Part V Plant Structure and Function
Plant Anatomy 
and Growth
Chapter 
20
Outline
	20.1	Plant Organs
	•	The organs of a plant are the leaves, the stem, and the root.338
	•	Growth occurs from both the shoot tip and the root tip.338
	•	The organization of organs differs in monocots and eudicots.340
	20.2	Plant Tissues and Cells
	•	In plants, epidermal tissue is protective; ground tissue forms the bulk of leaves, stems, and roots; vascular tissue transports water and nutrients.341
	20.3	Organization of Leaves
	•	Leaf veins terminate in the mesophyll of leaves, where photosynthesis takes place.342–43
	•	The stomata of leaves carry on gas exchange and allow water vapor to escape into the environment.342–43
	20.4	Organization of Stems
	•	The primary growth of a shoot produces new leaves and adds to stem length.344
	•	In woody stems, secondary growth consists of annual rings of xylem that increase the girth of stems.345
	20.5	Organization of Roots
	•	Primary growth adds to root length, and the pericycle produces lateral roots.347
	20.6	Plant Nutrition
	•	Plants require only inorganic nutrients to produce all the organic compounds of their bodies.348
	•	Roots are adapted to take up some minerals through mutualistic relationships with bacteria and fungi.348
	 20.7	Transport of Nutrients
	•	The cohesion-tension model accounts for transport of water and minerals in xylem.349
	•	The pressure-flow model accounts for transport of organic nutrients in phloem.350
If you want to see an interesting plant called the pitcher plant, take a walk in an acidic bog. Pitcher plants like to grow in areas where nitrogen and phosphorus, which are required nutrients for plants, are lacking. What’s their secret? Pitcher plants feed on animals, particularly insects, to get the nutrients they need. That’s right—like a few other types of plants, pitcher plants are carnivorous. 
A pitcher plant is named for its leaves, which are shaped like a container we call a pitcher. The pitcher attracts flying insects because it has a scent, is brightly colored, and provides nectar that insects can eat. When an insect lands on the lip of the pitcher, a slippery substance and downward-pointing hairs encourage it to slide into a pit. There, juices secreted by the leaf begin digesting the insect. The plant then absorbs these nutrients into its tissues. Although the pitcher is designed to attract insects, animals as large as rats have been found in pitcher plants! The unique adaptations of pitcher plants allow them to thrive in environments where other plants don’t have a chance of surviving.
Some animals are able to turn the tables on the pitcher plant by taking advantage of the liquid they provide. Larvae of mosquitoes and flies have been known to develop safely inside a pitcher plant. 
Carnivorous plants such as the pitcher plant are fairly unusual in the plant world, but so are many others, such as the coastal redwoods—the tallest trees on planet Earth. In this chapter, you will learn about the organs and structures of flowering plants, the nutrition they need, and how they transport nutrients within plant tissues.
20.1		Plant Organs
A flowering plant, whether a cactus, a daisy, or an apple tree, has a root system and a shoot system (Fig. 20.1). The root system simply consists of the roots, while the shoot system consists of the stem, the leaves, and the organs of sexual reproduction. The three vegetative organs—the leaf, the stem, and the root—perform functions that allow a plant to live and grow. The flower, which functions during reproduction, is discussed in Chapter 21. 
Activity at the terminal bud, the bud at the tip of a main shoot or a branch shoot, causes the shoot to increase in length, while activity at the tip of the root causes the root system to increase in length. This is called primary growth, and it would be equivalent to you increasing in height by growing from your head and your feet! 
Leaves
Leaves are usually the chief organs of photosynthesis, and as such they require a supply of solar energy, carbon dioxide, and water. Broad and thin foliage leaves have a maximum surface area for the collection of solar energy and the absorption of carbon dioxide. Leaves receive water from the root system by way of vascular tissue that terminates in the leaves. Without carrying on photosynthesis, a plant would not be able to grow. 
The wide portion of a foliage leaf is called the blade. The petiole is a stalk that attaches the blade to the stem. Not all leaves are foliage leaves. Some are specialized to protect the plant, attach to objects (tendrils), or even capture insects (Fig. 20.2). Some trees, called evergreens, retain their leaves, and others, called deciduous, lose their leaves during a particular season of the year.  
Stems
A stem is the main axis of a plant. Terminal bud, also called an apical bud, activity produces new leaves and tissues during primary growth. Lateral (side) branches grow from a lateral bud, also called an axillary bud, located at the angle where a leaf joins the stem. A node occurs where a leaf or leaves are attached to the stem, and an internode is the region between the nodes (see Fig. 20.1). Nodes and inter-nodes are used to identify the stem, even if it happens to be an underground stem. In some plants, the nodes of horizontal stems asexually produce new plants.
In addition to supporting the leaves, a stem houses vascular tissue that transports water and minerals from the roots through the stem to the leaves and transports the products of photosynthesis, usually in the opposite direction. 
Stems may have functions other than transport. In some plants (e.g., cactus), the stem is the primary photosynthetic organ and serves as a water reservoir (Fig 20.2a). 
Roots
Just as plants are diverse, so are roots (Fig. 20.3). All root systems support the plant by anchoring it in the soil, as well as absorbing water and minerals from the soil for the entire plant. As a rule of thumb, the root system is at least equivalent in size and extent to its shoot system. Therefore, an apple tree has a much larger root system than a corn plant. Also, the extent of a root system depends on the environment. A single corn plant may have roots as deep as 2.5 meters (m), while a mesquite tree that lives in the desert may have roots that penetrate to a depth of 20 m.
