1.Introduction to Plant Body

The object in Figure 35.1 is not the creation of a computer genius with a flairfor the artistic. It is a head of romanesco, an edible relative of broccoli. Roma-

nesco’s mesmerizing beauty is attributable to the fact that each of its smaller buds resembles in miniature the entire vegetable (shown below). (Mathematicians refer to such repetitive patterns as fractals.) If romanesco looks as if it were generated by a computer, it’s because its growth pattern follows a repetitive sequence of instructions. As in most plants, the growing shoot tips lay down a pattern of stem . . . leaf . . . bud, over and over again. These repetitive developmental patterns are genetically determined and subject to natural selection. For example, a mutation that shortens the stem segments between leaves will generate a bushier plant. If this altered architecture enhances the plant’s ability to access resources such as light and, by doing so, to produce more offspring, then this trait will occur more frequently in later generations—the population will have evolved.

Romanesco is unusual in adhering so rigidly to its basic body organization. Most plants show much greater diversity in their individual forms because the growth of most plants, much more than in animals, is affected by local environmental conditions. All adult lions, for example, have four legs and are of roughly the same size, but oak trees vary in the number and arrangement of their branches. This is

▲ Figure 35.1 Computer art?

because plants respond to challenges and opportunities in their local environment by altering their growth. (In contrast, animals typically respond by movement.) Illumination of a plant from the side, for example, creates asymmetries in its basic body plan. Branches grow more quickly from the illumi-nated side of a shoot than from the shaded side, an architec-tural change of obvious benefit for photosynthesis. The highly adaptive development of plants is critical in facilitating their acquisition of resources from their local environments.

Are Plants Computers?

All plants are able to harvest diffuse resources and concentrate them in cells and tissues, but their forms and strategies are diverse. These baobab trees in Madagascar may live to be hundreds of years old despite drought conditions, in part by storing water in their enormous trunks.

Photosynthetic plants carry out the most remarkable biochemistry of any terrestrial organisms. Using the energy in sunlight and the simplest of starting materials—carbon dioxide, water, and ions containing nitrogen, phosphorus, potassium, and other key atoms—plants synthe-

size thousands of different carbohydrates, proteins, nucleic acids, and lipids. They use these com-pounds to build bodies that may live for thousands of years.

This feat is even more impressive when you consider that the simple starting materials that plants need to grow are tiny and diffuse—carbon dioxide molecules, water molecules, nitrate ions, and other resources are usually found at low concentrations over a large area. To gather the raw materi-als required for their sophisticated biosynthetic machinery, a plant’s roots and shoots grow outward, extending the individual into the soil and atmosphere.

In essence, a plant’s body harvests diffuse resources and con-centrates them in cells and tissues. The structure of its body is dy-namic, because most plants exhibit indeterminate growth; that is, they grow throughout their lives. A 4750-year-old bristlecone pine has roots and shoots that are still growing. In response to favorable conditions, a plant sends shoots and roots in the most promising directions, seeking light and the simple compounds it requires.

The contrast between the plant and animal way of life is strik-ing. Most animals move around, eat concentrated sources of food, and avoid stressful conditions. But plants stay in one place, extend their roots and shoots to harvest diffuse resources, make their own food, and cope with stress where they stand.

What Is the Basic Body Plan of Plants?

Plants live by harvesting energy from sunlight and by col-

lecting water and mineral nutrients from the atmosphere

and the soil. Because these resources are sometimes limited,

plants must collect them from large areas, both above and

below ground. The plant is further challenged by its inabil-

ity to move; a plant cannot, for example, relocate from a

dry, shady location to one that is wet and sunny.

The plant body plan allows plants to respond to these

challenges:

• Stems, leaves, and roots enable a plant anchored to one

spot to capture scarce resources effectively, both above

and below the ground.

• Plants can grow throughout their lifetimes, enabling

them to respond to environmental cues. A plant can

redirect its growth to exploit opportunities in its imme-

diate environment; for example, it can extend its roots

toward a water supply.

Plant organs are organized into two systems :

• The root system anchors the plant in place, absorbs water

and dissolved minerals, and stores the products of photo-

synthesis from the shoot system. The extreme branching

of plant roots and their high surface area-to-volume ratios

allow them to absorb water and mineral nutrients from the

soil efficiently.

