallophyta, the Bryophyta, and the Tracheophyta.

The word "Thallophyta" means "Thallus plants." These plants are primitive and undifferentiated into roots, stems, and leaves. The bacteria, fungi, and algae belong to this group. Bacteria and fungi are important to the gardener because they cause many diseases of garden plants. Algae are sometimes a problem in greenhouses and in wet, shady locations.

The word "Bryophyta" means "moss plants." This group contains the mosses and the liverworts. Mosses are primitive plants that like shade and moist sites. Liverworts are flat, creeping plants that grow in moist, shady sites. They are frequently found growing along a stream. In Japan mosses are grown for their landscape effect. In the moist Japanese climate mosses make a fine ground cover under pines. We use mosses in terrariums, but for the most part we make little use of these charming plants in outdoor gardens. When mosses grow in a shady lawn, we try to get rid of them.

The Tracheophyta have well-differentiated roots, stems, and leaves and well-developed tissues for conducting food and water. The group contains the ferns and their allies as well as the seed-producing plants. This group of higher plants is the one that the gardener is primarily concerned with. The Tracheophyta are subdivided into three classes: Filicinae, Gymnospermae, and Angio-spermae. The Filicinae include the ferns and the fern allies like the horsetails and lycopodiums, or ground pines. These reproduce by spores and do not bear seeds.

The Gymnospermae and the Angiospermae are the seed plants and are considered the highest forms of the plant kingdom. The Gymnospermae include the conifers, cycads, ginkgoes, and so on. The word "Gymnospermae" means "naked seed." In this group the seeds are not enclosed in a fruit. In the conifers the seeds are borne at the base of the scales that constitute the cones. As the cones mature, the scales separate and the seeds drop to the ground.

In the Angiospermae the seeds are produced inside fruits which assume a variety of forms and may be either dry or fleshy. The Angiospermae are divided into two subclasses: Monocotyledoneae (monocots) and Dicotyledoneae (dicots). These names refer to the number of seed leaves, or cotyledons. The Monocotyledoneae have other distinguishing characteristics besides the single cotyledon: The leaves are parallel veined. The vascular bundles (water and food conducting tissues) are often scattered as in corn, and there is no well-differentiated cambium (a single layer of cells capable of cell division. The flower parts (sepals, petals, stamens, and carpels) usually occur in 3's or multiples of 3. The Dicotyledoneae is the largest group of seed plants. In addition to having two cotyledons, or seed leaves, the leaves are netted veined, the vascular bundles form a cylinder and develop a well-defined cambium, and the flower parts do not occur in 3's or multiples of 3.

Below the class and subclass categories, plants are classified into orders and families and finally into the correct genus and species. It is important that the gardener know both the scientific and common name of a plant. Knowing the family to which a plant belongs can also help since many of the plants in a family have similar cultural requirements.

The scientific name always consists of the genus and the species names and sometimes the name of the botanical variety. The common name may be easier for some people to remember, but, unfortunately, different common names are used for the same plant in different parts of the country and occasionally in the same locality. For example, the several species of Amelanchier are variously known as serviceberry, sarvisberry, Juneberry, saskatoon, shad-bush, snowy mespilus, and so on.

The scientific name for a given plant, once the taxonomists have agreed upon it, is the same throughout the world. Scientific names are not difficult to pronounce and are easy to remember. We have no trouble remembering names like petunia, zinnia, tradescantia, and impatiens, which are generic names that are also used as common names.

Every plant has a genus and species identification. This system of classification was devised by Linnaeus in the eighteenth century and is called the binomial system. Let us take a familiar group of plants like the maples. All maples belong to the genus Acer. The species name is often descriptive: Acer rubrum is the red or swamp maple, Acer saccharum the sugar maple. In the oaks the genus name is Quercus. Quercus alba is the white oak, Quercus coccinea the scarlet oak.