The cylindrical shape of roots and their slimy surface allow them to penetrate the soil as they grow and permit water to be absorbed from all sides. The absorptive capacity of a root is also increased by its many root hairs, -located in a special zone near the root tip. Root hairs are so numerous that they increase the absorptive -surface of a root tremendously. It has been estimated that a single rye plant has about 14 billion root-hair cells, and if placed end to end, the root hairs would stretch 10,626 kilometers. Root-hair cells are constantly being replaced, so this same rye plant forms about 100 million new root-hair cells every day. You probably know that a plant yanked out of the soil will not fare well when transplanted; this is because small lateral roots and root hairs are torn off. Transplantation is more apt to be successful if you take a part of the surrounding soil along with the plant, leaving as many of the lateral roots and the root hairs intact as possible.
Figure 20.3a shows the diversity of roots. A carrot plant has one main taproot, which stores the products of photosynthesis. Grass has fibrous roots that cling to the soil (Fig. 20.3b), and corn plants have prop roots that grow from the stem for better support (Fig. 20.3c). Perennial plants are ones that are able to regrow next season because their roots survive, even though the shoot system dies back.
Monocot Versus Eudicot Plants
Figure 20.4 shows how flowering plants can be divided into two major groups. One of these differences concerns the cotyledons, which are embryonic leaves present in seeds. The cotyledons wither after the first true leaves appear. Some embryos have one cotyledon, and these plants are known as monocotyledons, or monocots. Other embryos have two cotyledons, and these plants are known as eudicotyledons, or eudicots (true dicots). The cotyledons of -eudicots supply nutrients for seedlings, but the cotyledons of monocots store some nutrients and act as a transfer tissue for nutrients stored elsewhere. 
Although the distinction between monocots and eudicots may seem of limited importance, it can be associated with many differences in their structure. Vascular plants contain two main types of transport tissue, the xylem for water and minerals and the phloem for organic nutrients. In a sense, xylem and phloem are to plants what veins and arteries are to animals. In the monocot root, transport tissue, also called vascular tissue, occurs in a ring, but in the eudicot, phloem is located between the arms of the xylem. In a stem, transport tissue occurs in the vascular bundles. The vascular bundles are scattered in the monocot stem, and they occur in a ring in the eudicot stems.
In a leaf, transport tissue forms leaf veins. In monocots, the veins are parallel, while in eudicots, the leaf veins form a net pattern. Monocots and eudicots also have different numbers of flower parts, and this difference will be discussed further in Chapter 21. 
The -eudicots are the larger group and include some of our most -familiar flowering plants—from dandelions to oak trees. The monocots include grasses, lilies, orchids, and palm trees, among others. Some of our most significant food sources are monocots, including rice, wheat, and corn.
20.2		Plant Tissues and Cells
Unlike humans, flowering plants grow their entire life because they have meristem (embryonic) tissue, composed of cells that divide. Apical meristem is located in the terminal bud of the shoot system and in the root tip. When apical meristem cells divide, one of the daughter cells remains a meristem cell, and the other differentiates into one of the three types of specialized tissues of a plant:
	1.	Epidermal tissue forms the outer protective covering of a plant. 
	2.	Ground tissue fills the interior of a plant and helps carry out the functions of a particular organ. 
	3.	Vascular tissue transports water and nutrients in a plant and provides support. 
Another type of meristem tissue is associated with vascular tissue and is called vascular cambium. When the meristem cells of vascular cambium divide, they give rise to new vascular tissue, which causes a plant to increase in girth (called secondary growth). 
Epidermis and Ground Tissue
The entire body of a plant is covered by a layer of epidermis composed of closely packed epidermal cells. The walls of epidermal cells that are exposed to air are covered with a waxy cuticle to minimize water loss. The cuticle also protects against bacteria and other organisms that might cause disease. As mentioned in Section 20.1, roots have root hairs, long slender projections of epidermal cells (Fig. 20.5a). These hairs increase the surface area of the root for absorption of water and minerals; they also help anchor the plant firmly in place. In leaves, the epidermis often contains stomata (sing., stoma). A stoma is a small opening surrounded by two guard cells (Fig. 20.5b). When the stomata are open, gas exchange and water loss occur. 
In the trunk of a tree, the epidermis is -replaced by cork, which is a part of bark (Fig. 20.5c). New cork cells are made by a meristem called cork cambium. As the new cork cells mature, they increase slightly in volume, and their walls become encrusted with suberin, a lipid -material, so that they are waterproof and chemically inert. These nonliving cells protect the plant and make it resistant to attack by fungi, bacteria, and animals.
Ground tissue forms the bulk of leaves, stems, and roots. Ground tissue contains three types of cells (Fig. 20.6). Parenchyma cells are the least specialized of the cell types and are found in all the organs of a plant. They may contain chloroplasts and carry on photosynthesis, or they may contain colorless plastids that store the products of photosynthesis. Collenchyma cells are like parenchyma cells except they have thicker primary walls. The thickness is uneven and usually involves the corners of the cell. Collenchyma cells often form bundles just beneath the epidermis and give flexible support to immature regions of a plant body. The familiar strands in celery stalks are composed mostly of collenchyma cells. Sclerenchyma cells have thick secondary cell walls impregnated with lignin, which makes plant cell walls tough and hard. If we compare a cell wall to reinforced concrete, cellulose fibrils would play the role of steel rods, and lignin would be analogous to the cement. Most sclerenchyma cells are nonliving; their primary function is to support the mature regions of a plant. The long fibers in plants, composed of strings of sclerenchyma, make them useful for a number of purposes. For example, cotton and flax fibers can be woven into cloth, and hemp fibers can make strong rope.
