Plants and Their Environment
David Trinklein, Division of Plant Sciences
Plants are living organisms that contain chlorophyll and use it to manufacture their own food. Their cell walls are more or less rigid and support both the individual cells and the whole structure. Even when plants have reached what we regard as their full, mature size, they continue to expand and develop new leaves, flowers, fruit and shoots.
Unlike animals, plants cannot move when the environment changes. They are at the mercy of the climate and the gardener because they are rooted in place. Even though it appears that many plants, especially larger ones, are quite tolerant of change, they sometimes do not show adverse effects until long after the event. For example, tree roots are often damaged or killed by suffocation during building projects or flooding. An established tree may still have strength to leaf out and may appear to thrive for several years. But in its weakened state, the tree is more likely to blow down, become infested or simply decline.
To understand why plants respond as they do to natural influences and to cultivation, gardeners must understand something about their structure and how they grow. This publication provides such an introduction.
Gardeners tend to group plants by their horticultural uses: fruits, vegetables, flowers, trees, shrubs, turf and so on. These categories are a convenient way to think and learn about plants.
Plants can also be categorized by the length of their life cycles. Annual, biennial and perennial are terms that describe how long a plant will live and also indicate when it is likely to bloom.
An annual plant's entire life cycle from seed germination to seed production occurs in one growing season, and then the plant dies. Many flowering plants that we consider to be annuals are not annuals in their native habitats. They would continue to grow and flower in future years if freezing temperatures did not kill them. Tuberous begonia (Begonia × tuberhybrida) is an example of an ornamental plant treated as an annual in the Midwest, although it is a perennial in the southern states.
Annuals may be further subdivided into summer and winter annuals:
- Winter annuals begin their life cycle in late summer or fall, survive in the vegetative state through winter, and then flower quickly in the spring before dying.
- Summer annuals begin their life cycle in spring or summer and complete their entire life cycle before fall or winter.
A biennial plant starts from seed and produces vegetative structures and food storage organs in its first full season. A rosette of basal leaves persists through winter. During the second season, the plant's life cycle is completed with flowers, fruit and seed. The plant then usually dies. These plants will often reseed themselves. Examples of biennials are carrots, beets, cabbage, celery, onions, hollyhock, Canterbury bells and Sweet William.
Sometimes plants that typically develop as biennials may complete their entire cycle of growth from seed germination to seed production in only one growing season. Conditions of drought, unusual variations in temperature or other climatic changes can cause the plant to pass through the physiological equivalent of two full growing seasons in one year.
A perennial plant is a plant that lives for more than two years. Typically, perennials die back in the fall and return in the spring because of some sort of overwintering structure, such as a rhizome or crown. Examples include flowers such as daylilies, blackeyed-susan and coneflower; and vegetables such as asparagus and rhubarb.. Plants often characterized as weeds such as common milkweed and morning glory are also perennials.
Perennials are classified in various ways:
Plants that die back to the ground each winter and have new stems that grow from the roots each spring.
Plants where the top persists, such as shrubs or trees.
Plants that shed their leaves and are leafless during a portion of the year.
Plants with leaves that persist throughout the year.
Plants that do not survive cold winters. Tender perennials are often grown as annuals.
Plants that tolerate cold temperatures.
The structure and appearance of plants' flowers, leaves, fruit and seed play a large part in how we think of them and also provide useful information about their classification. For example, the flowers of a daisy indicate a probable relationship with other plants that have similar flowers. The majority of grasses are easily recognized by their long leaf blades.
More than 500,000 different kinds of plants and plantlike organisms exist in the world. Of these, the flowering plants classified as angiosperms are the most abundant and familiar to us. Gymnosperms are the other main group of seed-bearing plants. There are also more primitive plants such as mosses and ferns that reproduce by spores.
Angiosperms have seeds encased in closed ovaries that become plants' familiar fruits, pods, grains or capsules. They represent virtually all crop plants and those we think of as flowers.
The angiosperms are further classified into two groups according to the number of seed leaves, called cotyledons, that emerge from a germinating seed:
- Monocotyledonous plants (monocots) are those such as grasses whose seed first extends a single cotyledon or seed leaf. Lilies and other bulb plants are also monocots.
