Plants and Their Environment | MU Extension

Missouri Master Gardener Core ManualDavid 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.

AnnualAn 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:

BiennialA 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.

PerennialA 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:

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.

AngiospermsAngiosperms 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:

GymnospermsGymnosperms 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 Linn, 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

Every living organism plant, animal, insect and so forth can be classified into the following categories or taxa:

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.

FamilyA 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.

GenusWhen 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:

SpeciesSpecific 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:

Variety (botanical), subspecies, formSometimes 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.).

Examples

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.

Examples

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 termsSeveral more terms may be used to define particular plants or plant groups:

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:

Figure 1Plant 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).

Dermal tissuesThere are two types of dermal tissues epidermis and periderm.

Vascular tissuesVascular tissues make up the water- and food-conducting system of a plant. They consist of the xylem and phloem.

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 rootsOne 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.

Figure 2Longitudinal section of root.

Root structureA root has no nodes and never directly bears leaves or flowers (Figure 2). Lengthwise, it has four main parts:

More on rootsThe quantity and distribution of plant roots are important because these two factors have a major influence on the root's ability to absorb moisture and nutrients. The depth and spread of the roots depend on the plant's inherent growth characteristics and on the texture and structure of the soil. Roots will penetrate more deeply into a loose, well-drained soil, where there is adequate soil oxygen, than into a dense, poorly drained soil. A solidly compacted layer in the soil, sometimes called a hardpan, will restrict or terminate root growth.

During early development, a seedling plant absorbs nutrients and moisture from the soil within a few inches of the location of the seed from which the plant grew. As plants become well established, the root system develops laterally and usually extends to several times the spread of the branches. The greatest concentration of fibrous roots occurs in the top 12 inches of soil, but significant numbers of laterals may grow downward from these roots to provide an effective absorption system several feet or more underground.

Stems are generally the bulkiest and most obvious part of the plant. They support the leaves, buds, flowers and fruit. Water, nutrients, the products of photosynthesis, and gases pass up and down stems, to and from the roots. In certain plants, stems function as storage organs for food manufactured through photosynthesis. They may spread out and root, making new plants. Portions of stem, often called cuttings or slips, are used in vegetative reproduction or propagation. Examples are ivy, blackberry and willow (Salix). We commonly use stems as food examples include asparagus, kohlrabi, broccoli, cauliflower, rhubarb and potatoes.

Figure 3Cross section of woody plant stem. Figure 4Cross section of woody plant stem.

Structure of stemsBark is the external covering of the stem of woody plants. Internally, the stem's three major parts are the xylem, phloem and cambium (Figure 3). The xylem tissue consists of tube-like cells that conduct water and dissolved minerals and gases in the stem, while the phloem tissue conducts food products. Xylem forms the inner rings to become sapwood and heartwood of woody stems. The cambium is dicotyledonous meristematic tissue with cells that divide and enlarge to force the stem to expand outward. New xylem is formed on the inner side of the cambium and new phloem on the outside. The cambium is a thin, actively growing layer that is vulnerable to girdling by wires, weed trimmers and even a tree's own roots.

Herbaceous plants have stems that differ in internal arrangement when compared with woody plants. Although monocots and dicots both contain xylem and phloem, their vascular systems are arranged differently (Figure 4). In the stem of a monocot, the xylem and phloem are paired into bundles that are dispersed throughout the stem. In herbaceous dicots, those vascular bundles are arranged in a circle in the stem.

Figure 5Typical woody stem.

External features of stemsStems grow either above- or belowground. They may be long with large distances between leaves and buds, or they may be compressed with almost no distance between leaves and buds. The location on the stem where a leaf or bud occurs is called a node (Figure 5). It is sometimes difficult to distinguish between stems and roots, but one sure way is to look for nodes. Stems have nodes; roots do not.

The internodes are the regions between nodes. The length of an internode depends on many factors. One of these is genetic oaks usually have shorter internodes than sycamores. Environment is also a great influence. For example, decreasing fertility will decrease internode length. Early-season growth, which is often the most vigorous, usually results in the greatest internode length. Too little light will cause stems to elongate, resulting in long, spindly growth. Paradoxically, plants that are growing vigorously tend to have longer internodes than weak plants. Internode length will also be affected by competition from surrounding stems or fruits. If the plant's energy (available water and food) is divided between three or four stems, or if fruits (seeds) are also developing on the stem, less energy is available for any one shoot, and internode length is shortened.

Look at the varying internode lengths in a full season's growth of a deciduous tree, such as an oak or an apple. An interesting exercise for a gardener is to look at a stem and then try to identify the conditions that may have affected growth.

Types of stemsTypical stems are the trunks and branches of shrubs and trees, and the stalks of nonwoody plants. Modified stems can be found both aboveground and belowground.

Parts of aboveground modified stems.

Parts of belowground modified stems.

A bud is an undeveloped shoot from which leaves or flower parts grow. The buds of deciduous trees and shrubs typically are protected by leathery bud scales or, in the case of some evergreens, a resinous covering. Some buds are termed "naked" because they have no covering. Herbaceous plants have naked buds in which the outer leaves are green and somewhat succulent.

Buds may require exposure to a certain number of days below a critical temperature before they will resume growth in the spring. This time period varies for different plants. During rest, dormant buds can withstand low temperatures, but after the rest period, buds become more susceptible to weather conditions and can be damaged easily by cold temperatures or frost.

