About 90% of the approximately 330,000 species of plants

32 Plant Structure, Growth, and Differentiation Oak seedling. A massive oak tree may grow from this young basket oak (Quercus prinus) seedling. Runk /Schoenberger from Grant Heilman KEY CONCEPTS Plants exhibit a variety of patterns of growth, reproduction, and longevity. The structure of plant cells, tissues, and organs matches their functions. The vascular plant body is differentiated into three tissue systems: the ground tissue system, the vascular tissue system, and the dermal tissue system. Primary growth lengthens roots and shoots throughout the life span of most plants. Woody plants have both primary growth and secondary growth, in which they increase in thickness. About 90% of the approximately 330,000 species of plants are flowering plants, vascular plants characterized by flowers, double fertilization, endosperm, and seeds enclosed within fruits (see Chapter 28). Because flowering plants are the largest, most successful group of plants, they are the focus of much of this chapter and of Chapters 33 to 37. Let us begin our study of the structure and functions of plants by examining a familiar species, the oak, and its fruit, the acorn. A new oak often begins its life after a squirrel has buried an acorn in the soil. The seed within first absorbs water from the surrounding soil. Germination occurs as a root emerges and works its way into the soil, absorbing additional water and anchoring the young plant. A shoot grows upward and breaks through the soil. At this point the young oak (see photograph) is a seedling that still depends on the food supply within the seed. The stem continues to elongate, and small leaves develop, expand, and begin to photosynthesize. The young tree grows taller by growth at the tips of its branches. The roots likewise elongate at their tips. As it ages, the tree also forms wood and bark, increasing in girth (thickness) along the sides of the more mature stems and roots. Growth and expansion of both the root and shoot systems continue throughout the oak s life. After several years, the oak tree becomes reproductively mature. Oaks reproduce by forming separate male and female flowers. The minute male flowers are found together on slender, drooping, caterpillar-shaped catkins (clusters); in contrast, the tiny female flowers are often solitary. After wind carries pollen from a male flower to a female flower, a sperm cell fertilizes the egg and 698

an acorn develops, partially enclosed by a scaly cup. Within the acorn is a seed containing an embryo of a miniature oak, along with a supply of stored food. Thus, the cycle of life repeats itself. In this chapter we examine the external structure of the flowering plant body; the organization of its cells, tissues, and tissue systems; and its basic growth patterns. PLANT STRUCTURE AND LIFE SPAN Learning Objectives 1 Distinguish between herbaceous and woody plants. 2 Discuss the differences among annuals, biennials, and perennials, and give an example of each. 3 Contrast two different life history strategies in plants. Plants range in size from the minute floating water-meal (Wolffia microscopica), the smallest flowering plant known, to Australian gum trees (Eucalyptus), some of Earth s tallest trees ( Fig. 32-1). The more than 300,000 species of flowering plants that live in and are adapted to the many environments on Earth represent remarkable variety. Yet all of these from desert cacti with enormously swollen stems, to cattails partly submerged in marshes, to orchids growing in the uppermost tree branches of lush tropical rain forests are recognizable as plants. Almost all plants have the same basic body plan, which consists of roots, stems, and leaves. Plants are either herbaceous or woody. Herbaceous plants are nonwoody. In temperate climates, the aerial parts (stems and leaves) of herbaceous plants die back to the ground at the end of the growing season. In contrast, the aerial parts of woody plants (trees and shrubs) persist. Botanically speaking, woody plants produce hard, lignified secondary tissues (the cell walls of secondary tissues contain lignin), and herbaceous plants do not. Lignin and the production of secondary tissues are discussed later in the chapter. Annuals are herbaceous plants (such as corn, geranium, and marigold) that grow, reproduce, and die in 1 year or less. Other herbaceous plants (such as carrot, Queen Anne s lace, cabbage, and foxglove) are biennials; they take 2 years to complete their life cycles before dying. During their first season, biennials produce extra carbohydrates, which they store and use during their second year, when they typically form flowers and reproduce. Perennials are herbaceous and woody plants that have the potential to live for more than 2 years. In temperate climates, the aerial (aboveground) stems of herbaceous perennials such as iris, rhubarb, onion, and asparagus die back each winter. Their underground parts (roots and underground stems) become dormant during the winter and send out new