- Dicotyledonous plants (dicots) have seed that forms a pair of cotyledons; examples are bean, petunia and tomato.
Gymnosperms are plants that develop exposed or naked seeds. These include the coniferous plants such as fir, pine and spruce. Ginkgo and the tropical cycads are also gymnosperms.
Modern plant taxonomy is based on a system developed by the Swedish physician and botanist Carl von Linné, who later changed his name to Carlos Linneaus. His classification is based on the flowers and reproductive parts of a plant. Because these are the parts of a plant least influenced by environmental changes, this system has been found to be the best.
Grouping plants with similar botanical structure helps us to understand how they are related to one another. Close relatives often have similar pest problems. Botanical similarities may also show, for example, how long certain plants can be expected to live and why they react as they do to certain conditions. In addition, their botanical, Latin or "proper" names help to avoid confusion when the same or similar common names exist for different plants.
Each plant is assigned two names. The genus or generic name can be likened to a person's last name, as in "Doe." The specific epithet or species name is that person's given name, "Jane" or "John." This combination of two names is the plant's botanical, scientific or Latin name.
For example, the botanical name for sugar maple is Acer saccharum (pronounced AY-ser sa-KAH-rum). The genus name Acer is a classical (Latin) name. The genus name for the Indian bean tree Catalpa is a Native American name. Other botanical names provide descriptions of the flower: for example, Antirrhinum (snapdragon) is from the Greek anti, which means "like," and rhinos meaning nose or snout. One familiar genus is Narcissus (daffodil) named for the mythological character who was turned into this flower when he drowned attempting to reach the person he saw reflected in a pool of water.
Specific epithets may have similar descriptive value, such as rubra for red and major for large or larger. In the sugar maple example, the word saccharum is from the Latin for sugar cane, and it is similar to words we know that mean sweet. Some species commemorate a botanist or plant explorer. The late 18th century Swedish naturalist Carl Peter Thunberg introduced many Asian plants. He is remembered in plant names, including the species Berberis thunbergii, the Japanese barberry, and a genus of the warm-climate, climbing blackeyed-susan, Thunbergia.
Words in many complete Latin names include botanical variety, subspecies and cultivar. These build upon the basic binomial naming system to further separate individuals that differ from one another in, for instance, flower color or growth habit. They are not so different as to require new specific names.
Botanical classification of four plants
|Colorado blue spruce||Ornamental pear||Cauliflower||Sweet corn|
|Cultivar||'Fat Albert'||'Bradford'||'Snow Crown'||'Silver Queen'|
Every living organism — plant, animal, insect and so forth — can be classified into the following categories or taxa:
- Genus (plural: genera)
- Species (abbreviated sp. [singular] or spp. [plural])
- Cultivar or variety
For plants, the kingdom is Plantae and division is Tracheophyta. Class is usually either Angiospermae or Gymnospermae, the angiosperms and gymnosperms that make up most of our cultivated plants. At the subclass and order level, further groupings of similar plants are named.
A family of plants shares similar characteristics. For example, the spring-flowering magnolia trees, whose deciduous forms are best known in the north, and the evergreen southern forms are in the same family, not surprisingly called Magnoliaceae. Different magnolia specimens can be "keyed out" using a botanical key. The combination of characteristics that identify this family are enclosed ovules, flowers that are not catkins, flowers with calyces, clear and separate "distinct" carpels (reproductive portion of flower), overlapping or imbricate sepals, and alternate, simple leaves.
In another example, peas belong to a large family of legumes called Fabaceae (formerly named Leguminosae). The edible pea flower is shaped much like the flower of a tree in the same family commonly referred to as redbud (Cercis canadensis). All legumes have similar flowers and fruiting structures even though they may be vastly different in form. Other legumes include alfalfa, beans, clover, honeylocust, Kentucky coffee tree, Siberian pea shrub and wisteria.
What do roses have in common with apple trees? They are members of the same family, Rosaceae. Their fruits are pomes. Plants in this family share susceptibility to the same diseases. For example, pears and roses are susceptible to fire blight, and both are subject to mildew during humid weather. Other plants in the Rosaceae family include cotoneaster, spirea, juneberry, quince and mountain ash.