Vascular Tissue
The two types of transport tissue, more properly called vascular tissue, have different functions. Xylem transports water and minerals from the roots to the leaves; phloem transports sugar, in the form of sucrose, and other organic compounds, such as hormones, often from the leaves to the roots. Xylem contains two types of conducting cells: vessel elements and tracheids (Fig. 20.7a). Both types of conducting cells are hollow and nonliving, but the vessel elements are larger, have perforated end walls, and are arranged to form a continuous pipeline for water and mineral transport. The end walls and side walls of tracheids have pits, depressions that allow water to move from one tracheid to another. 
The conducting cells of phloem are sieve-tube members, arranged to form a continuous sieve tube (Fig. 20.7b). Sieve-tube members contain cytoplasm but no nuclei. The term sieve refers to a cluster of pores in the end walls, collectively known as a sieve plate. Each sieve-tube member has a companion cell, which does have a nucleus. The two are connected by numerous plasmodesmata, and the nucleus of the companion cell may control and maintain the life of both cells. The companion cells are also believed to be involved in the transport function of phloem.
Vascular tissue, consisting of both xylem and phloem, extends from the root through the stem to the leaves and vice versa (see Fig. 20.1). In the root, the vascular tissue is located in a central cylinder; in the stem, it is in vascular bundles; and in the leaves, it is found in leaf veins.
20.3 	 	Organization of Leaves
Figure 20.8 shows that leaves can be organized variously. In simple leaves, the blade is not divided as it is in compound leaves. Notice that it is possible to tell from the placement of the lateral bud whether you are looking at several individual leaves or one compound leaf.
Figure 20.9 shows a typical eudicot leaf of a temperate-zone plant in cross section. At the top and bottom are -layers of epidermal tissue that often bear protective hairs and/or glands that secrete irritating substances. These appendages discourage insects from eating leaves. The epidermis characteristically has an outer, waxy cuticle that helps keep the leaf from drying out. The cuticle prevents loss of water and also prevents gas exchange because it is not gas permeable.  
The interior of a leaf is composed of mesophyll, the tissue that carries on photosynthesis. Notice how leaf veins terminate at the mesophyll. This termination is important because the leaf veins transport water and minerals to a leaf and transport the product of photosynthesis, a sugar, away from the leaf. Mesophyll has two distinct regions: palisade mesophyll, containing elongated cells, and spongy mesophyll, containing irregular cells bounded by air spaces. The loosely packed arrangement of the cells in the spongy layer increases the amount of surface area for gas exchange and water loss.
The lower epidermis of eudicot leaves and both surfaces of monocot leaves contain stomata. Carbon dioxide for photosynthesis enters a leaf at the stomata, and the by-product of photosynthesis, oxygen, exits a leaf at the stomata. Also, water brought into a leaf by leaf veins evaporates from spongy mesophyll and exits at the stomata. 
20.4		Organization of Stems
During the primary growth of stems, the shoot apical meristem in the terminal bud produces new cells that elongate and then differentiate to become the tissues and organs of the shoot system, namely the leaves and the stem. The shoot apical meristem is protected by the immature leaves that envelop it. Also, note the lateral buds from which lateral (side) branches develop (Fig. 20.10). 
Nonwoody Stems
A stem that experiences only primary growth is nonwoody. Plants that have nonwoody stems, such as zinnias and daisies, are termed herbaceous plants. As with leaves, the outermost tissue of a herbaceous stem is the epidermis, which is covered by a waxy cuticle to prevent water loss. Herbaceous stems have distinctive vascular bundles, where xylem and phloem are found. In each bundle, xylem is typically found toward the inside of the stem, and phloem is found toward the outside.
In the herbaceous eudicot stem, the vascular bundles are arranged in a distinct ring that separates the cortex from the central pith, which stores water and the products of photosynthesis. The cortex is sometimes green and carries on photosynthesis. In the herbaceous monocot stem, the vascular bundles are scattered throughout the stem. The cortex is a narrow band of cells beneath the epidermis, and ground tissue occupies the rest of the stem. Figure 20.11 contrasts herbaceous eudicot and monocot stems.
The vascular tissue of a stem supports the growth of the shoot system. The sclerenchyma cells of vascular tissue and the strong walls of vessel members and tracheids help support the shoot system as it increases in length. Further, the vascular bundles bring water and minerals to the leaf veins, which distribute them to the mesophyll cells of leaves, the primary organs of photosynthesis in most plants. The vascular bundles also distribute the products of photosynthesis to the root system and to any immature leaves that are not yet photosynthesizing. 
Woody Stems
Plants with woody stems experience both primary and secondary growth. -Secondary growth, which occurs only in trees and shrubs, increases the girth of stems, branches, and roots. Secondary growth occurs because of a change in the location and activity of vascular cambium, which, as you may recall from Section 20.2, is a type of meristem tissue. In herbaceous plants, vascular cambium is present between the xylem and phloem of each vascular bundle. In woody plants, the vascular cambium forms a ring of meristem that divides parallel to the surface of the plant and produces new xylem and phloem each year. Eventually, a woody eudicot stem has three distinct areas: the bark, the wood, and the pith. Vascular cambium occurs between the bark and the wood (Fig. 20.12).
Bark
The bark of a tree contains cork, cork cambium, cortex, and phloem. It is very harmful to remove the bark of a tree, because without phloem, organic nutrients cannot be transported. Although secondary phloem is produced each year by vascular cambium, it does not build up in the same manner as xylem. 
