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Chapter 1: horticulture and plant science.

KEY TERMS

abscissic acid active transport adenosine triphosphate (ATP) adventitious root anther authority, author citation auxin basal plate binomial nomenclature bulb cambium chlorophyll chloroplast complete corm cuticle cytokinin dicot (dicotyledon) dioecious ethylene [F.sub.1] hybrid filament floret genus gibberellin guard cell hermaphroditic horticulture imperfect incomplete inflorescence intergeneric hybrid interspecific hybrid intraspecific hybrid leaf (leaflet) lenticel meristem monocot (monotyledon) monoecious nomenclature organelle osmosis ovary ovule pedicel peduncle perfect petal petiole petiolule phloem pistil pistillate rachis receptacle rhizome root cap root hair sepal sessile source-to-sink movement species spur stamen staminate stigma stipule stolon stomate storage organ style taxonomy thorn/spine trichome tuber tuberous root tunic venation xylem zone of cell elongation

What Is Horticulture? Horticulture [Latin hortus garden + cult(us) till] is the cultivation of flowers, fruit, vegetables, or ornamental plants; the science and art of cultivating such plants. The fruits and vegetables that nourish our bodies and the flowers and ornamental plants that adorn our world with beauty and nourish our souls bring another type of satisfaction in their cultivation. Although most of us no longer need to grow our own food, many still do plant a small vegetable garden or cultivate a fruit tree or two. The knowledge that we can plant a seed or a small plant and in a few weeks harvest fresh, wholesome produce from it provides a deep satisfaction in itself. But there is another kind of satisfaction that comes from successfully growing a garden, an orchid, or an ornamental tree, and there is also a special kind of frustration that comes from trying and not succeeding in doing these things. Our desire to grow plants has deep roots in human history.

Humans have collected and used plants for much longer than they have been cultivating them, and they have been cultivating them for thousands of years. For example, chile peppers were used in Peru at least 8,000 years ago, and there is some evidence that larger seeds of flax, which was used for fiber and oil, had been repeatedly selected and saved as long as 9,000 years ago. Cultivation of grapes dates to 4000 BCE (Before the Current Era) and corn dates to 5000 BCE. Cacao has been cultivated in the Americas for more than 2,000 years.

This history may help explain why gardening is such a popular hobby. For many, cultivating plants is just simply irresistible. Many people wish to cultivate plants but find they are not successful or that their skills are too limited to meet their desires: for it is true that cultivation of desirable plants and control or elimination of undesirable ones is not necessarily easy. Plant cultivation--gardening--operates by its own set of rules, and there is much to know before one can truly master them all. One famous farmer, Thomas Jefferson, stated that "Though an old man, I am but a young gardener."

THE WORLD OF HORTICULTURE

Plants can be classified into groups based on the manner in which they are used or the purpose for which they are used. For example, horticultural plants may be used for ornamental purposes or for eating. Ornamental crops include shade trees, flowering trees, shrubs, herbaceous flowering perennials, ornamental grasses, flowering annuals, bulbs, vines, and groundcovers, including turf. Edible crops include vegetables, tree fruit, nuts, small fruits and berries, culinary herbs, and condiments. Horticultural plants are also used for cosmetics, bath products, and medicinal purposes.

Those who engage in horticultural activities for a living may be greenhouse or nursery managers. Arborists provide care and management of trees. The study of fruit culture is termed pomology, whereas the study of vegetable culture is termed olericulture. Viticulture is the culture of grapes. Landscape designers, managers, and architects are concerned with the aesthetic presentation of plants as are florists and floral designers. Turf managers provide care for residential or commercial properties or manage athletic fields or golf courses.

THE SCIENTIFIC FOUNDATIONS OF HORTICULTURE

The basic knowledge of horticultural science lies in the study of botany and plant physiology. Botany looks at the anatomy and morphology of plants, including how leaves are arranged along a stem and how cells function within leaves and stems. Plant physiology examines the inner workings of plants, including the basic physiological processes of photosynthesis, respiration, absorption and translocation, and transpiration. The horticulturist who strives to have a good understanding of the basic science of plants will be able to answer many questions about how plants work and why they work the way they do. Whereas the foundations of plant science have been laid down for us by both anonymous and luminous predecessors, research into the inner workings of plants is ongoing.

TAXONOMY AND NOMENCLATURE

Classification of plants provides a structural framework in which to reference our knowledge. Taxonomy is the science or technique of classification, naming, or identification of an organism. Nomenclature is specifically the system of naming of plants. For centuries people all over the world have been naming plants. They have named them after their appearance, according to what they were used for, whether for medicinal or culinary purposes, or even for what dangers they presented. Plants are still referred to by colorful common names such as bleeding heart, liverwort, or sheep's bane. The main drawback to this way of referring to plants is that common names vary by region and language. For example, the edible weed, Lambsquarters (Chenopodium album L.), which originated in Asia and Europe, but is widely distributed throughout the United States and Canada, is also commonly referred to as goosefoot and pigweed. Pigweed is also the common name for another edible weed, green amaranth (Amaranthus retroflexus L.) that also grows all over the North American continent.