When groups of similar plants are categorized into families, the next lower level of classification is the genus. Plants in the same genus often share similar fruits, flowers, roots, stems, buds and leaves. The genus name is always capitalized and italicized or underlined. Examples:
- Acer = maple
- Begonia = begonia
- Catharanthus = vinca
- Solanum = tomato
- Quercus = oak
Specific definition comes with the species name, or specific epithet. At this level, marked features that are carried from generation to generation distinguish the group. Specific names are not capitalized, but they are italicized or underlined. Examples:
- Acer rubrum (red maple)
- Begonia x semperflorens (fibrous-rooted begonia)
- Catharanthus roseus (vinca)
- Solanum lycopersicon (tomato)
- Quercus alba (white oak)
Variety (botanical), subspecies, form
Sometimes the specific name is followed by a botanical variety, subspecific name or form that denotes a fairly consistent, naturally occurring variation within the species. This second specific name is preceded by the abbreviation var., ssp., or forma (f.).
- Picea pungens var. glauca (Colorado blue spruce)
- Gleditsia triacanthos var. inermis (thornless honeylocust)
Cultivar (short for cultivated variety)
A cultivar is a group of plants that is clearly distinguished by certain characteristics that may be morphological (structural), physiological (functional), cytological (cellular) or chemical. The differences do not have to be visual for a variation to gain cultivar status — perhaps it is simply more hardy or disease resistant. When a plant is reproduced asexually (by cloning), it retains these distinguishing characteristics.
Cultivar names are always capitalized within single quotes or preceded by the abbreviation cv. In the nursery industry, the cultivar name is recognized as a plant's official name.
- Acer platanoides 'Crimson King' (Crimson King Norway maple)
- Fraxinus pennsylvanica cv. Marshall's Seedless (Marshall's Seedless green ash)
Along with cultivar designation, recent new cultivars may have other assigned names that are often trademarked (Golden NuggetAA dwarf Japanese barberry, Berberis thunbergii 'Monlers')
More plant identification terms
Several more terms may be used to define particular plants or plant groups:
A variation within a cultivar. For example, 'Spur Red Delicious' apple is a selected type of Red Delicious, which is a cultivar of apple.
The result of natural or deliberate crossbreeding between two or more dissimilar parents is known as a hybrid. Hybrids are denoted by the multiplication sign "x, " placed correctly without a space before the hybrid name. While we find most hybrids at the specific level within a genus, some are intergeneric (result from the cross-breeding of two genera). For example, the hybrid Forsythia xintermedia is from a cross between two species, Forsythia suspensa and Forsythia viridissima.The intergeneric hybrid xSolidaster (hybrid goldenrod) comes from its parent species, Solidago and Aster.
An individual or individuals that were started by asexual propagation and are genetically identical to each other.
Plants of uniform appearance grown from seed.
- F1 hybrid
A cross between two inbred lines..
Plant identification using plant keys
Dichotomous plant keys are used to identify plants through a series of choices between pairs of alternatives. Each pair refers to a specific plant characteristic such as arrangement of leaves on the stem, type of leaf margin or type of fruit. By selecting the option that accurately describes the plant, you will be led to the next choices until you determine the genus or species.
If a result is ambiguous, final verification can be made by comparison with a known example of that species. In their detailed comparisons, plant taxonomists often use preserved specimens stored in an herbarium.
Reference books for specific types of plants, such as ferns, wildflowers or shrubs, frequently contain their own specialized plant keys. Try to use keys that employ botanical rather than common names. Common names can be confusing for several reasons: one plant may have several common names; the common name for a plant often differs from one region to another; and the same common name can also apply to more than one plant. Botanical names, by contrast, are unique and relatively permanent.
Several major plant keys are available, including the following:
- Bailey, L.H. 1949. Manual of Cultivated Plants Most Commonly Grown in the Continental United States and Canada. New York: Macmillan.
- Gleason, Henry A., and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. Bronx: New York Botanical Garden.
- Petrides, G.A. 1958. A Field Guide to Trees and Shrubs. Boston: Houghton Mifflin.
The plant cell
The plant cell is the basic organizational unit of plants (Figure 1). Each living plant cell contains a nucleus that controls all of the chemical activities in the cell. Within the nucleus, division of the DNA provides the way for the cell to pass on heritable information from one generation of cells to the next.
