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Chapter 3 Describing and identifying plants.


Upon completion of this chapter, you should be able to

* use scientific plant names correctly.

* define common taxonomic and professional terms.

* describe plants using taxonomic terms.

* use a plant key.


In Chapter 1, you were introduced to a system of classifying plants based on their genetic and evolutionary relationships. It is a natural system of classification and one that accommodates all plants. The systematic classification of plants is called taxonomy and those botanists who are especially interested in plant classification are called taxonomists.

To review briefly, every plant in nature is classified into the following categories and subcategories:

Every plant derives its scientiric name from its generic and specific classification, the smallest two categories to which a plant belongs. Since there are two parts to each scientific name, it is termed a binomial. Scientific names are derived from the classical Latin and Greek languages, predominantly Latin. The use of languages that are no longer spoken ensures that the language will not change and thus affect the scientific terminology.

A plant's scientific name gives it international recognition, since a specific scientific name can only be assigned to one plant, unique in some way from all others in the world. Although plants have common names by which they are known within a particular country or region, the common names have little acceptability to the scientific community because they are often misleading and are inconsistent from place to place.

To illustrate all of this, consider the trees we call the maples. All are in the Genus Acer because of certain genetic and structural similarities. Examples are the silver maple, amur maple, hedge maple, red maple, and Norway maple. To the residents of a particular region, the common names are an adequate identification for these plants, known in the Latin as Acer saccharinum, Acer ginnala, Acer campestre, Acer rubrum, and Acer platanoides, respectively. However, there is also Acer saccharum. It is known in some areas of North America as sugar maple and in other areas as hard rock maple, yet it is the same plant. Also, if desiring a maple with red, not green leaves, throughout the summer season, should a red maple, a scarlet maple, or a crimson king maple be selected? All imply a red color, but with two of the choices, the red color occurs only in the autumn. Only one, the crimson king maple, has red foliage all summer long. Also consider the plant known as the box elder. The name does not suggest a maple, yet it too is a maple, Acer negundo. The common names given plants are often based on physical similarities to more familiar plants. For example, the grape holly (Mahonia aquifolium) is neither a grape nor a holly, but it has leaves resembling an American holly and purple-clustered fruit suggestive of grapes. Common names may also be based on legend, sentiment, or a desire to offer tribute, as, for example, the Judastree, the Tree of Heaven, or the Jacqueline Kennedy Tea Rose.

Professional horticulturists need to know both the scientific and common names of the plants they grow or work with. The layman will normally use the common names in dialogue with the horticulturist, but professionals will use the scientific names in business matters to ensure understanding.

Scientific names are always underlined or printed in italics. The genus is capitalized, but the species epithet is not. To be totally correct, the genus and species are often followed by an abbreviated name or initial in reference to the person who first described the species. For example, in Nerium oleander L., the L after the species name refers to Swedish-born Karl von Linne who described the oleander in 1753. More often called Linnaeus, after the Latin form, he was the first to establish the binomial system of nomenclature for classifying plants and animals. During his seventy-one years (1707-78), Linneaus described over thirteen hundred different plants from all corners of the globe. He truly earned his title as the father of taxonomy (Figure 3-1).

In the everyday use of scientific names, the reference to the pioneer taxonomist may be omitted; however, still another subcategory beyond the species can occur and is often needed to fully distinguish a certain plant. Within the cultivated species that are the economic backbone of ornamental horticulture are certain groups of plants sufficiently different in appearance from others of the same species to warrant special designation. Taxonomists term such a group a variety. When it is an intentionally cultivated variety, it is termed a cultivar. Consider the Norway maple. It has the scientific name of Acer platanoides and has distinctive morphological (structural) features, including deep green foliage. Within the species, one member shares all of the morphological characteristics except that its foliage has a reddish tinge. Its common name is the Schwedler maple or Schwedler Norway maple. Its scientific name is Acer platanoides 'Schwedleri.' The cultivar portion of the name is capitalized, not italicized, and set off by single quotation marks. Another way to indicate the cultivar is as Acer platanoides cv Schwedleri. Either method is considered correct.



In addition to their botanical classification and possession of scientific and common names, most plants can be grouped into an assortment of other categories based on physical appearance rather than genetic relationship. Such groupings are artificial, yet serve many needs of professional plant workers, especially the ornamental horticulturists. An understanding of the terms and descriptions to follow is basic to the many career fields in ornamental horticulture.

Woody plants are those having a corky outer surface of bark covering their older stems. The woody plants usually survive the winter, and the woody stems normally increase in diameter each year.

Herbaceous plants are more succulent plants. They lack bark covering, and their twigs usually do not increase much in diameter. They are often unable to survive the winter in cold climates above ground.