• The shoot system of a plant consists of the stems, leaves,

and flowers. Broadly speaking, the leaves are the chief

organs of photosynthesis. The stems hold and display the

leaves to the sun and provide connections for the transport

of materials between roots and leaves.

Shoots and roots are composed of repeating modules called

phytomers. Each phytomer in the shoot consists of a node car-

rying one or more leaves; an internode, which is the interval of

stem between two nodes; and one or more axillary buds, each

of which forms in the angle (axil) where a leaf meets the stem.

A bud is an undeveloped shoot that can develop further to pro-

duce another leaf, a phytomer, a flower, or a flowering stem.

The axillary buds (also called lateral buds) are distinguished

from the bud at the end of a stem or branch, which is called a

terminal bud. If it becomes active, an axillary bud can develop

into a new branch, or an extension of the shoot system. The ar-

rangement of leaves along the stem (called the phyllotaxy) is

characteristic of the plant species.

Plant roots also have a modular construction. In the roots,

each phytomer consists of a root segment between two

branches.

Most angiosperms are either monocots or eudicotsAs we saw in Section 29.3, most angiosperms belong to one of

two major clades. Monocots are generally narrow-leaved flow-

ering plants such as grasses, lilies, orchids, and palms. Eudi-cots are broad-leaved flowering plants such as soybeans, roses,

sunflowers, and maples. These two clades, which account for

97 percent of flowering plant species, differ in several basic

characteristics:

• Monocots have one cotyledon (leaf in the embryo),

whereas eudicots have two.

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• In eudicots, the vascular bundles in the stem are arranged

in concentric circles; in monocots they are scattered.

• In monocots, the major leaf veins are usually parallel; in

eudicots they are reticulate, meaning they form a network.

• Eudicots usually have taproot systems; monocots have fi-

brous root systems.

• Monocot flowers have parts (petals and sepals) that occur

in threes; eudicots have floral parts that occur in fours or

fives.

• Monocot pollen grains each have one furrow or pore; eudi-

cot pollen grains have three.

Vascular plants, with few exceptions, rely on both systems for survival. Roots are almost never photosynthetic; they starve unless photo-synthates, the sugars and the other carbohydrates produced during photosynthesis, are imported from the shoot system. Conversely, the shoot system depends on the water and minerals that roots absorb from the soil.min

The iterative (repeating) unit of the vegetative shoot consists of the internode, node, leaf, and axillary bud, but not reproductive structures. An axillary bud is a lateral shoot apex that allows the plant to branch or replace the main shoot if it is eaten by an herbivore. A vegetative axillary bud has the capacity to reiterate the development of the primary shoot. When the plant has shifted to the reproductive phase of development, these axillary buds may produce flowers or floral shoots.

The shoot system

consists of stems

and leaves, in

which photosynthesis

takes place.

The root system

anchors the plant

and provides water

and nutrients for the

shoot system.

Eudicot

Terminal bud

Axillary bud

Leaf

Blade

Petiole

Node

Internode

Node

Internode

Phytomer

Branch

Stem

Roots

34.1 Vegetative Organs and Systems The basic plant body

plan, with root and shoot systems, and the principal vegetative

organs are similar in eudicots and monocots, although there are

also some differences between the two clades.Monocot

Shoot apex

Flower

Stipule

Axillary bud

Internode

Node

Vascular system

Primary root

Lateral root

Root apex

Root

Shoot

Petiole

Vein

Blade

Leaflet Leaf

Tendril

Figure 36.1 Diagram of a plant body. Branching root and shoot systems create the plant’s architecture. Each root and shoot has an apex that extends growth. Leaves are initiated at the nodes of the shoot, which also contain axillary buds that can remain dormant, grow to form lateral branches, or make fl owers. A leaf can be a simple blade or consist of multiple parts as shown here. Roots, shoots, and leaves are all connected with vascular (conducting) tissue.

Roots and shoots are composed of three types of tissuesRoots, shoots, and leaves all contain three basic types of tissues: dermal, ground, and vascular tissue. Because each of these tis-sues extend through the root and shoot systems, they are called tissue systems.