Cork cambium is located beneath the -epidermis. When cork cambium first begins to divide, it produces tissue that disrupts the epidermis and replaces it with cork cells. Cork cells are impregnated with suberin, a waxy layer that makes them waterproof but also causes them to die. This is a protective strategy, because now the stem is less edible. In a woody stem, gas exchange is -impeded, except at lenticels, which are pockets of loosely arranged cork cells not impregnated with suberin.
Wood
Wood is secondary xylem that builds up year after year, thereby increasing the girth of trees. In trees that have a growing season, vascular cambium is dormant during the winter. In the spring, when moisture is plentiful and leaves require much water for growth, the secondary xylem contains wide vessels with thin walls. In this so-called spring wood, wide vessels transport sufficient -water to the growing leaves. Later in the season, moisture is scarce, and the wood at this time, called summer wood, has a lower proportion of vessels. Strength is required because the tree is growing larger, and summer wood contains -numerous thick-walled tracheids. At the end of the growing season, just before the cambium becomes dormant again, only heavy fibers with especially thick secondary walls may develop. When the trunk of a tree has spring wood followed by summer wood, the two together make up one year’s growth, or an annual ring (Fig. 20.12). You can tell the age of a tree by counting the annual rings. 
Wood is one of the most useful and versatile materials known. It is used for building structures and for making furniture, and in addition to its functionality, its beauty gives us pleasure. Much wood is also used for heating and cooking, as well as for producing paper, chemicals, and pharmaceuticals. The resins derived from wood also have commercial importance. 
20.5		Organization of Roots
Figure 20.13, a longitudinal section of a eudicot root, reveals zones where cells are in various stages of differentiation as primary growth occurs. The apical meristem of a root is in the zone of a cell division, protected by a root cap. When this apical meristem divides, it provides cells to the zone of elongation above. In the zone of elongation, the cells lengthen as they become specialized. The zone of maturation, which contains fully differentiated cells, is recognizable because many of its epidermal cells bear root hairs. 
Tissues of a Eudicot Root
Figure 20.13 also shows a cross section of a root at the region of maturation. The following specialized tissues are identifiable:
EpidermisThe epidermis, which forms the outer layer of the root, consists of only a single layer of largely thin-walled, rectangular cells. In the zone of maturation, many epidermal cells have root hairs. 
CortexLarge, thin-walled parenchyma cells make up the cortex of the root. The cells contain starch granules, and the cortex may function in food storage.
EndodermisThe endodermis is a single layer of rectangular cells that fit snugly together. Further, a layer of impermeable material on all but two sides forces water and minerals to pass through endodermal cells. In this way, the endodermis regulates the entrance of minerals into the vascular tissue of the root.
PericycleThe pericycle is the first layer of cells inside the endodermis. It has retained its capacity to divide and can start the development of branch, or lateral, roots (Fig. 20.14). 
Vascular tissueXylem appears star-shaped in eudicot roots because several arms of tissue radiate from a common center (see Fig. 20.13). The phloem is found in separate regions between the arms of the xylem.
Organization of Monocot Roots
Monocot roots have the same growth zones as eudicot roots, but they do not undergo secondary growth as many eudicot roots do. Also, the organization of their tissues is slightly different. The ground tissue of a monocot, called the pith, is centrally located and surrounded by a vascular ring composed of alternating xylem and phloem bundles. Monocot roots also have pericycle, endodermis, cortex, and epidermis.
Comparison with Stems
Both roots and stems have apical meristem regions that account for their primary growth. The branching of a stem occurs at a lateral bud, whereas the branching of a root occurs some distance from apical meristem and is initiated by the pericycle. Eudicot roots, like woody stems, experience secondary growth. Vascular cambium located between the arms of the xylem and the phloem produces secondary xylem and phloem. As secondary xylem builds up, a root grows in girth. 
20.6		Plant Nutrition
Plant nutrition is remarkable to us because plants require only inorganic nutrients, and from these they make all the organic compounds that compose their bodies. Of course, they require carbon, hydrogen, and oxygen, which they can acquire from carbon dioxide and water, but they also need elements that are absorbed as minerals. An element is termed an essential nutrient if a plant cannot live without it. The essential nutrients are divided into macronutrients and micronutrients, according to their relative concentrations in plant tissue. The following diagram and slogan show which are the macronutrients and which are the micronutrients:
All of these elements play vital roles in plant cells. For example, nitrogen and phosphorus are required to make nucleic acids, and nitrogen and sulfur are important parts of amino acids. Figure 20.15 shows how an insufficient supply of the macronutrient nitrogen can affect a plant. The micronutrients are often cofactors for enzymes in various metabolic pathways. 
It’s interesting to observe that humans make use of a plant’s ability to take minerals from the soil. For example, we depend on plants for supplies of calcium to build our bones and teeth, and for iron to help carry oxygen to our cells. Minerals such as copper and zinc, which we acquire by eating plants, are also cofactors for our own enzymes. 
Adaptations of Roots for Mineral Uptake
Minerals enter a plant at its root system, and two mutualistic relationships assist roots in fulfilling this function. The air all about us contains about 78% nitrogen (N2), but plants can’t make use of it. Most plants depend on bacteria in the soil to fix nitrogen—that is, the bacteria change atmospheric nitrogen (N2) to nitrate (NO32) or ammonium (NH41), both of which plants can take up and use. Some plants, such as legumes, have roots colonized by bacteria that are able to take up atmospheric nitrogen and reduce it to a form suitable for incorporation into organic compounds (Fig. 20.16a). The bacteria live in root nodules, and the plant supplies the bacteria with carbohydrates; the bacteria in turn furnish the plant with nitrogen compounds. 