Evergreens are plants that retain their leaves all year. While individual leaves drop and are replaced periodically, the overall appearance of the plant remains green.

Deciduous plants are those that drop their leaves and enter a period of dormancy once a year. Semievergreens may retain their leaves during the winter months, but the leaves discolor and often winter-burn. In southern climates such plants usually are evergreen.

Trees are woody plants, either evergreen or deciduous, that produce a canopy of leaves atop a single stem.

Shrubs are basically the same as trees except that they seldom get as tall and have multiple stems instead of one.

Vines may be woody or herbaceous. Their stems are unable to support the weight of the plants. If the plants are to grow upright, they must be supported by a trellis, fence, or wall.

Groundcovers can be woody or herbaceous, flowering or nonflowering, trailing or compact. At maturity, their height is 18 inches or less.

Annuals are plants that complete their life cycle from seed to fruit to death within one growing season.

Perennials are plants that live several years and, where necessary, can survive cold winter months in a dormant state. They do not die after flowering.

Biennials live two years. The first year is spent in a vegetative stage of development. After a period of winter temperatures, necessary to initiate flower development, the plant flowers during the second year, then dies.

Hardy plants are those that will survive the winter temperatures of a locale. Ornamental plants, especially the woody ones, are assigned hardiness zone ratings. These correspond to eleven zones on the North American continent with different average annual minimum winter temperatures. Zone 1 is the coldest zone and zone 11 is the warmest zone (Figure 3-2). For example, a plant rated as a zone 5 plant will survive the winters in zone 5 or warmer. It will not survive in zone 4 or cooler.

The hardiness zones were established by the United States Department of Agriculture (USDA). Other attempts to rate plant hardiness within regions have complicated this essentially simple rating system. It is important to know if the USDA rating system or some other one is being referred to when using hardiness ratings for plants.

Tender plants are those that will not survive temperatures below freezing.

Nursery plants are those produced in nursery fields, greenhouses, and container operations for use in residential, commercial, public, and institutional landscapes.

Greenhouse crops are usually herbaceous and often flowering plants grown in greenhouses for sale to retail flower shops and other outlets.

Bedding plants are used to create flower beds and flower borders. They may be annual or perennial but are nearly always herbaceous. They are often grown and sold as multiples in strips of peat moss pots or in shallow plastic trays. Others are sold as singles in 4-inch pots.

Foliage plants are prized more for their leaves and habit of growth than for their flowers or fruit. In temperate regions they are used primarily as indoor plants. In tropical and semitropical regions they are used outdoors.

Native plants are those that evolved in a given area.

Exotic plants are those brought into an area to which they are not native Very often their survival is dependent on care and tending by humans.

Naturalized plants are those brought into an area as exotics that adapt well enough to escape cultivation and grow as successfully as native plants.

Aquatic plants are rooted under water. Those of importance to ornamental horticulture, such as water lilies, send their leaves and flowers to the surface.

Specimen plants are visually distinctive due to some feature such as growth habit, flower, bark, or fruit color. The term is applied most often in the landscape profession.

Accent plants, another landscape term, refers to plants different from others with which they may be grouped. The difference is not as marked as with specimen plants, however. It may be only a difference of height, shape, or texture.


Many more horticultural plant categories exist. They are a part of the technical language of the many specialized branches of ornamental horticulture.


The groupings just listed are especially helpful for narrowing the choices when plants are being selected to serve a function, play a role, or solve a problem. There still remains the need to identify plants specifically; for example, to recognize the difference between red oaks and live oaks or between zinnias and dahlias, or to recognize deciduous plants by their winter twigs after leaves, flowers, fruit, or other helpful features are gone.

There are several kinds of identification. The simplest, and at the same time the most complex, is identification through recognition, often built on a lifetime of association and familiarity. If someone asked how you identify your best friends, it might be difficult to describe the mental process behind the recognition. The combination of body build, facial features, hair and eye color, manner of walking, voice pattern, and more, all identify a certain individual. Having a mental picture of that friend then allows us to take only one feature, such as the voice on the telephone, and reconstruct the total person in our mind's eye. Similar identification becomes possible with plants after repeated exposure to the same species. How the plants were first learned is forgotten and the ability to recognize those plants in nearly all situations becomes a part of our permanent learning.

Another, less precise, kind of identification occurs when a plant species is recognized as belonging to a group but the exact species name is not known. For example, you may know it's a spruce, but not know which spruce. Such a level of identification serves many professional horticulturists who have no particular need for precision. The exact basis for the ability to recognize at least the genus of the plant may be as lost in memory as that described earlier.