Scientists have long been interested in classifying and naming plants and animals, thus allowing a means of communicating more clearly with one another about the research they were doing or the discoveries they made. The most widely accepted and successful system of naming plants is the one that was developed more than 250 years ago that we are still using today. A Swedish doctor and botanist by the name of Carl Linne' refined it. Latin was the language of science, and Linn, used mainly Latin terms to describe plants. He liked that system so well, he renamed himself in the Latin translation of his name, Carolus Linnaeus (see box).

The greatest contribution of Linnaeus to nomenclature was the development of the two-name system, or binomial nomenclature. In Linneaus' system, the first name refers to a general grouping, or genus (plural genera). The second name is more specific, and is called the specific epithet. Together these two form the species name.

The genus and the specific epithet are always either written in italics or underlined. The genus is always capitalized and the specific epithet is not. Other terms that indicate broader groupings in this system of classification are Kingdom, Division, Class, Order, and Family, and those that indicate even more specific groups are variety, and cultivar (Table 1-1). Further refinements are sometimes made in subdivisions, subclasses, suborders, subfamilies, tribes and subtribes, subgenera, sections and subsections, series, subspecies, subvarieties, and forma and subforma.

Hybrids between plants of different species (interspecific hybrids) are known in the plant world, and they are designated with an x in front of the species name. Pelargonium xhortorum is the common garden geranium, a hybrid between P. zonale and P. inquinans. Intergeneric hybrids are designated with an x in front of the genus name. An example is the Leyland cypress, xCupressocyparis leylandii, a hybrid between Cupressus macrocarpa and Chamaecyparis nootkanensis. Hybrids between plants of the same species (intraspecific hybrids) have been developed for improved horticultural and agronomic traits, such as floral display, disease tolerance, and yield. These are commonly called hybrids, or, sometimes, [F.sub.1] hybrids, although not all cultivars are [F.sub.1] hybrids. (The [F.sub.1] designation refers to the first filial generation resulting from a controlled cross-pollination of inbred lines; see chapter 2 for a full discussion.)

Regardless of the method used to obtain them, improved plants with commercial potential are usually given a cultivar name. Cultivar is a contraction of the two words, cultivated variety. Cultivars are enclosed by single quotes, or preceded by cv., cvr., or the word cultivar. For example, a beefsteak-type tomato with improved traits for disease tolerance and good flavor is named Lycopersicon esculentum 'Celebrity' and could also be designated Lycopersicon esculentum cvr. Celebrity, or, simply, 'Celebrity.' The designation of cultivar is applied to horticultural plants that have been improved through breeding and differs from the designation variety, which is assigned to genetically different plants that occur naturally. For example, Viola cornuta var. alba is the white-flowered horned violet that occurs naturally in the wild.

Genera of plants are grouped together into families based on similar floral, fruit, or vegetative characteristics (Table 1-2). Plants within a family are often susceptible to the same diseases or pests. For example, apples, pears, and firethorn, members of the rose family, are all susceptible to fireblight. For general reference, plants are referred to by their genus and species name. This designation is usually enough to distinguish the plant under discussion across cultural and language barriers.

The naming of plants follows an established set of rules defined by the International Code of Botanical Nomenclature. These rules are very specific, and they are reviewed regularly at International Botanical Conferences. New plants must be described and named according to the Code to be officially accepted. The person who first accurately describes and names a plant is the authority. A complete and accurate binomial includes an abbreviation of the name of the authority, or the author citation. For example, the flowering dogwood, named by Linnaeus, is Cornus florida L. On occasion, plants have been inadequately described or incorrectly placed into a species. In an effort to correct this mistake, the plant in question may be renamed. This explains why some plants are known by two different scientific names, resulting in some confusion. Nevertheless, the rather rigid set of regulations that are used for naming plants provides a great deal of stability and accuracy in plant nomenclature.

PLANT MATURITY AND JUVENILITY

A plant life cycle begins with the seed, followed by germination, a period of vegetative growth, and onset of reproductive status, followed by flowering, pollination, and fertilization, resulting in seed formation. The period of vegetative growth that precedes the onset of reproductive status varies by species and environmental conditions. That period is the juvenile stage of the plant life cycle. Some woody plants go through several years of juvenility before achieving reproductive status. Many fruit trees, for example, do not flower at all for the first 3 to 10 years, Macadamia nut trees may not bear for 20 years. Other plants, however, reproduce within a few weeks after seed germination.

PLANT STRUCTURE

Plant anatomy is the physical structure of the plant and its organs, including roots, stems, and leaves. Morphology describes the shapes and composition of the structures.

Anatomy

Plants are commonly composed of roots and one or more stems and leaves (Fig. 1-1). The roots remove water and mineral nutrients from the soil. These are then delivered throughout the rest of the plant through the vascular system. The vascular tissue in plants is composed of three major types of tissue: xylem, phloem, and cambium. Water and mineral nutrients are transported upward in the plant to the stems and leaves through xylem. Leaves are the site of photosynthesis, where food is made in the form of carbohydrates. Carbohydrates are transported throughout the rest of the plant in an upward or downward direction in the plant tissue known as phloem. Xylem and phloem are arranged in a consistent manner in plants, depending on whether they are monocots or dicots, the two major classes of flowering plants (Fig. 1-2). Stems provide structure to plants. If they are green, they may also be a site for photosynthesis. Cambium is tissue formed from actively dividing cells. These undifferentiated cells may develop into either xylem or phloem. Cambium cells generate new xylem and phloem tissue.