A leaf bud is composed of a short stem with embryonic leaves and develops into leafy shoots. Leaf buds are often less plump than flower buds. Flower buds are made up of a short stem with embryonic flower parts.

Buds are classified by their location on the stem. Terminal or apical buds are located at the apex or tip of the stem. Lateral or axillary buds are found on the sides of the stem, usually in the leaf axil, the point of leaf attachment to the stem. Adventitious buds arise at other sites, including the internode of the stem, at the edge of a leaf blade, from callus tissue at the cut end of a stem or root, or laterally from the roots of plants.

The principal function of leaves is to absorb sunlight for the manufacture of plant sugars. This process is called photosynthesis. The typical leaf has a flattened surface to present a large area that efficiently absorbs light energy. In most cases, the leaf is supported by a stemlike appendage called a petiole. The base of the petiole is attached to the stem at the node. The angle formed between the petiole and the stem is called the leaf axil. A bud or cluster of buds is usually located in the axil.

Figure 6Cross section of dicot leaf.

Structure of leavesThe leaf blade is composed of several layers (Figure 6). On the top and bottom is a layer of small, tough epidermal cells. The primary function of the epidermis is to protect leaf tissues. The arrangement of the cells in the epidermis determines the texture of the leaf surface. Hairs that are present on some leaves are extensions of epidermal cells.

The thickness of the cuticle (the layer of cutin produced by epidermal cells) is a direct response to sunlight. The stronger the light, the thicker the cuticle. For this reason, plants grown in the shade should be moved into full sunlight gradually over a period of a few weeks to allow the cutin layer to build and to protect the leaves from rapid water loss and sunscald.

Cutin repels water and can shed pesticides if spreader/sticker agents or soaps are not used. This is the reason many pesticide manufacturers include some sort of spray additive to adhere to or penetrate the cutin layer.

On the surface of leaves are the stomata. Some plants have stomata on both surfaces; others have them only on the lower surface. Formed from epidermal guard cells that are capable of opening and closing, the stomata regulate the passage of water vapor, oxygen and carbon dioxide into and out of the leaf. The opening and closing of guard cells is determined by the environment. Conditions that cause large water losses from plants (high temperature, low humidity) stimulate closing, while mild weather conditions leave guard cells open. Guard cells close in the absence of light.

The middle layer of a leaf is known as the mesophyll. This is the location of the chloroplasts that contain the green pigment chlorophyll. Photosynthesis takes place here. In monocot plant leaves, the mesophyll consists of cells and air spaces. In dicot leaves, it is divided into a dense upper layer called the palisade and a lower, spongy layer of cells with air spaces.

Leaf types

Figure 7ASimple leaf type.

Figure 7BCompound leaf type.

Figure 8Leaf venation.

Leaf venation Venation describes the patterns in which the veins are distributed in the blade (Figure 8).

Figure 9Leaf shapes, simple, left, compound, above right, and conifers, below right.

Leaf shapesThe shape of the leaf blade and the type of leaf margin are important characteristics that help identify plants (Figure 9). Leaf blades vary a great deal. They may be simple (apple, oak) or compound (divided into several smaller leaflike segments, as in honeylocust). The smaller segments are called leaflets and are attached to a stalk (rachis) with a petiolule. Leaflets can also be arranged palmately (horse chestnut) or pinnately (ash). Pinnately compound leaves are said to be odd pinnate (ash) when ending in one leaflet and even pinnate when ending in two leaflets (locust). This terminology is important in identifying plants by their leaves.

Leaf modificationsLeaves have adapted to survive a wide range of environmental conditions. For instance, leaves exposed to strong sunlight are often smaller and have thicker cuticles than leaves of the same plant growing in shade. The reduced surface area and thicker cuticle reduces water loss. Leaves that develop in shade have a larger surface area to absorb light.

Chloroplasts respond to light by exposing as much pigment as possible in low light situations and by exposing less pigment in bright conditions. We see the results: dark green foliage on shade-grown plants and paler green foliage when the same plant grows in a sunnier location. Examples of this include hosta, Norfolk Island pine and weeping fig.

Leaves on plants that grow in dry environments will often be thick or narrow with few intercellular (air) spaces in the mesophyll, while guard cells are sunken below the level of the regular epidermis to minimize water loss. In some desert plants, such as cacti, the foliage leaves may be modified into thorns and photosynthesis occurs in chloroplast-containing cells in the stem.

Plants that grow underwater have just a few widely spaced mesophyll cells and big intercellular spaces for holding gases that are harder to acquire underwater.

Many conifers have leaves adapted for windy or low-moisture conditions. Needlelike leaves on pines have little wind resistance, and the flattened or scalelike leaves of junipers are waxy and well protected from the hot sun.

Distinct leaf modifications that occur on plants

Figure 10Leaf arrangements.

Leaf arrangement and attachmentLeaves at the nodes may grow in pairs opposite one another (maple) or alternate (birch) from side to side along the stem (Figure 10).

They also may be whorled, with three or more leaves arising from a node, such as hydrangea. Subopposite leaves are slightly offset from one another; these are relatively rare. An example is the katsura tree, Cercidiphyllum japonicum.

Although there are many different kinds of flowers, they are similar in their organization. The function of flowers is sexual reproduction. Thus, flowers often form the showiest part of the plant. Their color and fragrance attract pollinators such as insects or birds to assure the continuance of the species. Flowers that are neither showy nor scented rely on other methods for pollination wind, for example. Yet all have the same basic structures.

Figure 11Parts of a typical flower.

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Plants and Their Environment | MU Extension