Another mutualistic relationship involves fungi and almost all plant roots (Fig. 20.16b). This association is called a mycorrhizal association, literally fungal roots. The hyphae of the fungus increase the surface area available for water uptake and break down organic matter, releasing inorganic nutrients that the plant can use. In return, the root furnishes the fungus with sugars and amino acids. Plants are extremely dependent on mycorrhizal fungi. For example, orchid seeds, which are quite small and contain limited nutrients, do not germinate until a mycorrhizal fungus has invaded their cells. The visual appearance of plants that do not have adequate nitrogen convinces us how important these associations are (see Fig. 20.15). 
20.7		Transport of Nutrients
Water and Mineral Transport in Xylem
Water and minerals are taken up by root hairs at the same time, and they are both transported within xylem. The vessel elements, assisted by the tracheids, constitute a continuous system for water and mineral transport in a plant (see Fig. 20.7). In other words, a pipeline for water transport exists from the roots through the stem to the leaves. People have long wondered how plants, especially very tall trees, lift water to the leaves against gravity. Water enters root cells by osmosis, and sure enough, this does create a positive pressure called root pressure. But this pressure is not nearly enough to account for the movement of water and minerals all the way to the leaves. 
The mechanism by which water and minerals travel up the xylem is called the cohesion-tension model. To understand how it works, we have to turn our attention to the anatomy of leaves and the properties of water. A leaf, as you know, has small openings called stomata, and the stomata open to air spaces in the leaf. Dry air passing across the leaves causes water to evaporate from the surface of spongy mesophyll cells. Water vapor in the air spaces of a leaf exits a leaf at the stomata, a phenomenon called transpiration. The evaporation of water creates a tension that is sufficient to pull a water column up from the roots to the leaves (Fig. 20.17). Why? Because of the properties of water. Water molecules have hydrogen atoms bonded to one another, and they tend to stay together—if one moves up, they all move up. This tendency of water molecules to cling together is called the cohesion property of water. Also, water molecules tend to adhere to the sides of the xylem vessels. This adhesion property of water gives the water column extra strength and prevents it from slipping back.
The total amount of water a plant loses through transpiration over a long period of time is surprisingly large. At least 90% of the water taken up by roots is eventually lost at the leaves. A single corn plant loses somewhere -between 135 and 200 liters of water through transpiration -during a growing season. As transpiration occurs, the water column is pulled upward, first within the leaf, then from the stem, and finally from the roots.
Opening and Closing of Stomata
Stomata are able to open and close because of changes in turgor pressure within their guard cells (Fig. 20.18). When water enters the guard cells, turgor pressure is created; when water leaves the guard cells, turgor pressure is lost. For transpiration to occur, the stomata must stay open. Still, when a plant is under stress and about to wilt from lack of water, the stomata close. Now the plant is unable to take up carbon dioxide from the air, and photosynthesis ceases. 
In plants kept in the dark, stomata open and close on a 24-hour basis, just as if they were responding to the presence of sunlight in the daytime and the absence of sunlight at night. Such circadian rhythms (behaviors that occur every 24 hours) are areas of intense investigation at this time. 
Organic Nutrient Transport in Phloem
Phloem transports organic nutrients from stored supplies in roots, or from mature leaves that are photosynthesizing, to all other parts of a plant, including young leaves that have not yet reached their full photosynthetic -potential and flowers in the process of making seeds and fruits. Nutrients are transported to any part of a plant, including roots, when they lack a ready supply. 
Just as xylem is continuous throughout a plant, so is phloem. In phloem, the sieve-tube members align end to end, and plasmodesmata extend through sieve plates from one sieve-tube member to the other, allowing direct cytoplasmic exchange. Sieve tubes, therefore, form a continuous pathway for transport of organic nutrients, primarily the sugar sucrose. 
The mechanism by which nutrients are transported through phloem is called the pressure-flow model (Fig. 20.19). The process begins during the growing season when photosynthesizing leaves are making sugar, and are therefore a source of sugar. This sugar is actively transported into sieve tubes, and water follows passively by osmosis. The buildup of water within sieve tubes creates a positive pressure that starts a flow of phloem contents. The other parts of a plant, such as roots that are not photosynthesizing, are a sink for sugar, because they require a supply of sugar. Here, sugar is actively transported out of phloem, and water -follows passively by osmosis. In this way, phloem contents continue to flow from source (e.g., leaves) to a sink (e.g., root).
The experiment described in Figure 20.20 supports the pressure-flow model. Two bulbs within a beaker of distilled water are connected by a glass tube. The glass tube is analogous to phloem. The first bulb contains solute at a higher concentration than the second bulb. Each bulb is bounded by a differentially permeable membrane. Distilled water flows into the first bulb because it has the higher solute concentration. The entrance of water creates pressure, and causes water to flow toward the second bulb. This pressure not only drives water toward the second bulb, but it also provides enough force for water to move out through the membrane of the second bulb—even though the second bulb contains a higher concentration of solute than the distilled water. 
The beauty of the pressure-flow model is that it can account for the flow of sugar in either direction, depending on which is the source and which is the sink. For example, in the spring, recently formed leaves can be a sink, and roots that have held stored sugar over the winter can be a source. Thus, organic nutrients will flow from the roots to the leaves, instead of the other way around. Maple sugar is made from the sap of maple trees flowing upward in the spring.
The Chapter in Review
Summary
20.1		Plant Organs
A flowering plant has a root system and a shoot system. Both grow at their tips.
