Biology20Ch14 PDF

CHAPTER Specialized plant structures support 14 plant functions. P lants and animals live very different lives. While anima...

CHAPTER Specialized plant structures support 14 plant functions. P lants and animals live very different lives. While animals move Learning from place to place and eat other organisms for food, plants are Expectations rooted in soil and use sunlight to make their food. However, do not be fooled: plants engage much more actively with their environment than By the end of this chapter, you may realize. While animals can search for food, plants may co-operate you will: with fungi, giving their fungal partners sugar in exchange for nutrients Developing Skills of from the soil. Plants can elongate their stems or reposition their leaves Investigation and to capture sunlight. They can modify the growth of their roots to reach Communication a source of water and minerals. Plants change their growth based on the use appropriate terminology interaction of external factors, such as light, nutrients, temperature, or related to plants gravity, and internal factors, such as plant hormones. When attacked by a predator, an animal can flee or fight. Plants cannot identify, and draw biological diagrams of, the specialized flee, but they can fight back. Thorns, spines, and leaf hairs protect plant plant tissues in roots, stems, stems and leaves from attack by herbivores. Many of our medicines are and leaves, using a microscope plant products, used by plants to defend themselves. Some plants can and models even call in allies to help them defend themselves. Acacia trees, for investigate various techniques example, have hollow thorns where ants live and they provide specialized of plant propagation structures to feed the ants (Figure 14.1). In return, the ants attack herbivores that try to feed on the acacia and kill vines that grow around Understanding Basic Concepts the base of the acacia. If the ants are removed from an acacia, it quickly describe the structures of the falls victim to herbivore attack or is overgrown by surrounding vegetation. various types of tissues in vascular plants, and explain the mechanisms of transport involved in the processes by which materials are distributed throughout a plant compare and contrast monocot and dicot plants in terms of their structures and their evolutionary processes explain the reproductive mechanisms of plants in natural reproduction and artificial propagation (a) Figure 14.1 Ants and acacia trees have a mutualistic relationship where both the ant and the tree benefit. (a) There are feeding structures for the ants on the ends of young leaves, and (b) the ants live in hollowed thorns. (b) 380 Unit E ©P 14-BIOLOGY-11SE-Ch14.indd 380 7/19/10 3:46:24 PM  14.1 Plant Organs, Tissues, and Cells Section Summary Plant organs include roots, shoots, leaves, and flowers. The structure of monocot and dicot organs differs. Flowering plants reproduce sexually, producing seeds. In addition, many plants reproduce asexually. The three main tissue systems of plants are dermal, vascular, and ground tissue systems. Most animals have a characteristic adult form. You terminal bud were a newborn, a baby, a child, an adolescent, and you will be an adult. At each stage of life, a human has axillary bud blade two arms, two legs, two eyes, one nose, and one mouth. petiole leaf A fly was a maggot, a pupa, and then an adult. As an flower adult, a fly has six legs and one pair of wings. Plants are different from animals. Their adult appearance can vary a lot. A rose bush may have one stem or fifty. Each stem is able to produce flower buds, and two rose bushes may be very different in the number internode and arrangement of flowers in bloom. However, like animals, the specialized organs of plants have specific node functions. These structural adaptations enhance the stem survival and reproductive success of plants in the shoot system environments in which they live. root system Plant Organs The primary organ systems of a plant are the roots, shoots, leaves, and flowers (Figure 14.2). These organ systems differ between the two groups of angiosperms: dicotyledons and monocotyledons. Dicotyledons, Figure 14.2 A plant has a root system below the ground and a shoot system above. A shoot consists of stems, leaves, or dicots, are the larger group, containing broad-leaf and flowers. New shoots grow from buds throughout a species such as dandelions, canola, and maple trees. plant’s life. Monocotyledons, or monocots, contain species with long, thin leaves such as grasses, orchids, and lilies. Root and Shoot Systems Roots are structures that anchor a plant in the soil, absorbing minerals and water and providing structural support. Monocots have fibrous root systems. A fibrous root system consists of a mat of thin roots spread out below the soil surface, providing increased exposure to soil nutrients and water (Figure 14.3). In contrast, most dicots have a taproot system that is characterized by (a) (b) one large vertical root with many smaller branches. Carrots, turnips, and beets are examples of dicots Figure 14.3 (a) Monocots have a fibrous root system with very large, starch-storing taproots. consisting of a mat of thin roots. (b) Dicots have a taproot system consisting of one thick central root with thin branches. ©P Chapter 14 Specialized plant structures support plant functions. 381 14-BIOLOGY-11SE-Ch14.indd 381 7/19/10 3:46:27 PM  While most roots are below ground, some roots have interesting structures that are adaptations to the plant’s environment. For example, buttress roots form on some tall or shallowly rooted tree species to help support them (Figure 14.4). Mangrove trees have above-ground roots that help bring oxygen into the roots. Shoots are usually above-ground structures consisting of stems, leaves, and flowers. Stems are the parts of a plant that support leaves and flowers. Nodes are the points on the stem at which leaves are attached, and internodes are the portions of the stem between nodes. Stems play an important role in transporting materials in a plant. Vascular tissue runs vertically in the stem. It transports water and minerals from the roots up to the leaves, and food from the leaves to other parts of the plant, such as growing buds, flowers, and roots. Shoots have modified roles and may vary considerably in their appearance (Figure 14.5). The stems of cacti are specialized to store water. Tubers, such as potatoes, are swollen shoots that store starch. The thorns that protect a hawthorn bush are actually modified branches. An onion is Figure 14.4 Many trees in the an underground shoot, modified