Plant cells contribute to three tissue systems. Dermal tissue, primarily epidermis, is one cell layer thick in most plants, and it forms an outer protective covering for the plant. Ground tissue cells function in storage, photosynthesis, and secretion, in addition to forming fibers that support and protect plants. Vascular tissue conducts fluids and dissolved substances throughout the plant body. Each of these tissues and their many functions are described in more detail in later sections.

key point

The tissue systems are continuous throughout the plant. For example, the vascular tissue system in a leaf is continuous with the vascular tissue system in the stem to which it is attached.

Figure 33-2 Animation The three tissue systems in the plant bodyThis figure shows the distribution of the ground tissue system, vascular tissue system, and dermal tissue system in a herbaceous eudicot such as Arabidopsis.© Cengage Learning

Dermal tissue system

Vascular tissue system

Ground tissue system

Dermal tissue system

Vascular tissue system

Ground tissue system

Dermal tissue system

Vascular tissue system

Ground tissue system

(a) Leaf

(b) Stem

(c) Root

predict What do you think would happen if the vascular tissue system were to become discontinuous in the stem but the dermal and ground tis-sue systems were to remain intact?

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Meristem cell

Meristem cell

Differentiated cell

Differentiated cell

Cell division

Meristem cell

Cell division

Meristem cell Differentiated cell

Cell division

Figure 36.3 Meristem cell division. Plant meristems consist of cells that divide to give rise to a differentiating daughter cell and a cell that persists as a meristem cell.

Meristems elaborate the body plan throughout the plant’s life

When a seed sprouts, only a tiny portion of the adult plant ex-ists. Although embryo cells can undergo division and differen-tiation to form many cell types, the fate of most adult cells is more restricted. Further development of the plant body de-pends on the activities of meristems, specialized cells found in shoot and root apices, as well as other parts of the plant.

Overview of meristemsMeristems are clumps of small cells with dense cytoplasm and proportionately large nuclei that act as stem cells do in ani-mals. That is, one cell divides to give rise to two cells, of which one remains meristematic, while the other undergoes differen-tiation and contributes to the plant body (figure 36.3). In this way, the population of meristem cells is continually renewed. Molecular genetic evidence supports the hypothesis that ani-mal stem cells and plant meristem cells may also share some common pathways of gene expression. Extension of both root and shoot takes place as a result of repeated cell divisions and subsequent elongation of the cells produced by the apical meristems. In some vascular plants, including shrubs and most trees, lateral meristems produce an increase in root and shoot diameter.

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Apical meristemsApical meristems are located at the tips of stems and roots (figure 36.4) . During periods of growth, the cells of apical mer-istems divide and continually add more cells at the tips. Tissues derived from apical meristems are called primary tissues, and the extension of the root and stem forms what is known as the primary plant body. The primary plant body comprises the young, soft shoots and roots of a tree or shrub, or the entire plant body in some plants.

Both root and shoot apical meristems are composed of delicate cells that need protection (see figure 36.4). The root apical meristem is protected by the root cap, the anatomy of which is described later on. Root cap cells are produced by the root meristem and are sloughed off and replaced as the root

moves through the soil. In contrast, leaf primordia shelter the growing shoot apical meristem, which is particularly suscepti-ble to desiccation because of its exposure to air and sun.

The apical meristem gives rise to the three tissue systems by first initiating primary meristems. The three primary mer-istems are the protoderm, which forms the epidermis; the procambium, which produces primary vascular tissues (pri-mary xylem for water transport and primary phloem for nutri-ent transport); and the ground meristem, which differentiates further into ground tissue. In some plants, such as horsetails and corn, intercalary meristems arise in stem internodes (spaces between leaf attachments), adding to the internode lengths. If you walk through a cornfield on a quiet summer night when the corn is about knee high, you may hear a soft popping sound. This sound is caused by the rapid growth of the intercalary meristems. The amount of stem elongation that oc-curs in a very short time is quite surprising.

Lateral meristemsMany herbaceous plants (that is, plants with fleshy, not woody stems) exhibit only primary growth, but others also exhibit secondary growth, which may result in a substantial increase of diameter. Secondary growth is accomplished by the lateral meristems—peripheral cylinders of meristematic tissue withinthe stems and roots that increase the girth (diameter) of gym-nosperms and most angiosperms. Lateral meristems form from ground tissue that is derived from apical meristems. Monocots are the major exception (figure 36.5) .

