Exploring the Roots and Stems of Abelia Plant: Anatomy and Function

Horticulturists work with a wide array of plants, both as garden friend or foe. This range of different plants brings an enjoyable diversity to the garden and landscape, yet on close observation many similarities become evident. These similarities in basic plant structures and functions, along with the environmental factors that affect plant growth, are the basis of this chapter. The information provided in this chapter will primarily focus on the higher flowering plants because they are most significant in the garden and landscape. This chapter will cover terminology to help enhance your understanding of botanical references in the future and to gain a perspective on the practices discussed in later chapters.

Hundreds of millions of years of evolution have produced an amazingly diverse and complex array of plants. Land plants are primarily divided into two groups: gymnosperms and angiosperms. Angiosperms are flowering plants that produce seeds enclosed in a fruit. There are over 300,000 species of angiosperms distributed all over the world. The gymnosperms, numbering around 700 species, are primarily the evergreen species of the temperate zones. They produce naked seeds, which are usually borne in cones. Gymnosperms generally also have narrow or needle-like leaves, while angiosperms usually have broad leaves.

Angiosperms are first subdivided into two major subclasses based on their vascular (or vein) arrangement. The dicots include most of the broadleaf herbs, shrubs, and trees. Monocots include such orders as lilies, palms, and grasses.

Cells are the structural and functional units of life. Large organisms are made up of trillions of cells, while small organisms may be composed of only a single cell. Both plants and animals are made of cells, though plant and animal cells are somewhat different.

Plant cells consist of a cell wall containing cellulose, a chemical compound. Inside the cell wall, a plant cell has some of the same features as animal cells: a cell membrane, mitochondria (which are responsible for respiration, or energy production), a nucleus (which contains the genetic information for the organism and controls the activities of the cell), and numerous other organelles necessary to carry out the mechanisms of life. Plant cells also develop one (or more) large liquid-filled cavity called a vacuole.

Plant cells have special structures called chloroplasts. Chloroplasts are the sites of photosynthesis and contain chlorophylls and carotenoid pigments. Chlorophyll is responsible for the green color of plants. Carotenoids are yellow and orange pigments that are masked by the more numerous chlorophyll pigments in green leaves. Chloroplasts are found only in plants and green algae (Evert and Eichhorn, 2012).

Plant cells grow and divide in different directions, creating all the structures of the plant like roots, stems, leaves, and more.

Stems are structures that support buds and leaves and serve as conduits for water, minerals, and sugars. The three major internal parts of a stem are the xylem, phloem, and cambium. The xylem and phloem are the major components of a plant’s vascular system. The vascular system transports food, water, and minerals and offers support for the plant. Xylem tubes conduct water and minerals to the leaves, while phloem tubes conduct sugars and other metabolic products away from the leaves.

The xylem and phloem make up the vascular system of the stem and are arranged in distinct strands called vascular bundles that run the length of the stem. When viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, these individual bundles grow together (producing growth rings). In monocot stems, the vascular bundles are randomly scattered throughout the stem.

The difference in the vascular system of the two groups is of practical interest to the horticulturist because certain herbicides are specific to either monocots or dicots. An example is 2,4-D, an herbicide that only kills dicots. In contrast, dicots may be more readily grafted as it is easier to align the vascular rings of the two stem pieces compared to the scattered bundles in monocots.

A part of the stem where a bud is located is called a node. This is where leaves are attached to the stem, and buds are located in these leaf axils (angle between stem and bud/leaf).

The stem section between nodes is called the internode. The length of an internode may depend on many factors. Internode length varies with the season. Growth produced early in the season has the greatest internode length; length decreases as the growing season nears its end. Decreasing fertility will decrease internode length. Too little light will result in a long internode, causing a spindly stem. This situation is known as stretch or etiolation. Vigorously growing plants tend to have greater internode lengths than less vigorous plants. Internode length will also vary with competition from surrounding stems or developing fruit. If the energy for a stem has to be divided among three or four stems, or if the energy is diverted into fruit growth, internode length will be shortened.

A bud is a small package of partially preformed tissue which becomes leaves/stems or flowers. In some cases, buds contain partially preformed flower tissue (flower bud), and usually have a different appearance from a vegetative bud, a bud that contains partially preformed leaf and stem tissue. Some buds contain both floral and vegetative tissues.