for food storage. tropical rainforest have buttress Undeveloped shoots are called buds. A terminal bud is found at the roots because the soils are shallow. tip of a stem. Axillary buds are found in the angles formed by a leaf and the main stem. These angles are called axils. Growth from axillary buds forms the plant’s branches. Figure 14.5 Many plant shoots have modified roles. The Leaf Leaves are the primary food-manufacturing sites of a plant, capturing sunlight and converting light energy to chemical energy during photosynthesis. Most plant leaves are flattened and thin, allowing them to intercept and capture sunlight effectively. The main part of the leaf is the blade. A stalk, called a petiole, connects the leaf to the stem. The vein that runs through the petiole and into the blade consists of vascular tissue and support tissue. These veins carry water and nutrients into (a) (b) the leaf and transport sugars from the leaf to other parts of the plant. The venation, or arrangement of Figure 14.6 Leaf venation differs between monocots and dicots. veins, differs in the leaves of monocots and dicots. (a) Monocot leaves have parallel veins; (b) dicot leaves have a In a monocot leaf, several major veins run parallel branching veins. along the length of the leaf blade. A dicot leaf has a branching network of veins (Figure 14.6). 382 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 382 7/19/10 3:46:28 PM  Some plants have highly modified leaves. Leaves, such as the spines on a cactus, are modified so much that you may not recognize them as leaves. The tendrils on a pea plant or a grapevine are modified leaves that allow a plant to attach to and climb along a surface. Because an onion bulb is a modified shoot, the layers of an onion are actually leaves (Figure 14.7). The long leaves of grasses lack petioles altogether. Celery, on the other hand, has enormous petioles — the “stalks” that you eat. Flowers and Sexual Reproduction (a) Next time you see a flower, look at it carefully. Many familiar flowers are known for their pretty petals, but there is much more to a flower than petals. In fact, the centre stage for the plant’s reproductive action is tucked inside the flower. While flowers come in many shapes, colours, and sizes, most share the same basic pattern. A flower is a specialized shoot, unique to angiosperms, that usually consists of four different rings of modified leaves: sepals, petals, stamens, and pistils (Figure 14.8). The outermost ring, the sepals, covers and protects the flower bud before the blossom opens. An example is a rosebud. The next ring into the flower is composed of petals. Petals are often strikingly colourful — they are probably the structures you think of when you picture a flower. Some flower petals have “runway” markings that help guide insect pollinators toward the flower’s reproductive parts. anther (b) stamen filament Figure 14.7 (a) The red “petals” of a poinsettia are actually leaves. (b) An onion is a modified underground shoot, and the layers of the onion are modified leaves. petal sepal stigma Figure 14.8 A typical flower ovule style pistil consists of sepals, petals, pistils, ovary and stamens. In the centre of the flower are the stamens, the male reproductive structures, and the pistils, the female reproductive structures. Most flowers have multiple stamens surrounding one or more pistils. Some species, however, have stamens and pistils on separate flowers or even separate plants. Each stamen consists of a long stalk topped by a sac called an anther. Within the anthers, meiosis produces spores that develop into pollen grains. Each pollen grain is a male gametophyte, consisting of two cells surrounded by a thick protective wall. When smelling a flower, you may have dusted your nose with some of these tiny pollen grains. At the base of each female pistil is an ovary. Inside the ovary are structures called ovules. Ovules contain the female gametophyte. When fertilized, ovules develop into seeds. Leading to the ovary is a narrow structure called the style, which has a sticky tip called the stigma (plural, stigmata). A pistil may contain one or more carpels, with each carpel containing one ovule. ©P Chapter 14 Specialized plant structures support plant functions. 383 14-BIOLOGY-11SE-Ch14.indd 383 7/19/10 3:46:37 PM  During pollination, pollen grains released from anthers land on the stigmata of flowers (Figure 14.9). Typically, this is after the pollen has been carried by wind or an animal to another flower. Once on a stigma, a pollen grain absorbs water and extends a structure called a pollen tube. The pollen tube, which contains two sperm nuclei, grows toward the ovary through the style. When the pollen tube reaches the ovule in the ovary, a sperm cell fertilizes the egg cell in the ovule and forms a zygote, which develops into the plant embryo. The other sperm cell contributes to the development of a nutrient-rich tissue, called endosperm, that nourishes the growing embryo. Several pollen tubes may grow down Figure 14.9 Pollen has landed on the stigma of this gorse flower. a style at once, competing with one another to fertilize one ovule. (magnification 250⫻) Monocot flowers tend to have sepals, petals, and reproductive parts in multiples of three. For example, a lily flower has six petals and six stamens. Dicot flowers tend to have parts in multiples of four or five. However, there are exceptions to this general rule. For example, the flowers of mustard, a dicot, have four petals and six stamens. Concept Check 1. Compare and contrast the functions of roots and shoots. 2. Compare the arrangement of veins in monocot and dicot leaves. 3. Draw the reproductive structures of a flower. For each structure, include a label stating a brief description of its function. embryo cotyledon Seed Development and Dispersal After fertilization takes place, the ovule develops into a seed. Seeds have seed coat a tough outer layer called a seed coat that helps to protect the tiny embryo and endosperm inside (Figure 14.10). In many seeds, the endosperm is the food source for the developing embryo and may contain starch, proteins, and oils. Many plant products, such as wheat flour and popcorn, are made Figure 14.10 Slicing a string bean from endosperm. In the embryo, a miniature root and shoot take form. seed in half reveals the embryo An embryonic leaf, called the cotyledon, also develops (Figure 14.11). and cotyledons. A tough seed coat surrounds the seed. The cotyledon functions in the storage and transfer of nutrients to the embryo and is especially important in seeds without endosperm. In dicots there are two (di) cotyledons. In monocots, there is one (mono) cotyledon. seed coat seed coat endosperm embryo embryo cotyledons Figure 14.11 Internal structures of Bean Corn (a) dicot and (b) monocot seeds. (a) (b) After several cycles of mitosis, the growth and development of the plant embryo within the seed is temporarily suspended. This is the stage when the seed is usually dispersed from the parent plant. In many flowering plants, a fruit develops from the ovary of an angiosperm. Fruits protect seeds and help disperse seeds from the parent plant. You may think of fruits as being sweet and juicy, but there are many types of fruits. Green peppers, walnuts, cucumbers, maple tree keys, coconuts, and corn are all types of fruits. 