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35
Roots and Mineral Nutrition
Storage roots. Carrots (Daucus
carota) are biennials (plants that
live for 2 years). During the first
year’s growth, food is stored in
the fleshy root system; during
the second year, the shoot
elongates and produces flowers.
Carrots and other root crops
are important sources of human
food.
B
ranching underground root systems are often more extensive
than a plant’s aerial parts. The roots of a corn plant, for exam-
ple, may grow to a depth of 2.5 m (about 8 ft) and spread outward
1.2 m (4 ft) from the stem. Desert-dwelling tamarisk (Tamarix) trees
reportedly have roots that grow to a depth of 50 m (163 ft) to tap
underground water. The extent of a plant’s root depth and spread
R. Calentine/ Visuals Unlimited
varies considerably among different species and even among different individuals in the same species.
Because roots are usually underground and out of sight, people do not always appreciate the important functions they perform.
First, as anyone who has ever pulled weeds can attest, roots anchor a
plant securely in the soil. A plant needs a solid foundation from which
to grow. Firm anchorage is essential to a plant’s survival so that the
stem remains upright, enabling leaves to absorb sunlight effectively.
K E Y C ON CEP TS
Second, roots absorb water and dissolved minerals (inorganic nu-
Roots anchor the plant in the soil, absorb water and minerals, conduct these materials to the rest of the plant body,
and store carbon compounds and water.
trients) such as nitrates, phosphates, and sulfates, which are neces-
Primary tissues (epidermis, cortex, endodermis, pericycle,
xylem, and phloem) of roots develop from root apical meristems. Secondary tissues (wood and bark) of woody roots
develop from lateral meristems.
The endodermis absorbs mineral ions from the soil solution
by an active, energy-requiring process.
Many roots form associations with soil bacteria and fungi.
Soil, the layer of Earth’s crust that has been modified by
contact with weather, wind, water, and organisms, contains
most of the essential elements required by plants.
sary for synthesizing important organic molecules. These dissolved
minerals are then transported throughout the plant in the xylem.
Many roots perform the function of storage. Carrots (see photograph), sweet potatoes, cassava, and other root crops are important sources of human food. Surplus sugars produced in the
leaves by photosynthesis are transported in the phloem to the
roots for food storage (usually as starch or sucrose) until needed.
Other plants, particularly those living in arid regions, possess storage roots adapted to store water.
In certain species, roots are modified for functions other than
anchorage, absorption, conduction, and storage. Roots specialized to perform uncommon functions are discussed later in the
chapter.
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■
ROOT STRUCTURE AND FUNCTION
Learning Objectives
1
2
3
4
5
Distinguish between taproot and fibrous root systems.
Label cross sections of a primary eudicot root and a monocot
root, and describe the functions of each tissue.
Trace the pathway of water and mineral ions from the soil
through the various root tissues, and distinguish between the
symplast and apoplast.
Discuss the structure of roots with secondary growth.
Describe at least three roots that are modified to perform
uncommon functions.
Two types of root systems, a taproot system and a fibrous root
system, occur in plants (❚ Fig. 35-1). A taproot system consists of
one main root that formed from the seedling’s enlarging radicle,
or embryonic root. Many lateral roots of various sizes branch out
of a taproot. Taproots are characteristic of many eudicots and
gymnosperms. A dandelion is a good example of a common herbaceous plant with a taproot system. A few trees, such as hickory,
retain their taproots, which become quite massive as the plants
age. Most trees, however, have taproots when young and later develop large, shallow, lateral roots from which other roots branch
off and grow downward.
A fibrous root system has several to many roots of similar size
developing from the end of the stem, with lateral roots branching off these roots. Fibrous root systems form in plants that have
a short-lived embryonic root. The roots originate first from the
base of the embryonic root and later from stem tissue. The main
(b)
roots of a fibrous root system do not arise from pre-existing roots
but from the stem; such roots are called adventitious roots. Adventitious organs occur in an unusual location, such as roots that
develop on a stem, or buds that develop on roots. Onions, crabgrass, and other monocots have fibrous root systems.
Taproot and fibrous root systems are adapted to obtain water in different sections of the soil. Taproot systems often extend
down into the soil to obtain water located deep underground,
whereas fibrous root systems, which are located relatively close
to the soil surface, are adapted to obtain rainwater from a larger
area as it drains into the soil.
Roots have root caps and root hairs
Because of the need to adapt to the soil environment instead of
the atmospheric environment, roots have several structures, such
as root caps and root hairs, that shoots lack. Although stems and
leaves have various types of hairs, they are distinct from root hairs
in structure and function.
Each root tip is covered by a root cap, a protective, thimblelike layer many cells thick that covers the delicate root apical
meristem (❚ Fig. 35-2a; also see Fig. 32-7). As the root grows,
pushing its way through the soil, cells of the root cap are sloughed
off by the frictional resistance of the soil particles and replaced by
new cells formed by the root apical meristem. The root cap cells
secrete lubricating polysaccharides that reduce friction as the root
passes through the soil. The root cap also appears to be involved
in orienting the root so that it grows downward (see discussion
of gravitropism in Chapter 37). When a root cap is removed, the
root apical meristem grows a new cap. However, until the root
cap has regenerated, the root grows randomly rather than in the
direction of gravity.