The presence of leaves (regular or modified) or buds distinguishes a stem. Although typical stems are above-ground trunks and branches with great distances between leaves and buds, there are modified stems that can be found above ground and below ground. The above-ground modified stems are crowns, stolons, and spurs; and the below-ground stems are bulbs, corms, rhizomes, and tubers.

Spurs are short, stubby, side stems that arise from the main stem. They are common on such fruit trees as pears, apples, and cherries, and are capable of bearing fruit. If severe pruning is done close to fruit-bearing spurs, the spurs can revert to a long, nonfruiting stem.

Crowns (seen in strawberries, dandelions, and African violets) are another type of compressed stem having leaves and flowers on short internodes. A crown is a region of compressed stem tissue from which new shoots are produced, generally found near the surface of the soil. Crowns are located at soil level so that roots support them upright and the central growing point is never covered with soil. Many herbaceous perennials, such as Shasta daisy, also develop crowns that enlarge with branching over successive years. These crowns persist over winter with buds that develop into elongated aerial stems during the growing season. A stolon is a horizontal stem that is fleshy or semi-woody and lies along the top of the ground. The spider plant has stolons. A runner is a type of stolon. It is a specialized stem that grows on the soil surface and forms a new plant at one or more of its nodes.

Strawberry runners are examples of stolons. Remember, all stems have nodes and buds or leaves. The leaves on strawberry runners are small, but are located at the nodes, which are easy to see. The nodes on the runner are the points where roots begin to form.

Below-ground stems, such as the potato tuber, the tulip bulb, gladiolus corm, and the iris rhizome, store food for the plant.

Rhizomes are similar to stolons but grow underground. Some rhizomes are compressed and fleshy, such as those of iris; they can also be slender with elongated internodes, such as bentgrass. Bermudagrass is both an effective lawn grass and a hated weed principally because of the spreading capability of its rhizomes.

Tulips, lilies, daffodils, and onions are plants that produce bulbs — shortened, compressed, underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem. If you cut through the center of a tulip or daffodil bulb, you can see major plant parts within the bulb. Many bulbs require a period of low-temperature exposure before they begin to send up the new shoot. Both the temperature and length of this treatment are of critical importance to commercial growers who force bulbs for holidays.

Corms are not the same as bulbs. They have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to a dry, leaf-like covering. Examples of corms include gladiolus and crocus.

A tuber is an enlarged portion of an underground stem. The tuber, like any other stem, has nodes that produce buds. The eyes of a potato are actually the nodes on the stem. Each eye contains a cluster of buds.

Some plants produce a modified stem referred to as a tuberous stem. Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the crown, and fibrous roots are found on the bottom of the tuberous stem.

In addition, some plants, such as dahlia and sweet potato, produce an underground storage organ called a tuberous root, which is often confused with a bulb or tuber. However, these are roots, not stems, and have neither nodes nor internodes.

It may sometimes be difficult to distinguish between roots and stems, but one sure way is to look for the presence of nodes with their leaves and buds. Stems have nodes; roots do not.

A shoot is a young stem with leaves present. A twig is a stem that is less than one year old and has no leaves since it is still in the winter-dormant stage. A branch is a stem that is more than one year old and typically has lateral stems. A trunk is a main stem of a woody plant. Most trees have a single trunk.

Trees are perennial woody plants, usually with one main trunk and usually more than 12 feet tall at maturity.

Shrubs are perennial woody plants that have one or several main stems, and usually are less than 12 feet tall at maturity. The distinction between a small tree and large shrub is blurry, and often botanists will describe these plants as small trees or large shrubs.

A vine is a plant that develops long, trailing stems that grow along the ground unless they are supported by another plant or structure. Some twining vines circle their support clockwise (hops or honeysuckle), while others circle counter-clockwise (e.g., pole beans or Dutchman’s pipe vine). Clinging vines are supported by aerial roots (e.g.,English ivy or poison ivy). A tendril is a modified plant part (leaf, stem, or flower, depending on the plant) that encircles the supporting object (e.g., cucumber, gourds, grapes, and passionflowers). Some tendrils have adhesive tips (e.g., Virginia creeper and Japanese creeper).

The five major plant functions that are the basics for plant growth and development are photosynthesis, respiration, transpiration, absorption, and translocation.