Finally, there is the kind where the horticulturist has absolutely no idea what the plant is and must begin a methodical process of identification through searching and gradual elimination. Knowing that in all probability the plant has been identified, classified, named, and entered into the scientific literature offers some comfort when faced with the identification of an unknown. Even then it seems like a monumental task trying to identify one plant from among the thousands and thousands of species on the earth.

Sometimes there are shortcuts. For instance, if the plant is known to be a broad-leaved evergreen, or a flowering woody shrub, or a cactus, you can go to specialized texts in which a description or photo may be found. Perhaps someone from the state university or the local county agent can identify the plant. Such questions can be directed to them with clear conscience, but it is still no substitute for knowing how to find out yourself.

The tracing of unknown plants, especially the economically important ornamentals, requires the use of a dichotomous analytical key. There are hundreds of such keys, some dealing with a limited group of plants, others more encompassing. All present the searcher with a series of couplets and pursue identification through a process of elimination. Each couplet consists of two contrasting statements. The searcher chooses the statement that best describes the unknown plant. Beneath that statement will be another couplet, again requiring the selection of one and elimination of the other. The process continues until all species have been rejected except the one to which the plant belongs.

These keys are based in large part on physical features of the plant rather than physiological or evolutionary relationships. Very complex and inclusive keys such as the one published in 1949 by Liberty Hyde Bailey, Manual of Cultivated Plants Most Commonly Grown in the Continental United States and Canada (Revised edition, Macmillan, New York) often require information unknown to the searcher. For example, the couplets may require the choice between flowers with five petals or multiples of five and flowers with six petals or multiples of six. If the unknown plant has no flowers attached, the tracing can get off track very easily.

For the beginner, it is best to begin with a simpler key. Consider the following simplistic example to understand how keys work. Only genera (the plural of genus) are being identified.
1. Plant evergreen
 2. Leaves needle-like
  3. Needles separate
   4. Needles stiff and sharp, four sided
     .... Spruce (Picea)
   4. Needles flat with two white lines on
      underside .... Hemlock (Tsuga)
  3. Needles grouped in fascicles
     .... Pine (Pinus)
2. Leaves broad and flat
    5. Leaves oval in shape, not spiny
     6. Flowers large, compound, and showy
        .... (Rhododendron)
     6. Flowers small, fascicled, and bell-like
        .... Andromeda (Pieris)
    5. Leaves holly-like and spiny
       .... Oregon-grape (Mahonia)

1. Plant deciduous

Keys are seldom this simple but at least the concept of dichotomous couplets can be appreciated.


Since keys rely heavily on physical features of plants as the basis for separation, a trained horticulturist must have a working knowledge of the terms used to describe plants. Such terminology is the stuff of which dichotomous keys are made.


A leaf is described in terms of its shape, its margin, the presence or lack of lobes, its base, the patterns of its veins, its apex style, whether it is simple or compound, and whether it is smooth-surfaced or pubescent (covered with fine epidermal hairs) (Figures 3-3 to 3-9).



A branch or twig is described on the basis of the type of buds it possesses, the nature of its terminal bud, the arrangement of its buds and leaves, the shape of its leaf petiole scars, prominence of its lenticels and stipule scars, and the type of pith it produces (Figures 3-10 to 3-13).


The parts of a flower were illustrated and discussed in Chapter 1. In addition to the flower parts already described, plant keys may identify plants by the type of flower formed. Flowers may be described by the position of the ovary in relation to the flower parts (Figure 3-14).

* A hypogynous flower has a superior ovary, which means that the ovary is attached to the stem above the place where the other flower parts are attached.

* A perigynous flower also has a superior ovary, but the petals and sepals are fused to form a tube-like structure around but separate from the ovary.

* An epigynous flower has an inferior ovary, or one that is attached to the stem below where the other flower parts are attached. There is a floral tube, but it is united with the wall of the ovary.












Flowers may also be described as singular (one flower per stem as with a tulip), composite (multiple small ray and disc flowers as with the sunflower), or an inflorescence (clusters of small flowers arranged on an axis as with snapdragons or viburnums) (Figure 3-15).


A fruit is the ripened ovary of the flower. Generally, as a fruit develops the ovary enlarges beyond the size seen in a fresh flower before pollination. Since there are different types of flowers, it follows that there are also varying kinds of fruits.