growth each spring. (In dormancy, an organism reduces its metabolic state to a minimum level to survive unfavorable conditions.) Similarly, in certain tropical climates with pronounced wet and dry seasons, the aerial parts of herbaceous perennials die back and the underground parts become dormant during the unfavorable dry season. Other tropical plants, such as orchids, are herbaceous perennials that grow year-round. All woody plants are perennials, and some of them live for hundreds or even thousands of years. In temperate climates, the Carlyn Iverson (a) Duckweed (Spirodela), the larger green plants in the photograph, are tiny floating aquatic plants about 1 cm (0.4 in) across. If you look closely at the frog s body, you will see minute green dots. Each is a water-meal (Wolffia) plant. These tiny floating herbs, about 1.5 mm (0.06 in) wide, are the smallest known flowering plants. Gunter Ziesler/Peter Arnold, Inc. (b) This red tingle tree (Eucalyptus jacksonii) was photographed in Walpole Noralup National Park, Australia. Red tingles, which are rare and endemic to Western Australia, have massive bases, with girths up to 26 m (85 ft), and heights of 75 m (244 ft). Figure 32-1 Size variation in plants Plant Structure, Growth, and Differentiation 699

aboveground stems of woody plants become dormant during the winter. Many temperate woody perennials are deciduous; that is, they shed their leaves before winter and produce new stems with new leaves the following spring. Other woody perennials are evergreen and shed their leaves over a long period, so some leaves are always present. Because they have permanent woody stems that are the starting points for new growth the following season, many trees attain massive sizes. Consider a common plant such as a carrot. Is it herbaceous or woody? An annual, biennial, or perennial? Explain. Consider a common plant such as an oak. Is it herbaceous or woody? An annual, biennial, or perennial? Explain. THE PLANT BODY Learning Objectives Plants have different life history strategies Woody perennials often live for hundreds of years, whereas some herbaceous annuals may live for only a few weeks or months. Such characteristic features of an organism s life cycle, particularly as they relate to its survival and successful reproduction, are known as life history strategies. Biologists try to understand the relative advantages of each life history strategy, including any trade-offs involved in the allocation of various resources. It appears that in some environments a longer life span is advantageous, whereas in other environments a shorter life span increases a species chances for reproductive success that is, for long-term survival. When an environment is relatively favorable, it is filled with plants competing for available space. Because such an environment is so crowded, it has few open spots in which new plants can become established. When a plant dies, the empty area is quickly filled by another plant, but not necessarily by the same species as before. Thus, an adult perennial survives well, but young plants, whether perennials or annuals, do not. A plant with a long life span thrives in this type of environment because it can occupy a piece of soil and continue to produce seeds for many years. In a tropical rain forest, for example, competition prevents most young plants from becoming established, and woody perennials predominate. These perennials expend considerable energy in making woody tissues, but this energy investment allows them to thrive and reproduce for a long period. In a relatively unfavorable environment, many possible sites are usually available. This type of environment is not crowded, and young plants usually do not have to compete against large, fully established plants. Here, smaller, short-lived plants have the reproductive advantage. These plants are opportunists; that is, they grow and mature quickly during the brief periods when environmental conditions are most favorable. As a result, all their resources are directed into producing as many seeds as possible before dying. In deserts following a rainy period, for example, annuals are more prevalent than woody perennials. Thus, each species has a characteristic life history strategy, with some plants adapted to variable environments and others adapted to stable environments. The longer life span characteristic of woody perennials is just one of several successful life history plans. We return to life history strategies in our discussion of population ecology (see Chapter 52). Review Consider a common plant such as a marigold. Is it herbaceous or woody? An annual, biennial, or perennial? Explain. 4 Discuss the functions of various parts of the vascular plant body, including the nutrient- and water-absorbing root system and the photosynthesizing shoot system. 5 Describe the structure and functions of the ground tissue system (parenchyma tissue, collenchyma tissue, and sclerenchyma tissue). 6 Describe the structure and functions of the vascular tissue system (xylem and phloem). 