One of the major differences between plants and animals is the ability of plants to internally manufacture their own food (called autotrophy). To produce food for itself, a plant requires energy from sunlight, carbon dioxide from the air, and water from the soil. If any of these ingredients is lacking, photosynthesis, or food production, will stop. If any factor is removed for a long period of time, the plant will die. Photosynthesis literally means “to put together with light.”

Plants first store the energy from light in simple sugars, such as glucose (C6H12O6). Some of these sugars are converted back to water and carbon dioxide, releasing the stored energy through the process called respiration. This energy released from respiration is required for all living processes and growth. Simple sugars are also converted to other sugars and starches (carbohydrates) which may be transported to the stems and roots for use or storage, or may be used as building blocks for more complex structures (e.g., oils, pigments, proteins, cell walls).

Any green plant tissue is capable of photosynthesis. Chloroplasts in these cells contain the green pigment chlorophyll which traps the light energy. However, leaves are generally the site of most food production due to their special structure. The internal tissue (mesophyll) contains cells with abundant chloroplasts in an arrangement that allows easy movement of water and air. The protective upper and lower epidermis (skin) layers of the leaf include many stomata that regulate movement of the gases involved in photosynthesis into and out of the leaf.

Photosynthesis is dependent on the availability of light. Generally speaking, as sunlight increases in intensity, photosynthesis increases. This results in greater food production. Many garden crops, such as tomatoes, respond best to maximum sunlight. Tomato production is cut drastically as light intensities drop. Only two or three varieties of “greenhouse” tomatoes will produce any fruit when sunlight is minimal in late fall and early spring.

Water (H2O) plays an important role in photosynthesis in several ways. First, it maintains a plant’s turgor, the firmness or fullness of plant tissue. Turgor pressure in a cell can be compared to air in an inflated balloon. Water pressure or turgor is needed in plant cells to maintain shape and ensure cell growth. Second, water is split into hydrogen and oxygen by the energy of the sun that has been absorbed by the chlorophyll in the plant leaves. The oxygen (O2) is released into the atmosphere, and the hydrogen is used in manufacturing carbohydrates. Third, water dissolves minerals from the soil and transports them up from the roots and throughout the plant where they serve as raw materials in the growth of new plant tissues. The soil surrounding a plant should be moist, not too wet or too dry. Water is pulled through the plant by evaporation of water through the leaves (transpiration).

Photosynthesis also requires carbon dioxide (CO2) which enters the plant through the stomata. Carbon and oxygen are used in the manufacturing of carbohydrates. Carbon dioxide in the air is plentiful enough so that it is not a limiting factor in plant growth. However, since carbon dioxide is consumed in making sugars and is not released by plants at an equal rate, a tightly closed greenhouse in midwinter may not let in enough outside air to maintain an adequate carbon dioxide level. Under these conditions, improved crops of roses, carnations, tomatoes, and certain other crops can be produced if the carbon dioxide level is raised with CO2 generators or, in small greenhouses, with dry ice.

Although not a direct component in photosynthesis, temperature is an important factor. Photosynthesis occurs at its highest rate in the temperature range 65 to 85°F (18 to 27°C) and decreases when temperatures are above or below this range.

Carbohydrates made during photosynthesis are of value to the plant when they are converted into energy. This energy is used in the process of building new tissues (plant growth). The chemical process by which sugars and starches produced by photosynthesis are converted into energy is called respiration. It is similar to the burning of wood or coal to produce heat (energy). This process in cells is shown most simply as:

This equation is precisely the opposite of that used to illustrate photosynthesis, although more is involved than just reversing the reaction. However, it is appropriate to relate photosynthesis to a building process, while respiration is a breaking-down process.

Plant growth and distribution are limited by the environment. If any one environmental factor is less than ideal, it will become a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, it is important to understand the environmental aspects that affect plant growth. These factors are light, temperature, water, humidity, and nutrition. For more about how these factors impact plants, see Chapter 5: “Abiotic Stress.”

Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in the summer and the minimum in winter. The more sunlight a plant receives (up to a point), the better capacity it has to produce plant food through photosynthesis. As the sunlight quantity decreases, the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using cheesecloth shading above the plants. It can be increased by surrounding plants with white or reflective material, or supplemental lights.

Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors, producing a rainbow. Red and blue light have the greatest effect on plant growth. Green light is least effective to plants as they reflect green light and absorb none. It is this reflected light that makes them appear green to us. Blue light is primarily responsible for vegetative growth or leaf growth. Red light, when combined with blue light, encourages flowering in plants. Fluorescent, or cool-white, light is high in the blue range of light quality and is used to encourage leafy growth. Such light would be excellent for starting seedlings. Incandescent light is high in the red or orange range, but generally produces too much heat to be a valuable light source. Fluorescent “grow” lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible, but they are costly and generally not of any greater value than regular fluorescent lights.

Light duration, or photoperiod, refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized, it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period, but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod. Plants can be classified into three categories depending upon their flowering response to the duration of darkness. These are short-day, long-day, or day-neutral plants.

Short-day plants form their flowers only when the day length is less than about 12 hours in duration. Short-day plants include many spring- and fall-flowering plants, such as chrysanthemum and poinsettia. Long-day plants form flowers only when day lengths exceed 12 hours (short nights). They include almost all of the summer-flowering plants, such as rudbeckia and California poppy, as well as many vegetables, including beet, radish, lettuce, spinach, and potato. Day-neutral plants form flowers regardless of day length. Some plants do not really fit into any category, but may be responsive to combinations of day lengths. The petunia will flower regardless of day length, but flowers earlier and more profusely under long daylight. Since chrysanthemums flower under the short-day conditions of spring or fall, the method for manipulating the plant into experiencing short days is very simple. If long days are predominant, a shade cloth is drawn over the chrysanthemum for 12 hours daily to block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not present longer than 12 hours, artificial light is added until flower buds are initiated.

Temperature affects the productivity and growth of a plant, depending on whether the plant variety is a warm- or cool-season crop. If temperatures are high and day length is long, a cool-season crop such as spinach will bolt (flower prematurely) rather than produce the desired flower. Temperatures that are too low for a warm-season crop such as tomato will prevent fruit set. Adverse temperatures also cause stunted growth and poor quality. For example, the bitterness in lettuce is caused by high temperatures. The USDA classifies geographic areas into a series of “plant hardiness zones” based on the average annual minimum winter temperature, divided into 10-degree F zones. The most recent (2012) version is based on weather data from 1976–2005. To check your USDA hardiness zone, go to:

Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daytime temperatures are around 59°F (15°C). The Christmas cactus forms flowers as a result of short days and low temperatures. Daffodils are forced to flower by putting the bulbs in cold storage in October at 35 to 40°F (2 to 4°C). The cold temperatures allow the bulb to break dormancy. The bulbs are transferred to the greenhouse in midwinter where growth begins. The flowers are then ready for cutting in three to four weeks.

Thermoperiod refers to the daily range of temperatures a plant is exposed to. Plants produce maximum growth when exposed to a day temperature that is about 10 to 15° higher than the night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. This causes plant growth to slow down or even stop. For growth to occur, photosynthesis must be greater than respiration.

Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. Not all plants grow best in the same temperature range. For example, snapdragons grow best when night temperatures are 55°F (12°C); the poinsettia prefers 62°F (17°C). Florist cyclamen does well under very cool conditions, while many bedding plants prefer a higher temperature. Recently, it has been found that roses can tolerate much lower night temperatures than previously believed. This has meant a conservation in energy for greenhouse growers.

However, in some cases, a certain number of days of low temperatures are needed by plants to grow properly. This is true of crops growing in cold regions of the country. Peaches are a prime example; most varieties require 700 to 1000 hours between 45°F (7°C) and 32°F (0°C) before they break their rest period and begin growth. Lilies need 6 weeks at 33°F (1°C) before blooming.

Plants can be classified as either hardy or nonhardy (tender), depending on their ability to withstand cold temperatures. Winter injury can occur to nonhardy plants if temperatures are too low or if unseasonably low temperatures occur early in the fall or late in the spring. Winter injury may also occur because of desiccation (drying out) — plants need water during the winter. When the soil is frozen, the movement of water into the plant is severely restricted. On a windy winter day, broadleaved evergreens can become water-deficient in a few minutes; the leaves or needles then turn brown. Wide variations in winter temperatures can cause premature bud break in some plants and consequent bud-freezing damage. Late spring frosts can ruin entire peach crops. If temperatures drop too low during the winter, entire trees of some species are killed by the freezing and splitting of plant cells and tissue.