384 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 384 7/19/10 3:46:40 PM  Seed dispersal can occur in many ways (Figure 14.12). Some seeds travel by sticking onto a passing animal’s fur, as burrs. (A burr is actually a fruit, just not a tasty one!) Other seeds are tucked inside fleshy, edible fruits that are attractive to animals as food. The flesh of the fruit gets digested, but the indigestible seed coat protects the embryo. The seed passes through the animal’s digestive tract and is eventually deposited as part of the animal’s feces, sometimes many kilometres from the original plant. In fact, seeds of some species benefit from (a) being passed through an animal’s digestive system, as the digestive enzymes weaken the seed coat, allowing the roots and shoots to emerge. Some seeds, such as coconuts, travel on water (again, encased in the fruit). Others are so tiny and lightweight that they can be carried by the wind. A dandelion is one example of a plant whose seeds are dispersed by the wind. Some seeds, such as the touch-me-not, are ballistically propelled several metres from the plant. One species of tree in Africa can throw its seeds over 60 m. (b) Seed Germination When conditions are favourable, the plant embryo within a seed begins to grow. This process is called germination. Most seeds must soak up water in order to germinate. By taking up water, the seed expands and splits its seed coat. The water also triggers metabolic changes in the embryo that enable it to grow. If you have ever tried to grow garden vegetables, you may have noticed that simply exposing the seeds to a warm, moist environment was often enough. But the conditions for germination (c) vary among plant species. Some plants have more particular requirements. For example, some desert plants germinate only after a heavy rainfall. This allows the seedling to push more easily through the moistened soil, and ensures at least a temporary water supply that can be used by its growing tissues. In climates with harsh winters, some seeds will germinate only after being exposed to a long period of cold. This prevents them from germinating during a warm spell in the middle of winter. Some seeds require exposure to the intense heat of a brush fire before germinating. The fire clears dense shrubs and other growth that (d) would otherwise shade and compete with the seedling. After breaking out of the seed coat, the journey of a plant Figure 14.12 Seeds are dispersed in different shoot through the soil to the surface is a difficult one. Sand ways. Seeds may be (a) inside spiny fruit that and other hard particles in soil are abrasive to new plant tissues hitch a ride on animals, (b) inside tasty fruit and dispersed after passing through an sliding past them. Plants have adaptations that protect the animal’s body, (c) dispersed by the wind, developing shoot as it grows toward the surface. For example, or (d) ballistically propelled by the fruit. some dicots have a hooked shoot tip (Figure 14.13(a), next page). This protects the delicate shoot tip by holding it downward as the shoot moves through the soil. As the shoot breaks through the soil surface, its tip is lifted gently out of the soil and straightens out. ©P Chapter 14 Specialized plant structures support plant functions. 385 14-BIOLOGY-11SE-Ch14.indd 385 7/19/10 3:46:44 PM  In most monocots, a sheath surrounding the shoot pushes straight upward, breaking through the soil (Figure 14.13(b)). The delicate shoot then grows upward through this protective tunnel. After emerging into the light, the first leaves expand from the shoot and begin making food by photosynthesis. At this stage, the young plant is called a seedling. foliage leaves (a) Garden bean first true leaves cotyledon root foliage leaves (b) Corn Figure 14.13 A range of sheath adaptations protects plants during germination. (a) In some dicots, such as beans, the shoot tip avoids travelling “face-first” by being hooked downwards as it moves through the soil. (b) In some monocots, such as corn, a protective sheath penetrates root the soil ahead of the shoot. Vegetative Reproduction In addition to sexual reproduction, many plants are also capable of asexual reproduction. Asexual reproduction in plants is called vegetative reproduction. The offspring, or clones, produced during vegetative reproduction are genetically identical to the original plant. Vegetative reproduction can occur naturally or with human help. Some plants, such as cacti, drop stems or other shoots that establish new roots and become clones. Other plants, such as strawberry plants and many grasses, send out runners (Figure 14.14). Some trees and shrubs send out shoots from the base of their trunks or from underground stems. Figure 14.14 Strawberry plants These clones may persist long after the original plant dies. reproduce vegetatively via runners. The simplest way to clone a plant is to cut off a leaf or stem and place the cut end in water or soil. In many plants, the cells at the cut end of the petiole or stem become undifferentiated and then form new plant tissues and organs. The result is the formation of a new plant, genetically BIOLOGY SOURCE identical to the original one. Suggested Activity Biologists have been growing plants from single cells in the laboratory E4 Inquiry Activity Overview on for over 50 years. Unlike animal cells, many plant cells, grown under page 391 the right conditions, are capable of forming all the tissues and organs of the adult plant. The first plants to be cloned in this way were carrots. 