7 Describe the structure and functions of the dermal tissue system (epidermis and periderm). The plant body of flowering plants (and other vascular plants) is usually organized into a root system and a shoot system ( Fig. 32-2). The root system is generally underground. The aboveground portion, the shoot system, usually consists of a vertical stem that bears leaves and, in flowering plants, flowers and fruits that contain seeds. Each plant typically grows in two different environments: the dark, moist soil and the illuminated, relatively dry air. Plants usually have both roots and shoots because they require resources from both environments. Thus, roots branch extensively through the soil, forming a network that anchors the plant firmly in place and absorbs water and dissolved minerals (inorganic nutrients) from the soil. Leaves, the flattened organs of photosynthesis, are attached more or less regularly on the stem, where they absorb the sun s light and atmospheric carbon dioxide (CO 2 ) used in photosynthesis. The plant body consists of cells and tissues As in other organisms, the basic structural and functional unit of plants is the cell. Plants have evolved a variety of cell types, each specialized for particular functions. Like animal cells, plant cells are organized into tissues. A tissue is a group of cells that forms a structural and functional unit. Simple tissues are composed of only one kind of cell, whereas complex tissues have two or more kinds of cells. In vascular plants, tissues are organized into three tissue systems, each of which extends throughout the plant body ( Fig. 32-3). Each tissue system contains two or more kinds of tissues ( Table 32-1 on page 703). Most of the plant body is composed of the ground tissue system, which has a variety of functions, including photosynthesis, storage, and support. The vascular tissue system, an intricate conducting system that extends throughout the plant body, is responsible for conduction of various substances, including water, dissolved minerals, and 700 Chapter 32 www.thomsonedu.com/biology/solomon

and cause new leaf and bud primordia to appear. Farther from the stem tip, the immature cells differentiate into the three tissue systems of the mature plant body. Outer bark (periderm) Inner bark (secondary phloem) Secondary growth takes place at lateral meristems Trees and shrubs undergo both primary and secondary growth. These plants increase in length by primary growth and increase in girth by secondary growth. The increase in girth, which occurs in areas that are no longer elongating, is due to cell divisions that take place in lateral meristems, areas extending along the entire length of the stems and roots except at the tips. Two lateral meristems, the vascular cambium and the cork cambium, are responsible for secondary growth, which is the formation of secondary tissues: secondary xylem, secondary phloem, and periderm ( Fig. 32-9). The vascular cambium is a layer of meristematic cells that forms a long, thin, continuous cylinder within the stem and root. It is located between the wood and bark of a woody plant. Division of cells of the vascular cambium adds more cells to the wood (secondary xylem) and inner bark (secondary phloem). The cork cambium, located in the outer bark, is composed of a thin cylinder or irregular arrangement of meristematic cells. Cells of the cork cambium divide and form cork cells toward the outside and one or more underlying layers of cork parenchyma cells that function in storage. Collectively, cork cells, cork cambium, and cork parenchyma make up the periderm. We are now ready to give a more precise definition of bark. Bark, the outermost covering over woody stems and roots, consists of all plant tissues located outside the vascular cambium. Bark has two regions, a living inner bark composed of secondary Bark Wood (secondary xylem) Vascular cambium Figure 32-9 Animated Secondary growth The vascular cambium, a thin layer of cells sandwiched between the wood and bark, produces the secondary vascular tissues: the wood, which is secondary xylem, and the inner bark, which is secondary phloem. The cork cambium produces the periderm, the outer bark tissue that replaces the epidermis in a plant with secondary growth. phloem and a mostly dead outer bark composed of periderm. Chapters 34 and 35 present a more comprehensive discussion of secondary growth. Review What is the role of plant meristems? Does animal growth involve meristems? What is primary growth? Secondary growth? What are apical meristems? Lateral meristems? SUMMARY WITH KEY TERMS Learning Objectives 1 Distinguish between herbaceous and woody plants (page 699). Plants are either herbaceous (nonwoody) or woody. In temperate climates, the aerial parts of herbaceous plants die back, whereas the aerial parts of woody plants persist. 