Although secondary growth increases girth in many nonwoody plants, its effects are most dramatic in woody

plants, which have two lateral meristems. Within the bark of a woody stem is the cork cambium—a lat-eral meristem that contributes to the outer bark of the tree. Just beneath the bark is the vascular cambium—a lateral meristem that produces secondary vascular tissue. The vascular cambium

forms between the xylem and phloem in vascular bundles, adding secondary vascular tissue to both

of its sides.

Secondary xylem is the main component of wood. Sec-ondary phloem is very close to the outer surface of a woody stem. Removing the bark of a tree damages the phloem and may eventually kill the tree. Tissues formed from lateral mer-istems, which comprise most of the trunk, branches, and older roots of trees and shrubs, are known as secondary tissues and are collectively called the secondary plant body.

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Young leaf primordium

Shoot apical meristem

Older leaf primordium

Lateral bud primordium

dermal tissue ground tissue vascular tissue

Root apical meristem

Root cap

400 μm

100 μm

Figure 36.4 Apical meristems. Shoot and root apical meristems extend the plant body above and below ground. Leaf primordia protect the fragile shoot meristem, while the root meristem produces a protective root cap in addition to new root tissue.

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Learning Outcomes Review 36.1The root system anchors plants and absorbs water and nutrients, whereas the shoot system, consisting of stems, leaves, and fl owers carries out photosynthesis and sexual reproduction. The three general types of tissue in both roots and shoots are dermal, ground, and vascular tissue. Primary growth is produced by apical meristems at the tips of roots and shoots; secondary growth is produced by lateral meristems that are peripheral and increase girth.

■ Why are both primary and secondary growth necessaryin a woody plant?

The Importance of Surface Area/Volume

Relationships

Before exploring the nature of root and shoot systems in more detail, it’s important to recognize a key structural relationship that is critical to their function.

Thick structure (64 cells)

Tubelike structure

(64 cells)Flattened structure

(64 cells)

Surface area = 240,000 μm2

(96 cell surfaces x 2500 μm2/cell surface)

Volume = 8,000,000 μm3

(64 cells x 125,000 μm3/cell)

Surface area/volume = 0.0300/μm Surface area/volume = 0.0425/μm Surface area/volume = 0.0525/μm

Surface area = 340,000 μm2

(136 cell surfaces x 2500 μm2/cell surface)

Volume = 8,000,000 μm3

(64 cells x 125,000 μm3/cell)

Surface area = 420,000 μm2

(168 cell surfaces x 2500 μm2/cell surface)

Volume = 8,000,000 μm3

(64 cells x 125,000 μm3/cell)

50 μm

FIGURE 37.2 The Morphology of Roots and Leaves Gives Them a High Surface-Area-to-Volume Ratio. Inthis example, the “thick structure” represents a tree trunk or potato-like storage organ; the “tubelike structure” represents a root; the “flattened structure” represents a leaf. Note that each schematic structure has the same number of cells and the same total volume—but a very different surface area.

Root and shoot systems both function in absorption—of wa-ter and key ions, or of light, respectively. Absorption takes place across a surface. But the cells that use the absorbed molecules and light occupy a volume. Thus, a plant body is more efficient as an absorption-and-synthesis machine when it has a large surface area relative to its volume.

FIGURE 37.2 illustrates this point. In this example, the cells in a plant are represented by cubes; the side of each cell is 50 μm long. Thus, each face of a cell has a surface area of 50 * 50 = 2500 μm2;each cell has a volume of 50 * 50 * 50 = 125,000 μm3. Follow the calculations in the figure and note that:⦁ If 64 cells are arranged in a cube, the surface area/volume rela-

tionship is 0.0300/μm.⦁ If 64 cells are arranged in a long tube, the surface area/volume

relationship is 0.0425/μm.⦁ If 64 cells are arranged in a flat sheet, the surface area/volume

relationship is 0.0525/μm.

This simple exercise has an important punch line: Tubes and sheets have much more surface area relative to their volume than cubes. It’s no surprise, then, that the absorptive regions of a root system are tubelike, and the absorptive regions of a shoot sys-tem are the flattened structures called leaves. Storage tissues such as tubers and seeds have a low surface-area-to-volume ratio be-cause they are not involved in absorption.