386 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 386 7/19/10 3:46:52 PM  Individual cells taken from a carrot root and grown in culture medium developed into new carrot plants, all genetically identical to the original plant. Vegetative reproduction gives us a way to propagate useful crops or decorative plants, without needing to wait for seeds produced by the desired plant to develop. It also ensures that plants will be genetically identical with the desired traits. In many woody species, a branch from one plant can be grafted (a) onto the stem of another plant belonging to the same or a closely related species. Grafting is widely used by fruit growers, allowing them to combine a high-quality fruit-bearing stem with a tough and hardy root or to put several varieties of a fruit on one plant (Figure 14.15). Concept Check 1. Describe three methods of seed dispersal. 2. Explain how two different adaptations of seed germination in dicots and monocots protect the developing shoot. (b) 3. Give two examples of vegetative reproduction in plants. Figure 14.15 (a) Shoots of several varieties of apples can be grafted onto a trunk, resulting in (b) a plant with several varieties of fruit. A Plant’s Main Tissue Systems Plants have three main tissue systems: dermal, vascular, and ground tissue systems. Figure 14.16 shows the three tissue systems as they occur in a young, non-woody plant. Dermal Tissue The dermal tissue is the outer covering or “skin” of the plant. The epidermis, the dermal tissue of non-woody organs, such as young roots, consists of one or more layers of cells. The epidermis covers and protects all the young leaf parts of the plant. Some epidermis is specialized. For example, the epidermis of leaves and stems secretes a waxy cuticle, an adaptation that helps plants retain water. Many plant species have epidermal hairs that trap or poison insects, protecting the plant from insect herbivores. stem Located in the epidermis of leaves and some other tissues are pores called stomata (plural, stoma). Gas and water exchange between the environment and the interior of the plant occurs through stoma. Vascular Tissue Vascular tissue transports water, mineral nutrients, and root organic molecules between roots and shoots. Vascular tissue also contributes to the structural support of the Key plant. There are two types of vascular tissue. Xylem Dermal tissue system transports water and dissolved minerals upward from Vascular tissue system roots into shoots. Phloem transports food made in Ground tissue system mature leaves to the roots and the parts of the shoot system that don’t carry out photosynthesis, such as Figure 14.16 The three main tissue systems are developing leaves, flowers, and fruits. present throughout a plant. ©P Chapter 14 Specialized plant structures support plant functions. 387 14-BIOLOGY-11SE-Ch14.indd 387 7/19/10 3:46:54 PM  Vascular tissue is located in the centres of roots, but in the stems it is arranged in many separate strands called vascular bundles. A monocot stem has vascular bundles scattered throughout its tissue. The vascular bundles of a dicot stem are arranged in a ring (Figure 14.17). vascular bundle cortex epidermis (a) (b) Figure 14.17 As viewed in cross section, (a) the vascular bundles of a monocot stem are scattered throughout the ground tissue. (magnification 5⫻) (b) In contrast, the vascular tissues of a dicot stem are organized in a ring. (magnification 10⫻) Ground Tissue BIOLOGY SOURCE Filling the spaces between the dermal and vascular tissues is ground tissue. It makes up most of a young, non-woody plant and functions in photosynthesis Suggested Activity in the shoot and in storage and support throughout the plant. The ground E6 Inquiry Activity Overview on tissue of the root consists primarily of a mass of cells called the cortex. page 391 Types of Plant Cells The plant tissues you have been reading about are made up of three basic cell types: parenchyma, collenchyma, and sclerenchyma (Figure 14.18). The most abundant type of cell, the parenchyma cell, has thin cell walls and, typically, large central vacuoles. These cells perform a variety of functions in the plant, including food storage, photosynthesis, and cellular respiration. Fruits are made up mostly of parenchyma cells. The food-conducting cells of phloem are also parenchyma cells. (a) (b) (c) Figure 14.18 Plant tissues consist of three basic cell types. (a) Parenchyma cells have thin cell walls. (magnification 150⫻) (b) Collenchyma cells have unevenly thickened cell walls. (magnification 250⫻) (c) Sclerenchyma cells have lignin-rich cell walls. (magnification 275⫻) 388 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 388 7/28/10 10:22:21 AM  Collenchyma cells have unevenly thickened cell walls. Grouped in strands or cylinders, collenchyma cells provide support in parts of the plant that are still growing. Young stems and petioles often have collenchyma just below their surface. The “strings” of a celery stalk, for example, are collenchyma. These living cells elongate with the stems and leaves they support as these regions grow. Sclerenchyma cells are specialized for support. Sclerenchyma cells grow and then die within a mature part of a plant. But that does not mean they become useless to the plant after dying. Their lignin-rich cell walls are left behind, creating a “skeleton” that supports the plant. For example, the water-conducting cells of xylem are specialized sclerenchyma cells. It is important to note that a particular type of plant tissue is not made up of just one type of plant cell. For example, a celery stalk is mostly parenchyma, but collenchyma forms the long strands. Sclerenchyma makes the gritty texture you feel when you bite into the mostly parenchyma fruit of a pear. The Cellular Structure of a Leaf Leaves are designed to capture sunlight and allow gas exchange between the surrounding air and the cells inside the leaf that carry out photosynthesis. The upper and lower surfaces of the leaf are covered by tightly packed epidermal cells (Figure 14.19). These cells are covered by a waxy cuticle, reducing the amount of water that is lost by the leaf. The epidermal layer on the lower surface of the leaf contains stomata flanked by guard cells. The guard cells regulate the opening and closing of the stomata, controlling the diffusion of gases into the leaf and the loss of water vapour from the leaf. cuticle mesophyll upper epidermal tissue palisade mesophyll xylem vascular tissue phloem spongy mesophyll lower epidermal tissue stoma guard cells Figure 14.19 Cellular structure of a dicot leaf ©P Chapter 14 Specialized plant structures support plant functions. 