2 Discuss the differences among annuals, biennials, and perennials, and give an example of each (page 699). Annuals, such as corn, geranium, and marigold, are herbaceous plants that grow, reproduce, and die in 1 year or less. Biennials, such as carrot and Queen Anne s lace, take 2 years to complete their life cycles before dying. Perennials, such as asparagus and oak trees, are herbaceous and woody plants that have the potential to live for more than 2 years. 3 Contrast two different life history strategies in plants (page 699). Plants have a variety of life history strategies to ensure their successful reproduction and survival. Long-lived trees thrive in a tropical rain forest, where competition prevents most small, short-lived plants from becoming established. Small, short-lived plants thrive in a relatively unfavorable environment, such as a desert following a rainy period. 4 Discuss the functions of various parts of the vascular plant body, including the nutrient- and water-absorbing root system and the photosynthesizing shoot system (page 700). The vascular plant body typically consists of a root system and a shoot system. The root system is generally underground and obtains water and dissolved minerals for the plant. Roots also anchor the plant firmly in place. The shoot system is generally aerial and obtains sunlight and exchanges gases such as CO 2, oxygen, and water vapor. The shoot system consists of a vertical stem that bears leaves (the main organs of photosynthesis) and reproductive structures (flowers and fruits in flowering plants). Buds, undeveloped embryonic shoots, develop on stems. 712 Chapter 32 www.thomsonedu.com/biology/solomon

Although separate organs (roots, stems, leaves, flower parts, and fruits) exist in the plant, tissue systems are integrated throughout the plant body, providing continuity from organ to organ. The plant body is composed of three tissue systems: ground, vascular, and dermal. Learn more about the plant body by clicking on the figure in ThomsonNOW. 5 Describe the structure and functions of the ground tissue system (parenchyma tissue, collenchyma tissue, and sclerenchyma tissue) (page 700). The ground tissue system consists of three tissues with a variety of functions. Parenchyma tissue is composed of living parenchyma cells that have thin primary cell walls. Functions of parenchyma tissue include photosynthesis, storage, and secretion. Collenchyma tissue consists of collenchyma cells with unevenly thickened primary cell walls. This tissue provides flexible structural support. Sclerenchyma tissue is composed of sclerenchyma cells sclereids or fibers that have both primary cell walls and secondary cell walls. Sclerenchyma cells are often dead at maturity but provide structural support. Learn more about parenchyma, collenchyma, and sclerenchyma cells by clicking on the figure in ThomsonNOW. 6 Describe the structure and functions of the vascular tissue system (xylem and phloem) (page 700). The vascular tissue system conducts materials throughout the plant body and provides strength and support. Xylem is a complex tissue that conducts water and dissolved minerals. The actual conducting cells of xylem are tracheids and vessel elements. Phloem is a complex tissue that conducts sugar in solution. Sieve tube elements are the conducting cells of phloem; they are assisted by companion cells. Learn more about xylem and phloem by clicking on the figure in ThomsonNOW. 7 Describe the structure and functions of the dermal tissue system (epidermis and periderm) (page 700). The dermal tissue system is the outer protective covering of the plant body. The epidermis is a complex tissue that covers the herbaceous plant body. The epidermis that covers aerial parts secretes a waxy cuticle that reduces water loss. Stomata permit gas exchange between the interior of the shoot system and the surrounding atmosphere. Trichomes are outgrowths, or hairs, that occur in many sizes and shapes and have a variety of functions. The periderm is a complex tissue that covers the woody parts of the plant body in woody plants. 8 Discuss what is meant by growth in plants, and state how it differs from growth in animals (page 710). Growth in plants, unlike that in animals, is localized in specific regions called meristems. Growth involves three processes: cell division, cell elongation, and cell differentiation. 9 Distinguish between primary and secondary growth (page 710). Primary growth is an increase in stem or root length. Primary growth occurs in all plants. See root and shoot development by clicking on the figures in ThomsonNOW. Secondary growth is an increase in stem or root girth (thickness). Secondary growth typically occurs in long cylinders of meristematic cells throughout the length of older stems and roots. Learn more about secondary growth by clicking on the figures in ThomsonNOW. 