Leaves carry on photosynthesis.
Stems support leaves, conduct materials to and from roots and leaves, and help store plant products.
Roots anchor a plant, absorb water and minerals, and store the products of photosynthesis.
Monocot Versus Eudicot Plants
Flowering plants are divided into eudicots and monocots according to:
	•	Number of cotyledons in the seed.
	• 	Arrangement of vascular tissue in leaves, stems, and roots.
	• 	Number of flower parts:
20.2		Plant Tissues and Cells
Plants have three types of specialized tissue:
Epidermal tissue is composed of only epidermal cells.
Ground tissue contains:
•	Parenchyma cells, which are thin-walled and capable of photosynthesis when they contain chloroplasts.
•	Collenchyma cells, which have thicker walls for flexible support.
•	Sclerenchyma cells, which are hollow, nonliving support cells with secondary walls.
Vascular tissue consists of:
•	
Xylem, which contains vessels composed of vessel elements and tracheids. Xylem transports water and minerals.
•	
Phloem, which contains sieve tubes composed of sieve-tube members, each of which has a companion cell. Phloem transports organic nutrients.
20.3		Organization of Leaves
	•	A leaf has an upper epidermis, with stomata mostly in the lower epidermis. 
	•	Stomata allow water vapor and oxygen to escape and carbon dioxide to enter the leaf. 
	•	Mesophyll (palisade and spongy) forms the body of a leaf and carries on photosynthesis. 
20.4		Organization of Stems
	•	Primary growth of a stem is due to the activity of the shoot apical meristem, which is protected within a terminal bud.
	•	Nonwoody eudicots have epidermis, cortex tissue, vascular bundles in a ring, and an inner pith.
	•	Monocot stems have scattered vascular bundles.
Woody Stems
	•	Secondary growth of a woody stem is due to vascular cambium, which produces new xylem and phloem every year.
	•	Cork replaces epidermis in woody plants.
	•	A woody stem has bark (contains cork, cork cambium, cortex, and phloem). Wood contains annual rings of xylem.
20.5		Organization of Roots
A eudicot root tip has three zones:
	•	Zone of cell division (contains root apical meristem) protected by the root cap.
	•	Zone of elongation.
	•	Zone of maturation (has root hairs).
In a eudicot root:
	•	Xylem appears star-shaped.
	•	Phloem is in separate regions between the arms of the xylem.
20.6		Plant Nutrition
Plants need only inorganic nutrients to make all the organic compounds that make up their bodies. Some nutrients are essential, being either macronutrients or micronutrients.
Adaptations of Roots for Mineral Uptake
Some roots have nodules where bacteria fix nitrogen and produce forms that plants can use. Most roots have mycorrhizal fungi.
20.7		Transport of Nutrients
Water and Mineral Transport in Xylem
The cohesion-tension model of xylem transport states that transpiration (evaporation of water from spongy mesophyll at stomata) creates tension, which pulls water upward in xylem. This method works only because water molecules are cohesive and adhesive.
Organic Nutrient Transport in Phloem
The pressure-flow model of phloem transport states that sugar is actively transported into phloem at a source, and water follows by osmosis. The resulting increase in pressure creates a flow, which moves water and sugar to a sink.
Thinking Scientifically
	1.	Legumes must be able to distinguish between nitrogen-fixing bacteria and all other bacteria in the soil in order to develop nodules. How do you suppose they can “perceive” differences among the diverse species of bacteria in the soil and engulf only those that will be beneficial? 
	2.	Welwitschia is a genus of plant that lives in the Namib and Mossamedes deserts in Africa. Annual rainfall averages only 2.5 centimeters (1 inch) per year. Contrary to what you might expect, the leaves of Welwitschia plants contain a large number of stomata (22,000 per square centimeter). Can you suggest an adaptive explanation for the abundance of stomata on these desert plants? 
Testing Yourself
Choose the best answer for each question.
	1.	It is possible to distinguish between stems and roots by looking for
	a. petioles on stems.
	b. petioles on roots.
	c. nodes on stems.
	d. nodes on roots.
	2.	In contrast to monocots, eudicots
	a. have parallel veins.
	b. produce root xylem and phloem in a ring.
	c. use endosperm as the major source of nutrients for seedlings.
	d. have flower parts in multiples of threes.
	e. produce a ring of vascular bundles in the stem.
	3.	Leaves require stomata because
	a. epidermal cells are sealed by a cuticle.
	b. vascular tissue is not near the epidermis.
	c. water cannot move across plant cell walls.
	d. sclerenchyma cells contain lignin.
	4.	Flexible support in immature regions of a plant is provided by
	a. vascular tissue.
	b. collenchyma cells.
	c. sclerenchyma cells.
	d. parenchyma cells.
	e. epidermal tissue.
	5.	Label the parts of a plant in the following diagram.
For questions 6–10, identify the plant organ that contains each feature. Each answer may be used more than once. Each question may have more than one answer.
Key:
	a. nonwoody stem
	b. woody stem
	c. root (nonwoody)
	d. leaf
	6.	Mesophyll
	7.	Vascular bundles
	8.	Ground tissue
	9.	Vascular cambium
	10.	Cuticle
	11.	During secondary growth, a tree adds more xylem and phloem through the activity of the
	a. apical meristems.
	b. vascular cambium.
	c. intercalary meristems.
	d. cork cambium.
	12.	Root nodules are important because they
	a. encourage the growth of mycorrhizal fungi.
	b. represent areas of extensive branch root growth.
	c. contain nitrogen-fixing bacteria.
	d. provide extra oxygen to the plant root system.