389 14-BIOLOGY-11SE-Ch14.indd 389 7/19/10 3:47:01 PM  The ground tissue of the leaf is made of mesophyll, a type of BIOLOGY SOURCE parenchyma cell that is specialized for gas exchange. Dicots have two Explore More layers of mesophyll cells. Under the upper epidermis is a layer of elongated How does the leaf structure of palisade mesophyll cells that are specialized for capturing light energy monocots and dicots differ? and carrying out photosynthesis. Next to the lower epidermis is a layer of loosely packed spongy mesophyll cells, also called aerenchyma. Air spaces within the spongy mesophyll allow carbon dioxide and oxygen to circulate within the leaf. These spaces are particularly large in the region above the stomata, to allow gas exchange with the air outside the leaf. Concept Check 1. List the functions of dermal, ground, and vascular tissues. 2. Describe characteristics of the three main plant cell types. 3. Which cell types provide structural support to the plant body? Comparing Monocots and Dicots BIOLOGY SOURCE The oldest angiosperm fossils are 125 million years old, dating from the Cretaceous period, when dinosaurs roamed Earth. Angiosperms are a Take It Further very successful group of plants, making up about 90 percent of known Pollen is usually transported plant species. Several evolutionary lines of flowering plants arose from between flowers by the wind or by the first angiosperms, but two groups have been extremely successful: insects. There are, however, many the monocots and the dicots. The evolutionary lines leading to monocots strange animal pollinators, including and dicots separated soon after the origin of the angiosperms. Together, bats, lizards, lemurs, and slugs. these two groups represent the great majority (97 percent) of modern Select one unusual pollinator and describe the pollination mechanism flowering plants. to your classmates. The terms monocot and dicot refer to the presence of one or two cotyledons, in the developing embryo. Table 14.1 summarizes the differences between monocots and dicots, as discussed in this section. Table 14.1 Features of Monocots and Dicots Feature Monocots Dicots Cotyledons One Two Roots Fibrous root system Taproot system Leaf venation Parallel veins Netted veins Leaf mesophyll One type of mesophyll Palisade and spongy mesophyll Flower parts Multiples of 3 Multiples of 4 or 5 Developing shoot Protected by sheath Hooked Vascular bundles in stem Scattered Arranged in a ring Secondary growth Absent Often present 390 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 390 7/19/10 3:47:02 PM  REQUIRED SKILLS Measuring E4 Inquiry Activity BIOLOGY SOURCE Drawing conclusions Plant Propagation Question Prelab Questions What part of a plant — leaf, stem, or root — regenerates Consider the questions below before beginning most readily into a new plant? this activity. 1. How does a plant benefit from the ability to Activity Overview propagate from a cutting? In this activity, you will cut leaves, stems, and roots from several plants and place the cuttings in water 2. Do you expect plant propagation to be more and in various commercial rooting solutions. You successful from leaves, stems, or roots? will determine which cuttings are able to form new 3. What ingredients do you think are put in commercial plant organs. rooting solutions? Your teacher will give you a copy of the full activity. E5 Skill Builder Activity BIOLOGY SOURCE Making Hand Sections of Plants Activity Overview In this activity, you will learn how to make cross sections of plant structures by hand. Your teacher will give you a copy of the full activity. REQUIRED SKILLS DI Key Activity Drawing conclusions E6 Inquiry Activity BIOLOGY SOURCE Reporting results The Structure of Plant Roots, Stems, and Leaves Question Prelab Questions How is structure related to function in the cells and Consider the questions below before beginning tissues of plant roots, stems, and leaves? this activity. 1. Look at the leaves on a celery petiole and Activity Overview determine whether it is a monocot or a dicot. What In this activity, you will cut and stain a thin cross section arrangement of vascular tissue do you expect to see of a young celery petiole and view cross section slides of in this cross section? a buttercup (Ranunculus) root and a lilac (Syringa) leaf. You will identify the cells and tissues that make up each 2. Name three tissues that you will find in both celery organ (Figure 14.20). and buttercup cross sections. Your teacher will give you a copy of the full activity. 3. What tissues do you expect to see in the dicot leaf? Figure 14.20 Cross section of a celery petiole (magnification 10⫻) ©P Chapter 14 Specialized plant structures support plant functions. 391 14-BIOLOGY-11SE-Ch14.indd 391 7/19/10 3:47:02 PM  14.1 Check and Reflect Key Concept Review 10. In your notebook, set up a table like the one below to show the differences between 1. Set up a table, similar to the one below, listing monocot and dicot structures. plant organs and their functions. For each plant organ, describe one structural feature Monocot and Dicot Structures that supports one of its functions. Monocot Dicot Plant Structure and Function Roots Plant Organ Function Structural Feature Stems Leaves Flowers Seeds 2. Give an example of (a) a modified leaf Connect Your Understanding (b) a modified shoot 11. Describe how plant cloning and grafting are 3. Name the four rings of modified leaves used in agriculture and horticulture. that make up a flower. State the function 12. Biologists generally define animal tissue as of each ring. a unit of many similar cells that perform a 4. Name the male and female gametophytes of specific function. How does this definition an angiosperm. of a tissue contrast with what biologists call 5. List four ways that seeds can be dispersed, and a “tissue system” in plants? give an example of each. 13. For each photograph below, identify the plant as a monocot or dicot. Give as many reasons as 6. Explain why each of the following statements you can for each answer. is incomplete or incorrect. (a) Within the ovaries of a flower, meiosis produces spores that develop into pollen grains. (b) Putting a seed in a warm, moist environment will cause it to germinate. (a) (b) (c) (d) 7. Explain the role of endosperm and cotyledon(s) in nourishing a young plant embryo. Question 13 ((c) magnification 20⫻) 8. Young seedlings are adapted to grow through soil without damage to the young shoot. Reflection Describe one such adaptation in 14. How has your understanding of plant (a) monocots reproduction changed after reading this section? (b) dicots 9. Prepare a table listing the three major tissue For more questions, go to BIOLOGY SOURCE systems of plants and stating their structural features and basic functions. 