10 Distinguish between apical meristems and lateral meristems (page 710). Primary growth results from the activity of apical meristems that are localized at the tips of roots and shoots and within the buds of stems. The two lateral meristems responsible for secondary growth are the vascular cambium and the cork cambium. TEST YOUR UNDERSTANDING 1. Plants that complete their life cycles in 1 year are called ; those that complete them in 2 years are ; and those that live year after year are. (a) annuals; perennials; biennials (b) biennials; annuals; perennials (c) annuals; biennials; perennials (d) perennials; annuals; biennials (e) perennials; biennials; annuals 2. Which of the following plant life history strategies would be successful in a relatively favorable environment such as a tropical rain forest? (a) long life span with flowers and seeds produced each year (b) long life span with flowers and seeds produced only when the plant is very young (c) short life span with flowers and seeds produced each year (d) short life span with flowers and seeds produced only when the plant is very young (e) very short life span with flowering at end of life 3. Most of the plant body consists of the tissue system. (a) ground (b) vascular (c) periderm (d) dermal (e) cortex 4. The cell walls of parenchyma cells (a) contain large quantities of pectin in the thickened corners (b) are rich in lignin but do not contain hemicelluloses and pectin (c) are predominantly cellulose, although they also contain hemicelluloses and pectin (d) contain cellulose, hemicelluloses, and lignin in approximately equal amounts (e) contain hemicelluloses, pectin, and lignin but no cellulose 5. Which tissue system provides a covering for the plant body? (a) ground (b) vascular (c) periderm (d) dermal (e) cortex 6. Storage, secretion, and photosynthesis are the functions of (a) collenchyma (b) vessel elements (c) lateral meristems (d) sclerenchyma (e) parenchyma Plant Structure, Growth, and Differentiation 713

7. The two simple tissues that are specialized for support are (a) parenchyma and collenchyma (b) collenchyma and sclerenchyma (c) sclerenchyma and parenchyma (d) parenchyma and xylem (e) xylem and phloem 8. Sclereids and fibers are examples of which plant tissue? (a) parenchyma (b) collenchyma (c) sclerenchyma (d) xylem (e) epidermis 9. Which of the following statements about the vascular tissue system is not true? (a) xylem and phloem are continuous throughout the plant body (b) xylem not only conducts water and dissolved minerals but also provides support (c) four cell types occur in phloem: sieve tube elements, companion cells, tracheids, and vessel elements (d) sieve tube elements lack nuclei (e) vessel elements are hollow, and their end walls have perforations or are entirely dissolved away 10. Conduction of water and minerals in xylem occurs in vessel elements and (a) sieve tube elements (b) tracheids (c) collenchyma (d) cork cells (e) phloem 11. Conduction of sugar in solution in the sieve tube elements is aided by (a) cork cells (b) sclerenchyma (c) parenchyma (d) guard cells (e) companion cells 12. The outer tissue that covers plants with primary growth is, whereas covers plants with secondary growth. (a) cuticle; cork parenchyma (b) periderm; phloem (c) epidermis; periderm (d) epidermis; collenchyma (e) cellulose; lignin 13. The noncellular waxy layer secreted by the epidermis over its aerial surface is called (a) lignin (b) cuticle (c) periderm (d) cellulose (e) trichome 14. Minute pores known as dot the surface of the epidermis of leaves and stems; each pore is bordered by two. (a) stomata; guard cells (b) stomata; fibers (c) sieve tube elements; companion cells (d) sclereids; guard cells (e) cuticle; guard cells 15. Localized areas within the plant body where cell divisions occur are known as (a) organs (b) fibers (c) meristems (d) cork parenchyma (e) stomata 16. Primary growth, an increase in the length of a plant, occurs at the (a) cork cambium (b) apical meristem (c) vascular cambium (d) lateral meristem (e) periderm 17. The two lateral meristems responsible for secondary growth are the (a) cork cambium and apical meristem (b) apical meristem and cork parenchyma (c) vascular cambium and apical meristem (d) vascular cambium and cork cambium (e) cork cambium and cork parenchyma CRITICAL THINKING 1. Grasses have a special meristem situated at the base of the leaves. Relate this information to what you know about the growth of grass after you mow the lawn. 2. A couple carved a heart with their initials into a tree trunk 4 ft above ground level; the tree was 25 ft tall at the time. Twenty years later the tree was 50 ft tall. How far above the ground were the initials? Explain your answer. 3. Sclerenchyma in plants is the functional equivalent of bone in humans (both sclerenchyma and bone provide support). However, sclerenchyma is dead, whereas bone is living tissue. What are some of the advantages of a plant having dead support cells? Can you think of any disadvantages? 4. Evolution Link. Flowering plants have both tracheids and vessel elements in their xylem. Conifers, which evolved earlier than flowering plants, have tracheids only. How do these differences in xylem structure help explain the difference in the success of these two plant groups? Additional questions are available in ThomsonNOW at www.thomsonedu.com/ login 714 Chapter 32 www.thomsonedu.com/biology/solomon