Morphology

Plant structures vary in their shapes and sizes. In addition to the stereotypical shapes we are all familiar with, there are many special adaptations that plants have made in response to various environments in which they have evolved. Some examples of these are the spines of a cactus (Fig. 1-3) or the blade-like succulent leaves of a century plant (Fig. 1-4). Both of these have evolved in dry desert environments. Each cactus or succulent has, in its own way, adapted to long periods with little moisture, one by storing moisture inside its leaves and the other by reducing the surface area presented to the dry air surrounding them and actually storing water and photosynthesizing in its stems!

Vegetative Structures

The vegetative structures that are clearly visible without the aid of a microscope are the leaves, stems, and roots. But plants have growing points where cell division occurs and new growth develops. These meristems are located in just a few places in a plant: shoot tips and root tips in the cambial layer, crowns of grasses, and at the bases of grass leaves (Fig. 1-5).

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ANATOMY OF STEMS. Stems provide structure to plants and may be a site for photosynthesis. Lenticels (Fig. 1-6, see page 13) are pores that are found in some stems that provide a means for exchange of oxygen and carb on dioxide. Inside the stem is the vascular tissue that conducts water and nutrients upward from the roots throughout the rest of the plant. Carbohydrates are transported throughout the plant from the site of photosynthesis to needed areas, such as newly developing leaves, flowers, fruit, and roots.

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Most people think of a plant stem as a fairly straight shoot with leaves coming off it. But there are many variations in plant stems (Fig. 1-7). Some stems serve as storage organs, such as those on barrel and saguaro cacti; others, called stolons, grow laterally along the ground, as in strawberries. Rhizomes are stems that grow underground. Some are fleshy, as for some iris species, and others are thin, as for bermudagrass. These underground stems often grow laterally underground and send up shoots along their length. There are shortened, compressed stems in the base of grasses and in some bulbs, such as tulips and daffodils. A spur is a specialized stem on fruit trees that bears flowers and fruit. A corm is a compressed, fleshy stem, and a tuber is a stem that functions as a storage organ, with eyes, or buds that can sprout new shoots.

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ANATOMY OF LEAVES. Leaves are usually green, flat surfaces where photosynthesis takes place and are referred to as the leaf blade, or lamina. The outline of the leaf blade is called the leaf margin, and the vascular system that carries nutrients to each cell is located in leaf veins. The stem of a leaf is called a petiole. Leaf blade shape (Fig. 1-8) is fairly uniform throughout most of a species to such a degree that it serves as an identifying feature of the plant. In addition, the shape of the leaf margin (Fig. 1-9, see page 16), leaf size, vein pattern (venation) (Fig. 1-10, see page 17), and arrangement of leaves along a stem are identifying features (Fig. 1-11, see page 18). As a matter of fact, many plant identification books rely primarily on leaf morphology for accurate identification. Thorns and spines are specialized leaves. Stipules are leaf-like appendages located at the base of a petiole or leaf. They may serve a protective function for axillary buds.

Upon examination with a microscope, a cross-section of a leaf reveals the following: a cuticle, hairs called trichomes, and stomates. Stomates (Fig. 1-12, see page 19) are found in greatest number on the undersides of leaves. They may be altogether absent on the upper sides. They are actually pores with two guard cells that open when they swell (become turgid) and close when they become flaccid from water loss. The guard cells allow for oxygen and carbon dioxide exchange and also allow water to escape from the plant.

The cuticle is not composed of cells but is simply a waxy coating on the leaf surface. It aids in reduction of water loss and helps protect leaves from attack by pests and pathogens. Trichomes are minute hairs of a variety of shapes. Some trichomes exude substances that are toxic to pests; others exude sticky substances that can trap pests.

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COMPOUND LEAVES. A compound leaf is composed of more than a single blade. Some compound leaves have numerous leaflets, attached to a central rachis by petiolules. They may have an even or odd number of leaflets, and the leaflets may be arranged in a variety of ways (Fig. 1-13, see page 19), such as palmate, pinnate, and trifolate (Fig. 1-14, see page 20).

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ANATOMY OF ROOTS. The root is the plant organ that typically grows underground, although aerial roots do occur. Plant roots serve several functions: they anchor a plant into the ground or growing medium, and they take up nutrients and water from the ground or growing medium. Roots have various morphologies, including fibrous, fleshy, or thickened taproots. The water and nutrients that they obtain from the soil or other medium is transported throughout the plant in the xylem tissue.

The growing tip of a root, or meristem, is protected by a mucilaginous sheath that helps keep it intact as growth forces it through the soil. There is often an accumulation of dead cells preceding the meristem in the soil that is called a root cap. Just behind the zone of cell division is a zone of cell elongation and then a zone where root hairs are present. Root hairs are single extensions of individual epidermal cells. Their function is to obtain nutrients and moisture from their surroundings. Vascular tissue begins developing in the zone of differentiation just beyond the zone of root hairs (Fig. 1-15, see page 20).