	13.	In contrast to transpiration, the transport of organic nutrients in the phloem
	a. requires energy input from the plant.
	b. always flows in one direction.
	c. results from tension.
	d. does not require living cells.
	14.	Label the parts of a leaf in the following illustration.
	15.	Label the parts of the eudicot stem in the illustration following.
	16.	Label the parts of the root in the following illustration.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
Wood is a renewable resource used for everything from constructing homes to making the paper on which this page is printed. However, some sources of wood are, for all practical purposes, nonrenewable. The old-growth forests of the Pacific Northwest are threatened by logging and will probably never be restored. Recently, the Bush administration eased restrictions on the logging of old-growth forests by allowing forest managers to begin logging without completing an inventory of rare plants and animals in the area. Opponents of this new policy say that it will double the amount of federal land on which logging occurs and will result in the extinction of hundreds of species. Representatives of the timber industry, on the other hand, applaud the change because previously they were required to spend years surveying the land before they could log it. Do you agree with the new policy?
Understanding the Terms
annual ring345
apical meristem341
bark345
blade338
cohesion-tension model349
collenchyma341
cork cambium345
cork cell345
cortex347
cotyledon340
cuticle341
deciduous339
endodermis347
epidermal tissue341
epidermis341
eudicot340
ground tissue341
herbaceous344
internode339
lateral bud339
leaf338
leaf vein340
meristem341
mesophyll342
monocot340
mycorrhizal association348
node339
palisade mesophyll342
parenchyma341
perennial plant339
pericycle347
petiole338
phloem340
pith347
pressure-flow model350
root cap347
root hair339
root nodule348
root system338
sclerenchyma341
shoot system338
sieve-tube member342
spongy mesophyll342
stem339
stomata (sing., stoma)341
terminal bud338
tracheid342
transpiration349
vascular bundle340
vascular cambium341
vascular tissue341
vessel element342
wood345
xylem340
Match the terms to these definitions:
a.	_______________	A plant that dies back and then regrows during the next season. 
b.	_______________	Vascular tissue that conducts organic nutrients and contains sieve-tube members. 
c.	_______________	Structures in a leaf that allow water vapor and oxygen to escape and carbon dioxide to enter.
d.	_______________	Secondary xylem that builds up year after year and becomes annual rings.
e.	_______________	Undifferentiated embryonic tissue in the active growth regions of plants.
Carnivorous plants have evolved an unusual way to get the nutrients they need.
The world’s tallest living tree, a coastal redwood, is taller than the Statue of Liberty.
Fertilizer contains the minerals plants need to grow. 
Figure 20.1The body of a plant.
A plant has a root system, which extends below ground, and a shoot system, which is composed of the stem and leaves. Activity at the terminal bud and the root tip allows a plant to increase in length, a process called primary growth. 
Figure 20.3Root diversity.
a. A taproot has one main root. b. A fibrous root has many slender roots with no main root. c. Prop roots are organized for support.
Figure 20.2
Leaf diversity.
a. The spines of a cactus protect the fleshy stem from herbivores. b. The tendrils of a cucumber are leaves modified to attach the plant to a support. c. Venus’s flytrap leaves serve as the trap for unsuspecting insects. After digesting this fly, the plant will absorb the nutrients released.
Figure 20.4Flowering plants are either monocots or eudicots.
Five features are typically used to distinguish monocots from eudicots: number of cotyledons; arrangement of vascular tissue in roots, stems, and leaves; and number of flower parts.
Check Your Progress
	1.	List the three vegetative organs in a plant.
	2.	What is the function of leaves, and what are some possible leaf specializations?
	3.	
List three functions of roots.
	4.	List the major differences between monocots and eudicots.
Answers:1. Vegetative organs are the leaves, stem, and root.2. Sites of photosynthesis; some leaves also store water, attach to objects, and capture insects.3. Anchorage, absorb water and minerals, and store products of photosynthesis.4. Monocots: embryo with single cotyledon; xylem and phloem in a ring in the root; scattered vascular bundles in the stem; parallel leaf veins; flower parts in multiples of three. Eudicots: embryo with two cotyledons; phloem located between arms of xylem in the root; vascular bundles in a ring in the stem; netted leaf veins; flower parts in multiples of fours or fives.
Figure 20.6Ground tissue cells.
a. Parenchyma cells are the least specialized of the plant cells. b. Collenchyma cells are much thicker and more irregular than parenchyma cells. 
c. Sclerenchyma cells have very thick walls and are nonliving—their only function is to give strong support.
Figure 20.5Modifications of epidermal tissue.
a. Root epidermis has root hairs, which help to absorb water. b. Leaf epidermis contains stomata (sing., stoma) for gas exchange. c. Cork, which is a part of bark, replaces epidermis in older, woody stems.
Figure 20.9Leaf structure.
Photosynthesis takes place in mesophyll tissue of leaves. The leaf is enclosed by epidermal cells covered with a waxy layer, the cuticle. The leaf veins contain xylem and phloem for the transport of water and solutes. A stoma is an opening in the epidermis that permits the exchange of gases.
Figure 20.8Simple versus compound leaves. 
a. The cottonwood tree has a simple leaf. b. The shagbark hickory has a compound leaf. c. The honey locust has a twice compound leaf. A lateral bud appears where a leaf attaches to the stem.
Figure 20.7Vascular tissue. 
a. Photomicrograph of xylem vascular tissue, with (left) drawing of a vessel (composed of vessel elements) and (right) drawing of tracheids. b. Photomicrograph of phloem vascular tissue, with drawing of a sieve tube. Each sieve-tube member has a companion cell.