Water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm (living material) of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stoma, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of the solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage.

Relative humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure, expressed as a percent.

For example, if a kilogram of air at 75°F could hold 4 grams of water vapor and there are only 3 grams of water in the air, then the relative humidity (RH) is:

Warm air can hold more water vapor than cold air; therefore, if the amount of water in the air stays the same and the temperature increases, the relative humidity decreases.

Water vapor will move from an area of high RH to one of low RH. The greater the difference in humidity, the faster water will move.

The relative humidity in the air space between the cells within the leaf approaches 100%; therefore, when the stomate is open, water vapor rushes out. As the vapor moves out, a cloud of high humidity is formed around the stomate. This cloud of humidity helps slow down transpiration and cool the leaf. If air movement blows the humid cloud away, transpiration will increase.

Many people confuse plant nutrition with plant fertilization. Plant nutrition refers to the needs and uses of the basic chemical elements in the plant. Fertilization is the term used when these materials are supplied to the environment around the plant. A lot must happen before a chemical element supplied in a fertilizer can be taken up and used by the plant.

Plants need 16 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are found in the soil. These six elements are used in relatively large amounts by the plant and are called macronutrients (nitrogen, potassium, and phosphorous are the primary macronutrients and magnesium, sulfur and calcium are the secondary macronutrients). There are seven other elements called micronutrients (or trace elements) and are used in much smaller amounts. Micronutrients are found in the soil: iron, zinc, molybdenum, manganese, boron, copper, and chlorine. All 16 elements, both macronutrients and micronutrients, are essential for plant growth.

Most of the nutrients that a plant needs are dissolved in water and then absorbed by the roots. Many nutrient combinations in fertilizers dissolve easily, and those nutrients can be readily absorbed. Sometimes two dissolved nutrients will combine into a product that has very low solubility. Availability of both nutrients to the plant is then severely reduced. This can occur with calcium and phosphorus and the micronutrients. Nutrient solubility is also affected by soil pH. High (alkaline) pH levels drastically reduce the solubility and availability of micronutrients (a factor in iron deficiency in azaleas), while low (very acidic) pH levels make some micronutrients (and non-nutrient minerals such as aluminum) so highly available as to injure the plant. Another consideration is the nutrient balance in the soil. For example, calcium and magnesium are absorbed similarly, but magnesium is absorbed more readily. The root does not select the nutrient to be absorbed; if both are present at the absorption site, the magnesium will be absorbed. This is why a soil test may indicate that, while there is sufficient calcium in the soil, a plant can suffer calcium deficiency because of an excess of magnesium competing for absorption.

The process whereby nutrients are absorbed varies. Water and nutrients can move between the outer cells of the root, but eventually they must cross a membrane to enter a cell. Water and some nutrients can do this easily (a passive process) while other nutrients are too large for the ‘holes’ in the membrane, and energy is needed to move these nutrients into the cell (an active process). Absorption is generally a combination of these processes: certain nutrients are actively absorbed, others enter passively to maintain a chemical balance.

Anything that lowers or prevents the production of sugars in the leaves can lower nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficiency problems may develop. The stage of growth or rate of growth may also affect the amount of nutrients absorbed. Many plants go into a rest period, or dormancy, during part of the year. During this dormancy, few nutrients are absorbed. Plants may also absorb different nutrients just as flower buds begin to develop.

Taxonomy is the science of biological classification of plants and animals. The purpose of taxonomy is to develop a convenient and precise method of classifying human knowledge. This method thus preserves knowledge and makes it accessible. In this chapter, we will learn how plants are classified, then learn how to go about identifying a plant by the use of a leaf key.

Table 1-1: Photosynthesis vs respiration

By now, it should be clear that respiration is the reverse of photosynthesis. Unlike photosynthesis, respiration occurs at night as well as during the day. Respiration occurs in all life forms and in all cells. The release of accumulated carbon dioxide and the uptake of oxygen occurs at the cell level. In animals, blood carries both carbon dioxide and oxygen to and from the atmosphere by means of the lungs or gills. In plants, there is simple diffusion into the open spaces around the cells, and exchange occurs through the stomata on leaves and stems, or through root hairs.