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Adventitious roots form from tissue where they do not normally occur, such as from stems. (Fig. 1-16, see page 21). They may be aerial roots, but sometimes they are roots that form from stem cuttings. This latter phenomenon allows plants to be reproduced asexually. Other times, plant stems touch the ground and then develop roots at their stem tips. In this manner, some plants cover a lot of ground as they are able to spread out as they grow.

SPECIALIZED VEGETATIVE STRUCTURES. In addition to the examples provided above, plants sometimes develop specialized structures. These are commonly storage organs that serve as a means of storing excess carbohydrates. But, for human purposes, they are often used as vegetative reproductive organs. Bulbs are composed of fleshy leaves, a shoot, and a compressed stem called a basal plate (Fig. 1-17, see page 22). Examples of bulbs are onions, garlic, and tulips. Some bulbs have a papery covering, or tunic. Corms are composed of a compressed stem and a basal plate. Gladiolus is a well-known corm-producing plant. Tubers are specialized underground stems that have leaf nodes, or eyes, that eventually sprout shoots. Potatoes and some anemones are tubers. Tuberous roots are fleshy roots and are found in sweet potatoes, dahlias, and daylilies, among other plants (Figs. 1-18 and 1-19, see page 22). Rhizomes are fleshy storage organs that may be large or quite small and often grow in a horizontal fashion. Each rhizome has one or more eyes, or buds, from which new shoots may form. Roots also grow from rhizomes. Pieces of rhizomes can be broken off to propagate new plants. It is important to include one shoot bud and some root material for propagation. Many species of iris are propagated this way.

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Whereas roots, stems, and leaves are considered vegetative plant organs, flowers are the sexual reproductive organs of flowering plants (angiosperms). Gymnosperms (conifers) and pteridophytes (ferns) lack flowers; their sexual reproductive organs are cones or spores, respectively.

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Floral Anatomy

Flowers vary widely in their forms and colors, but there are four basic floral organs that they have in common: sepals, petals, stamens, and pistils (Fig. 1-20, see page 23). Sepals are arranged in the outer whorl of a flower and are usually green, resembling leafy tissue. The collective term for all the sepals is a calyx. Petals are the second whorl from the outside and are usually brightly colored, even having striking markings and patterns, thought to attract pollinators. The collective term for all the petals is corolla, and perianth is the term used to refer to the sepals and petals together. A flower with all four of these floral organs is said to be complete. A flower with both male and female organs is said to be perfect.

Stamens comprise the next floral whorl and are composed of a thin stem, or filament, and a pollen container, or anther. The stamen is considered to be the male floral structure. Finally, in the center of a flower is the pistil, the female structure. A pistil is composed of a sometimes sticky tip where pollen lands, called a stigma, an often narrow neck, or style, and a base called an ovary. The ovary contains one or more ovules. Variations abound in nature, and many flowers lack one or more of the four basic floral organs. They are then called incomplete. If they lack one of the sexually reproductive structures, either stamens or pistils, they are considered imperfect. However, they may be perfect, yet lack sepals or petals, or both. A flower with stamens and no pistils is staminate. One with pistils that lacks stamens is pistillate. In some flowers, the floral organs are held by a receptacle. This arrangement is particularly clear in strawberries and raspberries, which both have a cone-like structure in the center of the fruit. In picking raspberries, the cone-like structure, or receptacle, remains on the plant when the ripe fruit are removed.

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FLORAL VARIATIONS. A plant that bears perfect flowers may be referred to as hermaphroditic. If staminate and pistillate flowers are borne on the same plant, as is the case with bananas, corn, and watermelon, it is said to be monoecious, from the Greek, meaning one house (mono- one + -ecious house). If staminate and pistillate flowers are borne on separate plants, as occurs in holly, ginkgo, and pistachio, the plant is said to be dioecious (di- two+ -ecious house). A compound flower is an inflorescence. Many arrangements are known (Figs. 1-21 and 1-22, see pages 23 and 24). The stem of a solitary flower is called a peduncle. However, compound flowers are composed of individual florets, which may be attached by pedicels to the peduncle, or they may be sessile, or lacking a pedicel. Compound flower heads, such as daisies and sunflowers, have two types of florets: ray florets, which are usually found on the outer ring and have a petal attached, and disk florets, which are commonly found in the center rings and lack showy petals. Ray florets are often sterile, whereas disk florets are usually perfect.

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PLANT PHYSIOLOGICAL PROCESSES

Plants make and use their own food and take up needed water and nutrients from the soil. Understanding how these processes work in plants opens the door to a deeper level of understanding of how plants grow and how best to manage and take care of them.

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Photosynthesis

Plants, unlike animals, are able to synthesize their own food for energy from their environment, most notably from water, air, and sunlight. Photosynthesis is the chemical process in which plants transform the sun's energy into carbohydrates. It is made possible by the molecule chlorophyll, at specific wavelengths of light energy in the presence of water and carbon dioxide. The generalized process is written as

Light energy + Carbon dioxide + Water ? Carbohydrates + Oxygen.