Fruits are catagorized into four major groups:

1. Simple fruit: develops from a single ovary

2. Aggregate fruit: develops from a single flower having a group of ovaries

3. Multiple fruit: develops from multiple ovaries of multiple flowers borne on a single stalk

4. Accessory fruit: develops from one or more ovaries and includes the calyx and/or receptacle.


Some of the fruit types are more common to ornamental plants than other types; however, a complete understanding of fruit types is preferable to a partial one. The simple fruits are described by the mature appearance of the ovary wall, the pericarp. Depending on the particular fruit type, the pericarp may be further divided into three separate layers:

1. exocarp (outer layer)

2. mesocarp

3. endocarp (inner layer)

The three layers are easily recognized in the drupes, or "pit" fruits, exemplified by plums and peaches. The thin outer skin is the exocarp, the fleshy part of the fruit is the mesocarp, and the hard pit is the endocarp. So distinctive are the many fruits formed by plants that keys exist which depend solely on the fruits as the basis for the separation (Figure 3-16).



By now, the accumulation of terminology may be weighing heavily on you. Rather than being put off by all of the new words, regard them as a new language, as an expansion of your existing knowledge. Like any new words, they are best learned and understood through repeated use. Obtaining a good plant key and a supply of unknown plants, then identifying one from the other, is the best means of learning the many terms. What begins as memorization can become permanent knowledge if approached properly.


The systematic classification of plants is termed taxonomy. Taxonomy provides each plant in nature with a binomial scientific name composed of the genus and species to which it belongs. While many plants have common names as well, the scientific binomial identifies the plant worldwide. Within a particular species, certain groups of plants are sufficiently distinctive to warrant further designation as varieties. Intentionally cultivated varieties are termed cultivars.

In addition to their botanical classification, plants are commonly grouped into assorted other categories based on their physical appearance rather than their genetic relationships. For example, plants may be categorized as:

* woody or herbaceous

* evergreen, deciduous, or semi-evergreen

* trees, shrubs, vines, or groundcovers

* annuals, perennials, or biennials

* hardy or tender

* nursery or greenhouse crops

* bedding or foliage plants

* native, exotic, or naturalized

* aquatic

* specimen or accent

Unknown plants can be identified by using a dichotomous analytical key. Hundreds of keys exist, some dealing with a limited group of plants, others more encompassing. All require the searcher to choose the plant's identifying features from among a series of couplets. Use of the keys requires that the searcher know the terminology used to describe leaves, twigs, flowers, and fruits.

Leaves are described in terms of their shapes, margins, lobing, bases, vein patterns, apex styles, surface pubescence, and whether they are simple or compound. Twigs are described on the basis of their bud types, terminal buds, bud and leaf arrangements, petiole scar shapes, prominence of their lenticels and stipule scars, and type of pith. Flowers are described by the position of the ovary in relation to the flower parts, or by the arrangement of flowers on the stem. Fruit are described as simple, aggregate, multiple, or accessory depending on the number of flowers and ovaries involved in their formation.

Achievement Review


1. For each of the following statements, list the word that will complete it correctly.

a. The systematic classification of plants is called --.

b. Botanists specializing in plant classification and relationships are called --.

c. The two-part scientific name of a plant is called a --.

d. Every plant derives its scientific name from its genus and --.

e. In the scientific name, only the -- is capitalized.

f. Every plant has only -- scientific name.

g. Scientific names are either italicized or --.

h. The father of taxonomy is --.

i. Within a single species of plants, differences may result in a subcategory known as a --.

j. When differences among members of a species are intentionally encouraged by horticulturists, the plants are termed --.

2. Explain the differences between the following types of plants:

a. woody and herbaceous plants

b. evergreen and deciduous plants

c. trees and shrubs

d. vines and groundcovers

e. annuals and perennials

f. biennials and perennials

g. hardy and tender plants

h. bedding plants and foliage plants

i. native and exotic plants

j. naturalized and exotic plants

k. specimen and accent plants

3. Label these three types of leaves and the types of apex, base, and margin of each.

4. Label these types of inflorescence.

5. Label these types of fruits.

6. From the definitions that follow, list the correct terms.

a. a flower that has an inferior ovary with a floral tube united with the wall of the ovary

b. a flower that has a superior ovary and a floral tube separated from the ovary

c. a flower that has a superior ovary and no floral tube

d. a flower having only one blossom per stem

e. a flower head composed of many small ray and disc flowers

f. a fruit that develops from a single ovary

g. a fruit that develops from a single flower having a group of ovaries

h. a fruit that develops from one or more ovaries and includes the calyx and/or receptacle

i. a fruit that develops from multiple ovaries of multiple flowers borne on a single stalk

j. the innermost part of the pericarp

k. the outermost part of the pericarp

l. the central part of the pericarp

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Title Annotation:SECTION I The Science of Ornamental Horticulture
Author:Ingels, Jack E.
Publication:Ornamental Horticulture, Science, Operations & Management, 3rd ed.
Geographic Code:1USA
Date:Jan 1, 2001
Previous Article:Chapter 2 The soil.
Next Article:Chapter 4 Plant growth regulators.

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