Transpiration is the process by which a plant loses water, primarily from leaf stomata. Transpiration is a necessary process that involves the use of about 90% of the water that enters the plant through the roots. The other 10% of the water is used in chemical reactions and in plant tissues. Transpiration is involved in the movement of water, minerals, and at times, stored sugars from the roots to other parts of the plant. This occurs in the xylem .

Transpiration and turgor pressure: The amount of water lost from the plant depends on several environmental factors such as temperature, humidity, and wind or air movement. An increase in temperature or air movement decreases humidity outside the leaf and increases the rate of transpiration. This presents a continuing danger to plants as the rate of water loss may not be matched by the rate of water absorption from dry soil into the roots. A water deficit in the plant may only lead to temporary wilting (loss of turgor pressure) from which the plant may rapidly recover when the transpiration rate decreases later in the day or stops overnight. The guard cells respond to the loss of turgor by shrinking and closing the stomata. While this significantly reduces further damaging water loss, it also impedes carbon dioxide entry for photosynthesis. Repeated temporary wilting can lead to stunted plants due to reduction of the food supply and other metabolic changes, especially in cell division and enlargement. A plant maintains adequate turgor pressure when the amount of water loss due to transpiration is equal to the amount of water absorbed into the plant.

Absorption is the process by which substances, particularly water and minerals, are moved into the plant. This occurs mainly through the roots in the tip region where root hairs are present, but it may also occur through leaf surfaces. Water absorption into roots may be a passive process due to a “pulling” action, drawing water through the xylem tubes to replace water lost from the leaves through transpiration. Other water absorption is an active process linked to active absorption of mineral nutrients. This is discussed later in the section on plant nutrition.

Translocation is the movement of sugars, amino acids, and other plant chemicals from the leaves to other parts of the plant through the phloem. This translocation is often an active process requiring respiration energy as the substances are moved upward and downward in the plants to growing areas or storage.

Plant growth and distribution are limited by the environment. If any one environmental factor is less than ideal, it will become a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, it is important to understand the environmental aspects that affect plant growth. These factors are light, temperature, water, humidity, and nutrition. For more about how these factors impact plants, see Chapter 5: “Abiotic Stress.”

Light has three principal characteristics that affect plant growth: quantity, quality, and duration.

Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in the summer and the minimum in winter. The more sunlight a plant receives (up to a point), the better capacity it has to produce plant food through photosynthesis. As the sunlight quantity decreases, the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using cheesecloth shading above the plants. It can be increased by surrounding plants with white or reflective material, or supplemental lights.

Light quality refers to the color or wavelength reaching the plant surface. Sunlight can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors, producing a rainbow. Red and blue light have the greatest effect on plant growth. Green light is least effective to plants as they reflect green light and absorb none. It is this reflected light that makes them appear green to us. Blue light is primarily responsible for vegetative growth or leaf growth. Red light, when combined with blue light, encourages flowering in plants. Fluorescent, or cool-white, light is high in the blue range of light quality and is used to encourage leafy growth. Such light would be excellent for starting seedlings. Incandescent light is high in the red or orange range, but generally produces too much heat to be a valuable light source. Fluorescent “grow” lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible, but they are costly and generally not of any greater value than regular fluorescent lights.

Light duration, or photoperiod, refers to the amount of time that a plant is exposed to sunlight. When the concept of photoperiod was first recognized, it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period, but the length of uninterrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photoperiod. Plants can be classified into three categories depending upon their flowering response to the duration of darkness. These are short-day, long-day, or day-neutral plants.

Short-day plants form their flowers only when the day length is less than about 12 hours in duration. Short-day plants include many spring- and fall-flowering plants, such as chrysanthemum and poinsettia. Long-day plants form flowers only when day lengths exceed 12 hours (short nights). They include almost all of the summer-flowering plants, such as rudbeckia and California poppy, as well as many vegetables, including beet, radish, lettuce, spinach, and potato. Day-neutral plants form flowers regardless of day length. Some plants do not really fit into any category, but may be responsive to combinations of day lengths. The petunia will flower regardless of day length, but flowers earlier and more profusely under long daylight. Since chrysanthemums flower under the short-day conditions of spring or fall, the method for manipulating the plant into experiencing short days is very simple. If long days are predominant, a shade cloth is drawn over the chrysanthemum for 12 hours daily to block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not present longer than 12 hours, artificial light is added until flower buds are initiated.