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The chemical process is written as

Light energy + 6C[O.sub.2] + 6[H.sub.2]O ? [C.sub.6][H.sub.12][O.sub.6] + 6[O.sub.2].

The sun's energy is locked in the carbohydrates in a chemical bond that will later be broken and the energy made available for plant growth processes.

Chlorophyll is a pigment that is located inside special organelles found within the cell. These organelles are called chloroplasts. During photosynthesis a water molecule is cleaved into hydrogen and oxygen. Some oxygen is released while some oxygen and the hydrogen and carbon are formed into a carbohydrate molecule from which sugar molecules will be formed. Energy is required for all processes within a plant, and this energy is supplied by the process of photosynthesis.

The light reaction of photosynthesis can only occur during the day, and it requires exposure to light energy in the specific wavelengths required to activate chlorophyll (see chapter 7). When light levels are low, as in shady or cloudy situations, photosynthesis occurs at a slower rate. Some plant processes, such as flowering and seed development, require high levels of energy in the form of carbohydrates; thus, shady environments may prevent them from occurring. Plants not adapted to low light conditions will not make enough carbohydrates for good growth. These plants will be weak and not very well developed. Often they will survive only a short while, eventually dying for lack of adequate carbohydrates for survival or growth.

During photosynthesis the plant takes up carbon dioxide in the form of a gas and releases oxygen in the form of a gas; thus, open stomates are necessary for free gas exchange. If conditions are too dry, causing plants to lose too much water through their stomates, the stomates will close. This causes photosynthesis to cease, thus causing plant growth to cease. A humid environment reduces moisture loss from the plant and thus reduces stress.

For the photosynthetic process to be successful, all the necessary components must be in place. One of these components is a healthy environment for root growth, such as moist-to-dry, well-aerated, fertile, and warm soil.

Respiration

Respiration is the process whereby the chemical bond formed during photosynthesis is broken, and the energy molecule adenosine triphosphate (ATP) is formed. ATP is the currency used in biochemical processes throughout the plant for many functions. It provides the energy for biochemical processes to take place. The general process is

Carbohydrate + Oxygen [right arrow] Energy + Carbon dioxide + Water.

The chemical formula is

[C.sub.6][H.sub.12][O.sub.6] + 6[O.sub.2] [right arrow] ATP + 6C[O.sub.2] + 6[H.sub.2]O.

As you can see, oxygen is required for this process. We usually think that plants give off oxygen and that is one reason we like to have them indoors. The oxygen that plants generated eventually allowed animal and human life to exist on earth, but plants also require oxygen for their own life processes. However, they tend to give off more oxygen in photosynthesis than they use during respiration, thus producing a net positive amount. If a plant uses as much oxygen as it gives off, it is not growing. If it uses more than it gives off, it is in decline. So healthy plants are healthy for humans too.

Respiration can take place in light or dark, as it does not require light energy. It is a process whereby stored energy is harvested from the chemical bonds in carbohydrates. Respiration occurs at a faster rate as temperatures warm up, so that during the day respiration rates tend to be faster than at night, when temperatures drop. Respiration also occurs at a higher rate in rapidly growing tissues. In dormant plants and seeds, respiration does not cease but continues at a slow rate. The rate can be further reduced by putting plant material such as seeds, cuttings, fruits, and vegetables in a cooler or refrigerator. This cooling reduces the rate at which stored carbohydrates are used, thus resulting in increased longevity for the seeds or cuttings. This principle is demonstrated every day in the florist shop where cut flowers are stored in coolers to maintain their freshness.

When a plant has enough carbohydrates for growth and everyday maintenance, then excess carbohydrates are stored. They may be stored in stems, roots, and specialized storage organs. These include taproots, tuberous roots, fleshy stems or roots, tubers, bulbs, corms, and rhizomes.

Absorption

Plants take up water and nutrients through their roots and sometimes through leaves but primarily through root hairs. Water is taken up by root hairs in a passive process called osmosis. In osmosis, water moves from an area of low concentration of solutes (free-flowing water in the soil) to an area of high concentration of solutes, the plant cell. Minerals may enter roots by osmosis or by active transport. The latter is a complex process requiring energy that is provided by respiration. Plants absorb nutrients in inorganic forms, such as nitrate (N[O.sub.3.sup.-]), ammonium (N[H.sub.4.sup.+]), phosphate ([PO.sub.4.sup.3-), and potassium ([K.sup+]). Water and the nutrients dissolved in it move upward in the plant through xylem tissue. Water can move upward through the trunk of a tree at a rate between 3 and 90 feet per hour, depending on the internal structure.

Transpiration

Transpiration is the loss of water through stomates. Humidity around the leaves can reduce the rate of water loss from a plant, thus reducing the water uptake in the roots. One can imagine a plumbing system in the plant whereby water enters through the roots and forms a continuous column through the plant to the stomates, where it transpires and is lost to the surrounding air. Because of this process, plants can have a cooling effect on the surrounding environment. Lawn grasses and trees cool the surrounding air as much as 10[degrees]F due to water loss through leaves. When soil moisture becomes low, stomates will close to avoid excess water loss. When stomates are closed, gas exchange for photosynthesis and respiration cannot occur. Thus, a dwarfing effect is often seen with plants in windy or arid locations, because of the large water losses that occur.