392 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 392 7/19/10 3:47:03 PM  14.2 Primary and Secondary Growth in Plants Section Summary Primary growth increases the length of plant roots and shoots. Secondary growth increases the thickness of woody plants. Meristems generate new dermal, vascular, and ground tissue. The timing of growth and development of parts of a plant is highly dependent on the environment. A whole plant can die, but sometimes only part of the plant dies. If you place your house plant in a shady part of the living room, it may lose half of its leaves. But it can grow more leaves when you put it in the sunny spot near the window. While you will reach your adult size and stop growing sometime within the next few years, most plants continue to grow their entire lives. This lifelong growth allows plants to modify the growth of their roots and shoots to increase their access to water, soil minerals, and sunlight. Meristems and Growth Plants, grow in two ways: in length and in girth, or thickness (Figure 14.21). Primary growth accounts for a plant’s lengthwise growth from root and shoot tips. Stems and roots of many plants also increase in girth. Growth in girth is called secondary growth. While all plants undergo primary growth, only woody dicots undergo secondary growth. Figure 14.21 Plants undergo primary growth in length and secondary growth in width. Tissues called meristems generate new dermal, vascular, and ground tissue in plants throughout their lives. A meristem consists of groups of cells that divide by mitosis, generating new cells that will later differentiate into one of the three main cell types: parenchyma, collenchyma, and sclerenchyma. Meristems have roles in both primary and secondary growth. In primary growth, meristems located in the tips of roots and buds of shoots are called apical meristems. Apical meristems produce the new cells that enable a plant to grow in length, both above and below ground, as well as to branch. ©P Chapter 14 Specialized plant structures support plant functions. 393 14-BIOLOGY-11SE-Ch14.indd 393 7/19/10 3:47:07 PM  The shoots of some monocots, such as grasses, have intercalary meristems at the base of each internode, causing continual elongation of the grass plant. Intercalary meristems cause blades of grass to grow from their base rather than their tips, which is why lawns need regular mowing (Figure 14.22). In secondary growth, two types of meristems, vascular cambium and cork cambium, produce tissue that increases the girth of trees and other woody plants. Figure 14.22 Grasses can be Primary growth mown without damaging their Primary growth allows roots to grow toward soil nutrients and water, meristerns. and allows shoots to gain access to sunlight. Perennial herbs regrow each spring by primary growth, after the shoot system dies back in the winter. Primary Growth of Roots BIOLOGY SOURCE The very tip of the root is the root cap, a cone of cells that protects the delicate, actively dividing cells of the apical meristem. The root’s apical Explore More meristem has two roles: it replaces the cells of the root cap that are What is inside a root and how does scraped away by the soil, and it produces the cells for primary growth. it grow? Cells produced during primary growth form three concentric cylinders of developing tissue (Figure 14.23). The outermost cylinder develops into the epidermis (dermal tissue) of the root. The middle cylinder is the bulk of the root tip. It develops into the root’s cortex (ground tissue). The innermost cylinder becomes the vascular tissue. Primary growth depends not only on the addition of new cells by the apical meristem, but on those new cells getting longer. The new cells become longer mainly by taking up water. This process of elongation is what actually forces the root tip through the soil. cortex (ground tissue) epidermis (dermal tissue) vascular cell differentiation cylinder cell elongation root hair Figure 14.23 New root cells are cell division generated in the apical meristem. Those cells produced toward the apical meristem bottom of the meristem replenish region root cap cells. Those toward the top differentiate into cells of the dermal, ground, and vascular root cap tissue, lengthening the root. 394 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 394 7/19/10 3:47:07 PM  Primary Growth of Shoots A shoot’s apical meristem is a dome-shaped mass of dividing cells at the very tip of the terminal bud (Figure 14.24). Elongation occurs just below this meristem. The elongating cells push the apical meristem upward, instead of downward as in the root. leaves apical meristem axillary bud meristems Figure 14.24 The micrograph of the tip of a Coleus plant shows the tightly packed cells characteristic of a meristem. (magnification 30⫻) As the apical meristem advances upward, some of its cells are left behind. These pockets of meristematic cells form axillary buds at the bases of new leaves. Axillary buds give rise to branches, which also show primary growth as they grow outward from the main stem. As in the root, the apical meristem forms three concentric cylinders of developing tissue. Similarly, each cylinder in the shoot develops into one of the shoot’s three main tissue systems — dermal, ground, or vascular tissues. Concept Check 1. Draw a simple non-woody plant and label the locations of its meristems. 2. Compare and contrast primary growth in a root and a shoot. 3. Describe the function of the root cap. Secondary Growth Secondary growth occurs only in woody dicots such as vines, shrubs, BIOLOGY SOURCE and trees. Monocots do not undergo secondary growth. Thousands of useful products are made from wood — from construction lumber to Suggested Activity fine furniture and musical instruments. Wood is the result of secondary E7 Inquiry Activity Overview on growth. Therefore, much of Canada’s economy is dependent on secondary page 398 growth. Secondary growth involves cell division in two meristematic tissues, called vascular cambium and cork cambium. ©P Chapter 14 Specialized plant structures support plant functions. 