Temperature affects the productivity and growth of a plant, depending on whether the plant variety is a warm- or cool-season crop. If temperatures are high and day length is long, a cool-season crop such as spinach will bolt (flower prematurely) rather than produce the desired flower. Temperatures that are too low for a warm-season crop such as tomato will prevent fruit set. Adverse temperatures also cause stunted growth and poor quality. For example, the bitterness in lettuce is caused by high temperatures. The USDA classifies geographic areas into a series of “plant hardiness zones” based on the average annual minimum winter temperature, divided into 10-degree F zones. The most recent (2012) version is based on weather data from 1976–2005. To check your USDA hardiness zone, go to:

Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daytime temperatures are around 59°F (15°C). The Christmas cactus forms flowers as a result of short days and low temperatures. Daffodils are forced to flower by putting the bulbs in cold storage in October at 35 to 40°F (2 to 4°C). The cold temperatures allow the bulb to break dormancy. The bulbs are transferred to the greenhouse in midwinter where growth begins. The flowers are then ready for cutting in three to four weeks.

Thermoperiod refers to the daily range of temperatures a plant is exposed to. Plants produce maximum growth when exposed to a day temperature that is about 10 to 15° higher than the night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. This causes plant growth to slow down or even stop. For growth to occur, photosynthesis must be greater than respiration.

Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. Not all plants grow best in the same temperature range. For example, snapdragons grow best when night temperatures are 55°F (12°C); the poinsettia prefers 62°F (17°C). Florist cyclamen does well under very cool conditions, while many bedding plants prefer a higher temperature. Recently, it has been found that roses can tolerate much lower night temperatures than previously believed. This has meant a conservation in energy for greenhouse growers.

However, in some cases, a certain number of days of low temperatures are needed by plants to grow properly. This is true of crops growing in cold regions of the country. Peaches are a prime example; most varieties require 700 to 1000 hours between 45°F (7°C) and 32°F (0°C) before they break their rest period and begin growth. Lilies need 6 weeks at 33°F (1°C) before blooming.

Plants can be classified as either hardy or nonhardy (tender), depending on their ability to withstand cold temperatures. Winter injury can occur to nonhardy plants if temperatures are too low or if unseasonably low temperatures occur early in the fall or late in the spring. Winter injury may also occur because of desiccation (drying out) — plants need water during the winter. When the soil is frozen, the movement of water into the plant is severely restricted. On a windy winter day, broadleaved evergreens can become water-deficient in a few minutes; the leaves or needles then turn brown. Wide variations in winter temperatures can cause premature bud break in some plants and consequent bud-freezing damage. Late spring frosts can ruin entire peach crops. If temperatures drop too low during the winter, entire trees of some species are killed by the freezing and splitting of plant cells and tissue.

Water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm (living material) of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stoma, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of the solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage.

Relative humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure, expressed as a percent.

RH=Water in the airWater the air could hold (at constant temperature and pressure)

For example, if a kilogram of air at 75°F could hold 4 grams of water vapor and there are only 3 grams of water in the air, then the relative humidity (RH) is:

Expressed as a percent = 75%

Warm air can hold more water vapor than cold air; therefore, if the amount of water in the air stays the same and the temperature increases, the relative humidity decreases.

Water vapor will move from an area of high RH to one of low RH. The greater the difference in humidity, the faster water will move.

The relative humidity in the air space between the cells within the leaf approaches 100%; therefore, when the stomate is open, water vapor rushes out. As the vapor moves out, a cloud of high humidity is formed around the stomate. This cloud of humidity helps slow down transpiration and cool the leaf. If air movement blows the humid cloud away, transpiration will increase.

Many people confuse plant nutrition with plant fertilization. Plant nutrition refers to the needs and uses of the basic chemical elements in the plant. Fertilization is the term used when these materials are supplied to the environment around the plant. A lot must happen before a chemical element supplied in a fertilizer can be taken up and used by the plant.

Plants need 16 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are found in the soil. These six elements are used in relatively large amounts by the plant and are called macronutrients (nitrogen, potassium, and phosphorous are the primary macronutrients and magnesium, sulfur and calcium are the secondary macronutrients). There are seven other elements called micronutrients (or trace elements) and are used in much smaller amounts. Micronutrients are found in the soil: iron, zinc, molybdenum, manganese, boron, copper, and chlorine. All 16 elements, both macronutrients and micronutrients, are essential for plant growth.