Translocation

Translocation is the movement of carbohydrates (sugars and starch), plant hormones, water, and nutrients throughout the plant. Carbohydrates, manufactured primarily in leaves (and some green stems), travel through phloem in any direction.

Carbohydrates move from source areas (where they are manufactured) to areas where they are required for specific activities, such as flowering and fruit development. The carbohydrate-requiring locations are called sinks, and the process of carbohydrate transfer is called source-to-sink movement. Roots are a primary sink in which carbohydrates may be stored for future use.

Plant Hormones

Many plant processes are controlled or affected by plant hormones. Some of the processes affected by hormones are seed germination, directional growth of plants, cell division, and cell elongation (Table 1-3).

Auxin (Greek auxein to grow) functions primarily to induce cell enlargement. Auxin also plays a prominent role in the phenomenon of apical dominance. It interacts with cytokinin in this function, such that the ratio of auxin to cytokinin determines whether apical dominance exists or is overcome. Higher auxin levels promote apical dominance, whereas higher cytokinin levels overcome it and induce axillary bud break. Auxins play a variety of other roles in plant growth responses. For example, they affect tropisms, or plant movement, particularly in response to light (plants grow toward light) and gravity (roots grow downward and shoots grow upward). When used in tissue culture, auxins promote root growth. They are used for vegetative propagation to promote adventitious roots.

Besides interacting with auxins in the control of apical dominance, cytokinins (Greek cyto- cell + kine- to cut) are involved in cell division. They are found in the highest amounts where cell division occurs, such as in meristematic regions and in seeds.

Ethylene is different from the other hormones because it is a gas. Ethylene influences seed and fruit ripening. If an unripe fruit, such as a peach, is placed in a closed paper bag, enough ethylene gas accumulates to speed ripening. Ethylene is also used to gas green bananas to induce the ripening process. Ethylene stimulates flower and fruit abscission (separation from plant) and flower opening. Ethylene is produced by the plant when pollination has occurred, thus beginning the process of petal abscission. This process provides an evolutionary advantage, because the petals are no longer needed to attract pollinators and the plant's energy is better used for seed development.

Abscissic acid is involved in fruit drop, or abscission. It is also involved in leaf and flower abscission. There is a thin layer of cells that are particularly sensitive to abscissic acid and ethylene, and when the plant generates these compounds, the plant organ separates from the plant at these abscission zones. Abscissic acid also plays a role in stimulating and maintaining dormancy, in stimulating stomatal closure under water stress, and in stimulating seeds to develop storage proteins during normal seed development.

Gibberellins are a group of compounds that include gibberellic acids. They are most important in the process of cell elongation, and, as such, they stimulate stem elongation, leaf expansion, and seed enlargement.

SUMMARY

Plants are complex organisms with specialized anatomy and physiology. Our understanding of them is aided by the use of the binomial system of nomenclature developed by Linnaeus. Plant vegetative structure includes the major organs of leaves, stems, and roots, with a vascular system composed of xylem and phloem. Cambium is present in vascular tissue as a source of cell division and new tissue. Plants exchange gases in photosynthesis and respiration through pores called stomates and lenticels. Flowers have four basic organs in common: sepals, petals, stamens, and pistils. The latter two are integral to sexual reproduction. There are many variations and specializations in both vegetative and floral structures.

The important physiological processes of plants are photosynthesis, respiration, absorption, transpiration, and translocation. These are responsible for plant food manufacture, energy use, taking up of minerals and water through the roots, loss of water through the leaves, and movement of sugars, or photosynthates, throughout the plant. Plant hormones are involved in many plant processes. An understanding of all of these components of how plants grow is fundamental to horticultural practices and horticultural science.

HANDS-ON ACTIVITIES

* Make a collection of different types of leaves--simple and compound. Identify the shapes, margins, and compound leaf arrangements. Include dicots and monocots.

* Look at a leaf under a dissecting microscope to see the cellular shapes and to find trichomes and stomata.

* Dissect a flower under a dissecting microscope and identify the floral organs. Look at a developing floral bud and notice the early development of sepals, petals, stamens, and pistil. Compare it to a developing vegetative bud.

* Dissect pistils from different flowers and compare the ovule arrangements. You may be able to do this without the aid of a microscope if the flowers you select are large enough. Otherwise, use a dissecting microscope to make your observations. No special dyes are necessary, but you will find a scalpel and forceps helpful.

* Make a collection of 2 to 10 flowers, dissect each with a scalpel, and mount the floral organs on card stock. You should show the sepal, petal, stamens, and pistil(s) from each flower.

* There are several exceptions to the rules that distinguish monocots from dicots. Research the rules and list them, including their exceptions. Provide a picture for a representative plant of each family having exceptions.