Check Your Progress
	1.	List the three types of tissue in a plant and the cells that make up these tissues.
	2.	Compare and contrast xylem with phloem.
	3.	What is the importance of the leaf tissue called mesophyll to a plant?
Answers:1. Epidermal tissue: epidermal cells; ground tissue: parenchyma, collenchyma, and sclerenchyma cells; vascular tissue: xylem (vessel elements and tracheids) and phloem (sieve-tube members).2. Xylem transports water and minerals from roots to leaves. Phloem transports organic compounds throughout the plant.3. Photosynthesis, which produces organic food for a plant, occurs in the mesophyll.
Figure 20.12Organization of a woody stem.
In a woody stem, vascular cambium produces new secondary phloem and secondary xylem each year. Secondary xylem builds up to form wood consisting of annual rings. Counting the annual rings tells you that this woody stem is three years old.
Figure 20.11Nonwoody (herbaceous) stems. 
a. In eudicot stems, the vascular bundles are arranged in a ring around well-defined ground tissue called pith. b. In monocot stems, the vascular bundles are scattered within the ground tissue. 
Figure 20.10Shoot tip.
The terminal bud contains the apical meristem that produces leaves and also cells that become the tissues of stems.
Check Your Progress
	1.	What transport tissues are in a vascular bundle?
	2.	
How are vascular bundles arranged in monocot stems? 
In eudicot stems?
	3.	Contrast primary growth with secondary growth.
	4.	
List the components of bark.
	5.	
Relate spring wood and summer wood to annual rings.
Answers:1. A vascular bundle contains xylem and phloem.2. Vascular bundles are scattered in monocot stems and form a ring in eudicot stems.
3. Primary growth is growth in length and is nonwoody; secondary growth is growth in girth and is woody.4. Bark is composed of cork, cork cambium, cortex, and phloem.5. An annual ring is composed of one year’s growth of wood—one layer of spring wood followed by one layer of summer wood.
Figure 20.14Branching of eudicot root.
These cross sections of a willow show the origination (a) and growth (b) of a branch root from the pericycle.
Figure 20.13Eudicot root tip.
The root tip is divided into three zones, best seen in a longitudinal section such as this. In the eudicot, xylem is typically star-shaped, and phloem lies between the points of the star. Water and minerals must eventually pass through the cytoplasm of endodermal cells in order to enter xylem; endodermal cells regulate the passage of minerals into xylem. 
Check Your Progress
	1.	Describe the relationship between the root apical meristem and the root cap.
	2.	
List the functions of the cortex, the endodermis, and the pericycle in a root.
	3.	
Compare and contrast roots with stems.
Answers:1. The root apical meristem is located at the tip of the root and is covered by the root cap.2. Cortex: food storage; endodermis: control of mineral uptake; pericycle: formation of branch roots.3. Both roots and stems have apical meristems and are capable of branching. Stems branch at lateral buds, while root branches originate in the pericycle. 
Figure 20.15Nitrogen deficiency.
This experiment shows that sunflower plants respond poorly if their growth medium lacks nitrogen.
Figure 20.16Adaptations of roots for mineral uptake.
a. Nitrogen-fixing bacteria live in nodules on the roots of plants, particularly legumes. b. Rough lemon plants with mycorrhizal fungi (right) grow much better than plants without mycorrhizal fungi (left).
Figure 20.17Cohesion-tension model of xylem transport.
How does water rise to the top of tall trees? Xylem vessels are water-filled pipelines from the roots to the leaves. When water evaporates from spongy mesophyll into the air spaces of leaves, this water column is pulled upward due to the cohesion of water molecules with one another and the adhesion of water molecules to the sides of the vessels.
Check Your Progress
	1.	Explain the significance of nitrogen-fixing bacteria in the soil to plants. 
	2.	Explain how both partners benefit from a mycorrhizal association.
Answers:1. The bacteria convert atmospheric nitrogen to nitrate or ammonium, which can be taken up by plant roots.2. The fungus obtains sugars and amino acids from the plant. The plant obtains inorganic nutrients and water from the fungus. 

Figure 20.19Pressure-flow model of phloem transport.
Sugar and water enter sieve-tube members at a source. This creates a positive pressure, which causes phloem contents to flow. Sieve-tube members form a continuous pipeline from a source to a sink, where sugar and water exit sieve-tube members.
Figure 20.18Opening and closing of stomata.
Stomata open when water enters guard cells and turgor pressure increases. Stomata close when water exits guard cells and turgor pressure is lost.

Figure 20.20Experiment supporting the pressure-flow model. 
When a concentrated sugar solution is placed in the first bulb, water enters and causes a flow of both water and sugar to the second bulb. The pressure is sufficient to cause water to exit the second bulb, even though the surrounding water contains no sugar. As water enters the first bulb, water level decreases (on left); as water exits the second bulb, water level increases (on right).
Check Your Progress
	1.	Explain why water is under tension in stems.
	2.	
Explain the significance of the cohesion and adhesion properties of water to water transport.
	3.	Explain how sugars move from source to sink in a plant.
Answers:1. Water is pulled upward in stems via evaporation from leaf surfaces, so it is under tension.2. When water molecules are pulled upward during transpiration, their cohesiveness allows them to cling together, creating a continuous column. Adhesion allows them to cling to the sides of xylem vessels, so the column of water does not slip down.3. Sugars build up in sieve tubes at sources, creating pressure as water flows in as well. The pressure is relieved at the other end when sugars and water are removed at the sink.
a. Taproot
b. Fibrous root system
c. Prop roots