Most of the nutrients that a plant needs are dissolved in water and then absorbed by the roots. Many nutrient combinations in fertilizers dissolve easily, and those nutrients can be readily absorbed. Sometimes two dissolved nutrients will combine into a product that has very low solubility. Availability of both nutrients to the plant is then severely reduced. This can occur with calcium and phosphorus and the micronutrients. Nutrient solubility is also affected by soil pH. High (alkaline) pH levels drastically reduce the solubility and availability of micronutrients (a factor in iron deficiency in azaleas), while low (very acidic) pH levels make some micronutrients (and non-nutrient minerals such as aluminum) so highly available as to injure the plant. Another consideration is the nutrient balance in the soil. For example, calcium and magnesium are absorbed similarly, but magnesium is absorbed more readily. The root does not select the nutrient to be absorbed; if both are present at the absorption site, the magnesium will be absorbed. This is why a soil test may indicate that, while there is sufficient calcium in the soil, a plant can suffer calcium deficiency because of an excess of magnesium competing for absorption.

The process whereby nutrients are absorbed varies. Water and nutrients can move between the outer cells of the root, but eventually they must cross a membrane to enter a cell. Water and some nutrients can do this easily (a passive process) while other nutrients are too large for the ‘holes’ in the membrane, and energy is needed to move these nutrients into the cell (an active process). Absorption is generally a combination of these processes: certain nutrients are actively absorbed, others enter passively to maintain a chemical balance.

Anything that lowers or prevents the production of sugars in the leaves can lower nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficiency problems may develop. The stage of growth or rate of growth may also affect the amount of nutrients absorbed. Many plants go into a rest period, or dormancy, during part of the year. During this dormancy, few nutrients are absorbed. Plants may also absorb different nutrients just as flower buds begin to develop.

Texture and growth of stems

Woody stems contain relatively large amounts of hardened xylem tissue in the central core and are typical of most fruit trees, ornamental trees, and shrubs.

A cane is a stem that has a relatively large pith (the central, strength-giving tissue of stem) and usually lives only one or two years. Examples of plants with canes include rose, grape, blackberry, and raspberry.

Herbaceous or succulent stems contain only small amounts of xylem tissue and usually live for only one growing season. If the plant is perennial, it will develop new shoots from a crown or underground part. An example of a plant with herbaceous stems is mayapple (Podophyllum peltatum), a native perennial that grows back each year from underground roots.

The edible portion of cultivated plants such as asparagus and kohlrabi is an enlarged succulent stem. The edible parts of broccoli are composed of stem tissue, flower buds, and a few small leaves. The edible part of potato is a fleshy, underground tuber. Although the name suggests otherwise, the edible part of the cauliflower is immature inflorescence (flowers) and flower stalk.

Plants are classified by the number of growing seasons required to complete a life cycle.

• Annuals pass through their entire life cycle from seed germination to seed production in one growing season, then die.
• Biennials are plants that start from seeds and produce vegetative structures and food storage organs the first season. In most biennials, during the first winter a hardy evergreen rosette of basal leaves persists. During the second season, flowers, fruit, and seeds develop to complete the life cycle. The plant then dies. Carrot, beet, cabbage, and celery are biennial plants. Hollyhock, Canterbury Bells, and Sweet William are biennials commonly grown for their attractive flowers.
  • Plants that typically develop as biennials may, in some cases, complete the cycle of growth from seed germination to seed production in only one growing season. This situation occurs when drought, variations in temperature, or other climatic conditions cause the plant to physiologically pass through the equivalent of two growing seasons in a single season.
• Perennial plants live for many years, and after reaching maturity, may produce flowers and seeds each year, though many only flower every few years. Perennials are classified as herbaceous if the top dies back to the ground each winter and new stems grow from the roots each spring. If significant xylem develops in the stem and the top persists, as in shrubs or trees, then they are classified as woody plants.

Plant Anatomy and Structure

FAQ

What are the structures and functions of roots and stems?

What are the functions of leaves roots and stems?

What is the anatomy of stem and root?

Do abelia have deep roots?