REVIEW QUESTIONS

1. What are the differences between monocots and dicots?

2. Compare and contrast photosynthesis and respiration.

3. In the name Cornus florida, which is the genus and which is the species epithet?

4. List the hormones produced by plants and one or two major effects they have in plants.

5. What is the difference between a cultivar and a variety?

6. What is the function of xylem? Of phloem?

7. What is cambium and what is its function?

8. Where are meristems located in plants?

9. Discuss absorption and translocation.

10. What are stomates and where are they located?

SELECTED REFERENCES

Cronquist, A. (1968). The evolution and classification of flowering plants. Boston: Houghton-Mifflin.

Janick, J., Schery, R. W., Woods, F.W., & Ruttan, V. W. (1981). Plant science. New York, W. H. Freeman.

Saigo, R. H., & Woodworth, B. (1983). Botany: principles and applications. Englewood Cliffs, NJ: Prentice-Hall.

Smith, J. P., Jr. (1977). Vascular plant families. Eureka, CA: Mad River Press.

Stein, J. (1975). The Random House college dictionary, rev. ed. New York: Random House.

Whittle, T. (1997). The plant hunters. New York: Lyons and Burford.

Carl Linne a.k.a. Carolus Linnaeus

Carl Linne was born a country boy in Sweden in 1705. As a university student, Linne was befriended by a priest and botanist. Linne tutored the priest's son in botany and through this experience discovered both a passion and a talent for botany. He was soon hired by the Academy of Science to explore and hunt plants in Lapland and followed that stint by studying plants in Northern Europe. Throughout this time Linne embraced his passion for plants, and he reveled in the discovery process. He traveled to Holland to earn his degree and then served as curator of the Hatrecamp Botanical Garden. At the time there was no uniform system of naming plants. In many cases, plant names were overly long and burdensome. For example, before Linne's system the wild briar rose was referred to by botanists as Rosa sylvestris inodora seu canina and as Rosa sylvestris alba cum rubore, folio glabro. Linne named it Rosa canina. The use of Latin in naming plants was common, and he Latinized his own name to become Carolus Linnaeus. He used the existing ideas of Sebastien Vaillant and Gaspard Bauhin to develop his system of plant classification, based on the sexual functioning of plants and having two names for each organism. He began classifying plants according to his new system, and in 1753 published Species Plantarum (The Species of Plants). His work brought order to the chaotic world of botanical nomenclature, and he was much appreciated throughout Europe for his contributions. Eventually Linnaeus' method of using floral organs alone was abandoned as being too restrictive for classification purposes, but he is still recognized for introducing the system of binomial nomenclature. Linnaeus, as he came to be known, was a much-admired and sought-after teacher of botany. He loved taking students to the field to study plants in their native habitats. He taught hundreds of students--many of whom were attracted to the botany program because of him. He even inspired 19 of his students to study plants and to go to foreign and often uninhabited places around the globe to seek, identify, and name plants. Some traveled around the world with Captain James Cook, others went to the American colonies, South America, Southeast Asia, Africa, and the Middle East, and one even studied and taught in Japan. In 1761 he attained the name Carl von Linn, when he was granted nobility.

Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 1-1
Example of Taxonomic Organization for Peony

RANK             ENDING         EXAMPLE

Kingdom                         Planta

Division         -phyta         Magnoliophyta

Class            -opsida        Magnoliopsida

Subclass         -idae          Dilleniidae

Order            -ales          Dilleniales

Family           -aceae         Paeoniaceae

Genus                           Paeonia

Species                         lactiflora

Cultivar                        'Gay Paree'

TABLE 1-2
Some Horticulturally Important Plant Families, Their Major
Characteristics, and List of Some Common Species

                 COMMON                   MAJOR
FAMILY           NAME                     CHARACTERISTICS

Compositae/      Aster                    Flowers are usually in a
  Asteraceae                                head of florets that may
                                            be distinguished as disk
                                            and ray florets. Fruit is
                                            an achene or cypsela.
                                            Many ornamental
                                            flowers and some
                                            vegetables.

Musaceae         Banana                   Tree-like herb, monoecious
                                            flowers; fruit is a
                                            leathery berry.

Ranunculaceae    Buttercup                Perianth may lack sepals or
                                            petals and sepals may
                                            be undifferentiated
                                            from one another.
                                            Occurs in cooler
                                            regions of Northern
                                            Hemisphere.

Cactaceae        Cactus                   Succulent herbs and
                                            shrubs often with spiny
                                            leaves or lacking leaves.

Geraniaceae      Geranium                 Perfect flowers, carpels are
                                            united, floral organs in
                                            5s or multiples of 5. Fruit
                                            a schizocarp, forming a
                                            beak-like shape

Cucurbitaceae    Gourd                    Annual or perennial herbs,
                                            palmate 5-lobed leaves.
                                            Fruit is a pepo, having
                                            leathery or hard
                                            exocarp (rind).

Liliaceae        Lily                     Most are perennial herbs,
                                            floral organs in 3s.
                                            Sepals resemble petals,
                                            giving appearance of
                                            6 petals, no sepals.
                                            Ornamental and food.