395 14-BIOLOGY-11SE-Ch14.indd 395 7/19/10 3:47:08 PM  BIOLOGY SOURCE Vascular Cambium The vascular cambium is a cylinder of actively dividing cells located Explore More between the xylem and phloem. The vascular cambium adds cells on How does secondary growth increase both sides, producing secondary xylem toward the inside of the stem the thickness of woody stems? and secondary phloem toward the outside of the stem (Figure 14.25). This secondary vascular tissue is added to the primary xylem and primary phloem produced by the apical meristem during primary growth. The secondary xylem that is laid down in the growing season of each year accumulates as wood. In a temperate climate like that of Ontario, the vascular cambium is dormant during winter. Year 1 Year 1 Year 2 Early Spring Late Summer Late Summer th grow th grow shed grow th epidermis epidermis cortex secondary primary phloem xylem cork vascular cambium (wood) cork secondary xylem primary xylem cambium bark (2 years of growth) secondary phloem Figure 14.25 Cell division in the vascular cambium and cork cambium contributes to secondary growth. With each added layer of xylem, the stem or the root thickens. Remember that secondary growth makes a tree thicker, but not taller. Over time, any object sitting beside a tree or nailed to it will get incorporated into it (Figure 14.26). Woody plants continue to grow in height via apical meristems. People first became aware of the role of phloem in moving sugar within trees by examining trees that had been girdled. Girdling occurs when a complete ring of bark is removed from the trunk circumference. When an actively growing and photosynthesizing tree is girdled, the bark above the cut area swells as phloem sap accumulates. The trunk and roots of the tree below the cut area are deprived of food, and the tree dies. Girdling is caused by gnawing animals, such as rabbits and rodents, or by humans when a wire or rope is tied too tightly Figure 14.26 The tree bark has around the tree trunk. grown around this sign. 396 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 396 7/19/10 3:47:09 PM  Cork Cambium As secondary growth begins and the stem or root thickens, the original soft dermal tissue and cortex cells of the young stem are shed. A meristem called cork cambium develops from parenchyma cells in the remaining cortex (Figure 14.27). It produces a tough outer layer of cork. As these cork cells die, they leave behind thick, waxy walls that help prevent water loss from the stem. Cork also functions as a barrier that helps protect the internal tissues from physical damage and pathogens. Figure 14.27 A cross section through a tree trunk reveals different layers of tissues. (Note that in the drawing, colours are used to distinguish the different layers.) Sapwood is new xylem that is still actively transporting water. Heartwood is old xylem that no longer transports water. rings BIOLOGY SOURCE heartwood secondary Explore More sapwood xylem What is cork, and how can it be grown and harvested sustainably? vascular cambium secondary phloem cork cambium bark cork Everything outside of the vascular cambium is called bark: the phloem, cork cambium, and cork. The older phloem dies as it is pushed outward. Along with the cork, this dead phloem helps protect the stem until the bark is shed. This dead tissue is harvested from the cork oak to make corks for bottles, cork flooring, and other cork products (Figure 14.28). The cork cambium produces a steady supply of new cork, keeping pace with growth from the vascular cambium. Because cork cambium is shed with the rest of the bark, new cork cambium continuously regenerates from Figure 14.28 Cork is sustainably harvested from the parenchyma cells in the still-living phloem left behind. cork oak by removing the outer layer of bark. ©P Chapter 14 Specialized plant structures support plant functions. 397 14-BIOLOGY-11SE-Ch14.indd 397 7/19/10 3:47:10 PM  BIOLOGY SOURCE Using Xylem to Determine the Age of Trees Examining an old tree trunk in cross section enables you to “read” the Take It Further history of the plant. You can estimate the tree’s age by counting its annual Tree rings can tell us what the growth rings. These rings result from the yearly activity of the vascular environments of trees were like in cambium. Environmental conditions during the growing season affect past years, centuries, or (for very old trees) millennia. Learn how tree rings xylem growth. The vascular cambium produces xylem cells that can carry can provide evidence of past drought, a lot of water — cells that are large and thin-walled — when temperatures floods, forest fires, insect attack, and are cool and water is plentiful, as in the typical spring. In contrast, the climate change. vascular cambium produces narrow, thick-walled cells under hot, dry conditions, as in the typical summer. Each tree ring represents a year’s growth. It consists of a cylinder of spring wood surrounded by a cylinder of denser summer wood. Differences in ring width reveal the variation in weather patterns from year to year, such as a particularly wet or dry spring. Concept Check 1. What two tissues does the meristematic vascular cambium tissue produce? 2. Describe how the cork cambium protects a woody plant. 3. In which tissue of a tree trunk are tree rings formed? Describe how tree rings can be used to determine a tree’s age. REQUIRED SKILLS Drawing conclusions E7 Inquiry Activity BIOLOGY SOURCE Reporting results The Secondary Tissues of a Woody Stem Question What secondary tissues are present in a woody stem of Tilia, the basswood tree? Activity Overview You will use a compound microscope to examine prepared slides of a basswood (Tilia) stem, drawing and labelling the cells and tissues of the woody stem (Figure 14.29). Your teacher will give you a copy of the full activity. Prelab Questions Consider the questions below before beginning this activity. 1. What is the difference between primary and secondary xylem and phloem? 2. Which tissue provides structural support for a woody plant? Figure 14.29 Cross section of the stem of a young basswood tree. 3. What tissues make up bark? 398 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 398 7/19/10 3:47:15 PM  14.2 Check and Reflect Key Concept Review 11. Examine the cross section of a tree trunk below. 1. Name and describe the structure that protects the tip of a growing root. 2. Describe how elongation occurs during the primary growth of roots and shoots. 3. Describe the two roles of the root’s apical A meristem in primary growth. B 4. Identify and describe the two types of tissue generated by the vascular cambium that contribute to secondary growth. 5. Distinguish between the location and function of apical meristems, vascular cambium, and cork cambium. Question 11 6. Examine the photo below of a root cross section and identify tissues A–C. (a) Approximately how old was this tree when it was cut down? (b) Match the letters on the photograph A with the descriptions below, and explain B each choice. i) In this year, there was probably C a drought. ii) In this year, spring was long and wet, and summer was short and hot. 12. Suppose two trees were damaged by two different Question 6 (magnification 30⫻) bark-eating animals. The first animal ate a ring of Connect Your Understanding bark all the way around 7. Explain why meristems are important to: the tree. The second (a) plant growth animal ate the same (b) Canada’s economy amount of bark as the first, but peeled it off as 8. Describe how mitosis and cell elongation a vertical strip. Did the combine to produce primary growth. two animals do the same 9. Explain why each of the following statements amount of damage to the is incomplete or incorrect. trees? Explain your answer. Question 12 (a) Tree trunks are made of dead cells. 