Minor differences in plant form or color are common within a plant species. A naturally occurring population that differs from the species is called a botanical variety. An example is the Black Hills strain of white spruce. Native across the northern United States and Canada, white spruce have denser foliage in the Black Hills of South Dakota than elsewhere. The scientific name of the white spruce is Picea glauca. The strain growing in the Black Hills is Picea glauca var. densata. Another example of a botanical variety is the Colorado blue spruce. The Colorado spruce is normally green, but occasionally trees with a bluish cast are found in nature and in nurseries. The scientific name of Colorado blue spruce is Picea pungens var. glauca.

To distinguish a horticultural variety from a botanical variety, the term "cultivar" is used. A single plant in a seedling population may differ from all the rest. If this plant has characteristics that would make it a desirable plant to grow for ornamental or food purposes, it is given a cultivar name and usually propagated vege-

tatively either from cuttings or by grafting (see Chapter 3). A few examples will help clarify the difference between a botanical variety and a cultivar. Let us first consider Picea pungens var. glauca (Colorado blue spruce). In a seedling population containing blue forms, a single plant may be distinct in form or color. This plant can be selected, propagated vegetatively, and introduced as a named cultivar. A cultivar name is always capitalized and either enclosed within single quotation marks or prefixed with the letters "CV." The Moerheim Spruce is a compact form of Colorado blue spruce that has very blue foliage. This is properly designated Picea pungens 'Moerheimi' or Picea pungens CV Moerheimi. Several seedlings of Norway maple develop a bloodred foliage. One of these is the Crimson King Norway maple. This is properly designated Acer platanoides 'Crimson King'. Most cultivars of trees and shrubs are increased from a single, selected parent plant by vegetative means.

In efforts to create new and better horticultural plants, man has often crossed two or more species of the same genus. The seedling population from a cross between two species is sometimes given a specific name. To distinguish this interspecific hybrid from a natural species, the species designation is preceded by an "x." Spiraea x bumalda (Bumalda spirea) is a hybrid species derived by crossing S. japonica x S. albiflora; Syringa x chinensis (Chinese Lilac) by crossing S. laciniata x S. vulgaris. Of the hundreds of other hybrid species that exist, some are natural hybrids and others were created by man. Usually, a seedling population of a hybrid species is somewhat variable. Individual plants that stand out as being superior to their sister seedlings may be vegetatively propagated and given a cultivar name. Spiraea x bumalda 'Anthony Waterer' (Anthony Waterer Spirea) is such a cultivar.

In the development of new cultivars in many genera of plants, several species are sometimes involved in the ancestry of a single selection. This is true with garden roses and many flowering crab-apples. In such instances only the genus name precedes the cultivar name. Malus 'Sparkler' is the proper designation for the recently introduced Sparkler crabapple. Rosa 'Peace' is the proper designation for the Peace rose, one of the Hybrid Tea roses.

Not all cultivars are vegetatively propagated. Some are grown from seed. This is particularly true of annual flowers and vegetables. Fj hybrid seed is the result of crossing two inbred lines. The seedlings resulting from such a cross will be very uniform and are given a cultivar name. Seeds saved from such hybrid plants will not produce uniform plants; seedlings in the second generation will revert to parental types. This is why one should not save seeds from Fj hybrids.

Cultivar names are also given to plants grown from open-pollinated seeds, provided the resulting seedlings are uniform and distinctive. Such seeds are generally the result of several generations of inbreeding to fix the desired characteristics. Seeds must then be produced in isolated fields to prevent cross-pollination from some other cultivar.

With commonly grown plants like the tomato and marigold, it is permissible to use the common rather than the scientific name. Tomato 'Big Boy' is as acceptable as Lycopersicum 'Big Boy'. In the chapters that follow, many species and cultivar names will be used; the preceding explanation should help gardeners understand the nomenclature.

Chapter i

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  • Dino
    How do gymnospermae and angiospermae differ from rest of plant kingdom?
    4 years ago

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