Cytoplasm is the other main part of the living plant cell. It is composed of many cell structures (organelles), water, pigments, sugar and various minerals. The cytoplasm is bound by a plasma membrane that regulates the flow of water and nutrients into and out of the cell.
The plant's cell wall is one of the fundamental differences between plant and animal cells. The somewhat rigid cell wall is made up of a number of chemical compounds, primarily the carbohydrate cellulose.
The second major difference between plant and animal cells is that many plant cells contain the green pigment chlorophyll. Chlorophyll is contained in chloroplasts, where photosynthesis, the food manufacturing process, takes place. A chloroplast is a type of organelle known as a plastid. There are also plastids that contain pigments other than chlorophyll.
Plant cells can have specialized functions, and there are many cell types. Plant cells are largely made up of water held within the vacuole, which exerts a pressure against the rigid cell wall. This pressure, called turgor pressure, gives the plant shape and structure. When insufficient water is available in the plant to maintain this pressure, the plant begins to droop or wilt.
Individual cells work together to form the whole plant. Tissues are organized groups of cells that are similar in appearance and function. An organ is a group of tissues that accomplishes a common function. Plants have two organ systems: roots and shoots. Shoots, in turn, have two main organs: leaves and stems. These organs are made up of various tissues that are called meristematic, which may be dermal or vascular.
Meristematic tissues are sites of cellular activity and division. This is where all of the cell division takes place. Meristematic tissues give rise to the other tissue systems and are named for their location. Animal tissues do not have these specific sites of cell division — rather, all animal cells can divide to create new tissues.
An apical meristem is located at the apex, or tip, of a shoot or root. The lateral meristems exist in the stems and roots of many plants. They help the plant grow in thickness or diameter. The vascular cambium is a lateral meristem that forms new xylem (water-conducting) cells on the inside and new phloem (food-conducting) cells on the outside. Active cambium cells are exposed when the outer skin or bark is peeled away from a dicot stem (monocots usually have no cambium).
There are two types of dermal tissues — epidermis and periderm.
The epidermis forms the outer covering of the plant and, in most cases, secretes a waxy coating called cutin, which forms the cuticle. This often shiny coating protects the plant from major water loss and protects the underlying cells. Specialized groups of epidermal cells form pores that allow water, carbon dioxide, oxygen and other gases to pass through to and from the underlying tissues. These pores, referred to as stomata (singular: stoma), are made up of two guard cells that open and close to regulate the flow of vapors and gases.
The periderm is created by the cambium and forms the bulk of the bark of woody plants. It is a layer of corky cells that, like the epidermis, prevents water loss and protects the cells beneath. The periderm also has pores to allow gas exchange for underlying tissues. These pores, which are visible to the naked eye, are called lenticels.
Vascular tissues make up the water- and food-conducting system of a plant. They consist of the xylem and phloem.
The xylem tubes are the water- and mineral-conducting channels and are made up of cells that are shaped into columns that can effectively move water through even a large tree.
Phloem tubes move food produced by photosynthesis to other parts of the plant.
Every plant has a unique form and structure and is made up of several distinct organs. All of these influence a plant's overall health and appearance. Gardeners need to consider all parts of the plant and the effects of the environment on these structures, which include roots, stems, buds, leaves, flowers, seeds and seedlings, and fruits.
Healthy roots are vital to the well-being and the continued development of most cultivated plants. Roots' structure and growth habits have pronounced effects on the size and vigor of a plant, its ability to adapt to various soil types, and its responses to cultural practices and irrigation. In addition, many plants spread through buds that develop on vigorous roots, and portions of root can be used for vegetative reproduction or propagation. Examples are phlox and lilac (Syringa). Roots that store carbohydrates are often used as food for us and for animals. Carrots, beets, sweet potatoes and turnips are examples.
Types of roots
One or more primary roots originate at the lower end of a seedling or cutting. From here, the root system develops, which is usually characteristic of the plant. Specific soil conditions can cause modifications in roots, however. For example, the taproot of a carrot growing in stony soil will be stunted and branched.
A taproot is formed when the primary root continues to elongate downward into the soil to become the dominant and most important feature of th