13. Use a Venn diagram to compare the growth (b) Once cork cambium is shed with the rest of plants and animals. of the bark, new cork cannot be formed. 10. Secondary growth occurs only in woody dicots. Reflection (a) Compare the arrangement of vascular 14. After reading this section, how has your bundles in monocot and dicot stems. understanding of how plants grow changed? (b) Using this information explain why secondary growth does not occur in monocot stems. For more questions, go to BIOLOGY SOURCE ©P Chapter 14 Specialized plant structures support plant functions. 399 14-BIOLOGY-11SE-Ch14.indd 399 7/19/10 3:47:17 PM  14.3 Plant Vascular Tissue Section Summary Root hairs and mycorrhizae increase the surface area of roots, helping them to absorb water and inorganic ions from soil. Root pressure and transpiration-pull are responsible for the upward movement of xylem sap within a plant. Leaf stomata open and close to regulate leaf transpiration and the movement of gases into and out of the leaf. Phloem sap moves through sieve-tube members from sugar sources to sugar sinks by the pressure-flow mechanism. From the outside, a tree trunk appears silent and unmoving, hardly even alive. But there is lots of activity inside. If you placed a stethoscope on a tree trunk in the early spring just before leaves appeared, you would hear the whoosh of sap running through the tree. The Upward Movement of Xylem Sap The tallest trees in Ontario are eastern white pines. The tallest of these trees stretch 50 m from soil to treetop. It is quite a feat that trees are able to carry water and nutrients, against the pull of gravity, from the soil, into their roots, and up their trunks into their leaves. How Roots Absorb Water and Minerals One function of plant roots is to absorb water and mineral nutrients from the soil. Root hairs are one way that plants increase the absorption of water and minerals. Root hairs are the tiny outgrowths of the root’s epidermal cells (Figure 14.30). They increase the root’s surface area, growing into the spaces between soil particles and greatly increasing absorption. In addition, the roots of most vascular plants form a symbiotic association with fungi called mycorrhizae. This association increases the ability of the root to absorb water and inorganic ions, especially phosphate. As much as 3 m of fungal hyphae (filaments) can extend Figure 14.30 Tiny root hairs give from each centimetre along a root. each of these radish roots a white, fuzzy appearance. The root hairs Once the water gets inside the root, two main forces operate in increase the root’s ability to absorb moving water upward from the roots and throughout the plant. They water and mineral nutrients. usually operate at different times of the day. Root Pressure Push The first force, called root pressure, helps push water up the xylem and usually operates at night. Cells in the root’s epidermis and ground tissue use energy from a chemical called adenosine triphosphate (ATP) to accumulate certain minerals. The minerals then move from cell to cell through specialized channels and eventually enter the xylem. Surrounding the vascular tissue is a layer of cells called the endodermis. 400 Unit E Plants: Anatomy, Growth, and Function ©P 14-BIOLOGY-11SE-Ch14.indd 400 7/19/10 3:47:19 PM  Epidermal cells have waxy cell walls that prevent water and minerals from leaking back out of the xylem. As minerals accumulate in the xylem, water follows by osmosis. Osmosis is the diffusion of water across a cell membrane. The osmotic movement of water builds up a positive root pressure. This pressure forces water and minerals up the xylem, pushing xylem sap upward. Transpiration Pull Root pressure accounts for only a small part of the sap’s upward movement. To get water to the top of the plant, another stronger force is involved. Rather than push water up the plant from the bottom, this force pulls it from the top. Drinking water with a straw is a useful analogy: the suction you create at the top is a pulling force somewhat like the pulling force in plants. In plants, transpiration generates the pull. It is the loss of water through leaves due to evaporation. This force, called transpiration-pull, is greatest during the day when transpiration rates are higher (Figure 14.31). Transpiration can pull xylem sap up a tree because of two properties of water: cohesion and adhesion. 1 Transpiration generates a pulling force on the column of transpiration water in the xylem. leaf vein stoma 2 Cohesion of water molecules extends this pulling force all the way down the roots. cohesion and adhesion in the xylem adhesion cohesion cell wall soil particle water uptake xylem from soil 3 Water in the soil is pulled into the roots. root hair endodermis Figure 14.31 The force of transpiration is so strong that it can pull water from the soil into the roots and all the way up the tree. ©P Chapter 14 Specialized plant structures support plant functions. 401 14-BIOLOGY-11SE-Ch14.indd 401 7/19/10 3:47:20 PM  Cohesion and Adhesion Cohesion is the tendency of molecules of the same kind to stick to one another. Water is a highly cohesive molecule, because areas of slight negative charge near the oxygen atom on one water molecule are attracted to areas of slight positive charge near the hydrogen atoms on other water molecules. These attractions are called hydrogen bonds. In water, hydrogen bonds make the water molecules stick to one another. The water molecules in the xylem tubes form continuous chains, extending all the way from the leaves down to the roots. Adhesion is the attraction between unlike molecules. Water molecules adhere, or stick to, cellulose molecules in the xylem walls. This assists the upward movement of xylem sap by counteracting the downward pull of gravity. Adhesion also prevents water from falling back down to the roots at night when transpiration rates are lower. Xylem Cells Water travels through the plant in two types of xylem cells. Tracheids are long cells with tapered ends. Vessel elements are wider, shorter cells with less tapered ends. The ends of tracheids or vessel elements overlap, forming tubes (Figure 14.32). The tubes are hollow because the cells have died. Only their cell walls, strengthen