Magnoliaceae     Magnolia                 Trees and shrubs with
                                            large, showy flowers.
                                            Calyx usually with
                                            3 sepals, corolla of
                                            usually 6 to many
                                            petals. Fruit a follicle,
                                            samara, (rarely) berry.

                 COMMON                   MAJOR
FAMILY           NAME                     CHARACTERISTICS

Labiatae         Mint                     Many are aromatic herbs.
                                            Usually square stems.
                                            Flowers are often
                                            whorled in leaf axils
                                            and are 2-lipped with 2
                                            fused upper petals and
                                            3 fused lower ones.

Cruciferae/      Mustard                  Flowers have 4 sepals and
  Brassicaceae                              4 petals; 6 stamen, with
                                            4 long and 2 short.
                                            Fruit is a silicle or
                                            silique

Solanaceae       Nightshade               Many members are
                                            poisonous. Many
                                            others are used as
                                            food, for
                                            ornamentation, and for
                                            medicinal purposes.

Orchidaceae      Orchid                   The most highly evolved
                                            family. Flowers are
                                            often very showy and
                                            have highly specialized
                                            petals. Pollen often in a
                                            sac called pollinia.
                                            Genera and species are
                                            often cross-compatible.

Leguminosae/     Pea                      Leaves simple or
  Fabaceae                                  compound; fruit is a
                                            legume or loment--
                                            indehiscent or
                                            dehiscent along one or
                                            two sutures.

Papaveraceae     Poppy                    Mostly herbaceous
                                            perennial or annual in
                                            subtropical and
                                            temperate North
                                            America. Fruit is a
                                            capsule. Milky or
                                            colored latex sap.

Rosaceae         Rose                     Floral organs usually in 5s,
                                            fruit a drupe, pome,
                                            achene, aggregate, or
                                            follicle. Many
                                            horticultural--food and
                                            ornamental--plants.

                 COMMON                   MAJOR
FAMILY           NAME                     CHARACTERISTICS

Nymphaceae       Water lily               Freshwater aquatic
                                            perennial. Perfect,
                                            solitary flowers of 3 to
                                            many sepals and 3 to
                                            many petals. Usually
                                            have nectaries, often
                                            fragrant.

Hamamelidaceae   Witch hazel              Trees and shrubs found
                                            mainly in subtropical
                                            areas of Africa, Asia,
                                            and Australia. Flowers
                                            are complete; fruit is a
                                            capsule.

FAMILY           COMMON SPECIES

Compositae/      Aster, sunflower, daisy,
  Asteraceae       coneflower, cosmos,
                   goldenrod, achillea
                   (yarrow), gazania,
                   lettuce, artichoke

Musaceae         Banana and plantain

Ranunculaceae    Hellebore, delphinium,
                   columbine,
                   ranunculus,
                   anemone

Cactaceae        Christmas cactus, giant
                   saguaro,
                   pincushion cactus

Geraniaceae      Geranium,
                   pelargonium,
                   stork's bill

Cucurbitaceae    Pumpkin, squash,
                   gourd, watermelon,
                   cantaloupe, luffa
                   sponge

Liliaceae        Lily, hosta, daylily, aloe,
                   alstromeria, tulip,
                   lily-of-the-valley,
                   amaryllis, onion,
                   garlic, agave

Magnoliaceae     Magnolia, tulip tree

FAMILY           COMMON SPECIES

Labiatae         Peppermint, spearmint,
                   oregano, thyme,
                   bee balm, catnip,
                   horehound, dead
                   nettle, lavender

Cruciferae/      Mustard, broccoli,
  Brassicaceae     Brussels sprouts,
                   shepherd's purse,
                   money plant

Solanaceae       Nightshade, tomato,
                   eggplant, potato,
                   pepper, chile,
                   tobacco,
                   mandrake, jimson
                   weed

Orchidaceae      Lady slipper, cattleya,
                   vanilla, vanda,
                   oncidium

Leguminosae/     Pea, bean, clover,
  Fabaceae         mimosa (silk tree),
                   locust, palo verde

Papaveraceae     Poppy, California
                   poppy

Rosaceae         Rose, apple,
                   strawberry, spirea,
                   firethorn, pear,
                   cinquefoil, plum,
                   apricot, almond

FAMILY           COMMON SPECIES

Nymphaceae       Water lily, giant
                   Amazon water lily

Hamamelidaceae   Witch hazel,
                   fothergilla, sweet
                   gum

TABLE 1-3
Plant Hormones and Their Effects in Plants

PLANT HORMONE    EFFECT IN PLANTS

Abscissic acid   Leaf and petal drop (abscission), stomate regulation.

Auxin            Apical dominance, stimulates root growth (in cuttings
                   and in tissue culture); tropisms.

Cytokinin        Growth by cell division, overcomes apical dominance,
                   stimulates shoot growth in tissue culture, stomate
                   regulation.

Ethylene         Ripening. Flower, fruit, and leaf drop.

Gibberellin      Stem elongation, leaf and seed enlargement, cellular
                   elongation.
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Author:Loehrlein, Marietta M.
Publication:Home Horticulture: Principles and Practices
Date:Jan 1, 2008
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