Root hairs are short-lived tubular extensions of epidermal
cells located just behind the growing root tip. Root hairs continually form in the area of cell maturation closest to the root tip
to replace those that are dying off at the more mature end of the
root-hair zone (❚ Fig. 35-2b; also see Fig. 32-7). Each root hair is
short (typically less than 1 cm, or 0.4 in, in length), but they are
quite numerous. Root hairs greatly increase the absorptive capacity of roots by increasing their surface area in contact with moist
soil. Soil particles are coated with a microscopically thin layer of
water in which minerals are dissolved. The root hairs establish an
intimate contact with soil particles, which allows efficient absorption of water and minerals.
Unlike stems, roots lack nodes and internodes and do not
usually produce leaves or buds. Although herbaceous roots have
certain primary tissues (such as epidermis, xylem, phloem, cortex, and pith) found in herbaceous stems, these tissues are arranged quite differently.
(a)
Figure 35-1
Animated
Root systems
(a) A taproot system develops from the embryonic root in the seed.
(b) The roots of a fibrous root system are adventitious and develop
from stem tissue.
The arrangement of vascular tissues
distinguishes the roots of herbaceous
eudicots and monocots
Although considerable variation exists in herbaceous eudicot and
monocot roots, they all have an outer protective covering (epidermis), a cortex for storage of starch and other organic molRoots and Mineral Nutrition
749
Runk /Rannels/Grant Heilman Photography
Root apical
meristem
(area of
cell division)
Root cap
Root hairs
Soil air
Dennis Drenner
Soil water
Soil particles
250 µm
(a) Root cap. LM of an oak (Quercus
sp.) root tip showing its root cap. The
root apical meristem is protected by
the root cap.
Figure 35-2
Animated
Epidermis
(b) Root hairs. Each delicate hair is an extension of a single cell of the root
epidermis. Root hairs increase the surface area of the root in contact with the
soil. This radish (Raphanus sativus) seedling is approximately 5 cm (2 in) long.
Structures unique to roots
ecules, and vascular tissues for conduction. Let us first consider
the structure of herbaceous eudicot roots.
In most herbaceous eudicot roots,
the central core of vascular tissue lacks pith
The buttercup root is a representative eudicot root with primary
growth (❚ Fig. 35-3). Like other parts of this herbaceous eudicot,
a single layer of protective tissue, the epidermis, covers its roots.
The root hairs are a modification of the root epidermis that enables it to absorb more water from the soil. The root epidermis
does not secrete a thick, waxy cuticle in the region of root hairs,
because this layer would impede the absorption of water from
the soil. Both the lack of a cuticle and the presence of root hairs
increase absorption.
Beginning with the root hairs, most of the water that enters
the root moves along the cell walls rather than enters the cells.
One of the major components of cell walls is cellulose, which
absorbs water as a sponge does. An example of the absorptive
properties of cellulose is found in cotton balls, which are almost
pure cellulose.
The cortex, which is composed primarily of loosely packed
parenchyma cells, composes the bulk of a herbaceous eudicot
root. Roots usually lack supporting collenchyma cells, probably
because the soil supports the root, although roots as they age
may develop some sclerenchyma (another supporting tissue; see
Chapter 32). The primary function of the root cortex is storage.
A microscopic examination of the parenchyma cells that form the
cortex often reveals numerous amyloplasts (see Figs. 35-3b and
3-9a), which store starch. Starch, an insoluble carbohydrate composed of glucose subunits, is the most common form of stored
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Chapter 35
energy in plants. When used at a later time, these reserves provide
energy for such activities as growth and cell replacement following an injury.
The large intercellular (between-cell) spaces, a common feature of the root cortex, provide a pathway for water uptake and
allow for aeration of the root. The oxygen that root cells need for
aerobic respiration diffuses from air spaces in the soil into the
intercellular spaces of the cortex and from there into the cells of
the root.
The water and dissolved minerals that enter the root cortex
from the epidermis move in solution along two pathways: the
symplast and apoplast (❚ Fig. 35-4). The symplast is the continuum of living cytoplasm, which is connected from one cell
to the next by cytoplasmic bridges called plasmodesmata (see
Fig. 5-27). Some dissolved mineral ions move from the epidermis
through the cortex via the symplast. The apoplast consists of the
interconnected porous cell walls of a plant, along which water
and mineral ions move freely. The water and mineral ions can
diffuse across the cortex without ever crossing a plasma membrane to enter a living cell.
The inner layer of the cortex, the endodermis, regulates the
movement of water and minerals that enter the xylem in the
root’s center. Structurally, the endodermis differs from the rest
of the cortex. Endodermal cells fit snugly against each other,
and each has a special bandlike region, called a Casparian strip
(❚ Fig. 35-5), on its radial (side) and transverse (upper and lower)
walls. Casparian strips contain suberin, a fatty material that is
waterproof. (Recall from Chapter 34 that suberin is also the waterproof material in cork cell walls.)
Until the endodermis is reached, most of the water and dissolved minerals have traveled along the apoplast and therefore
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Most crops, unless they have been genetically selected to
tolerate high salt, are not productive in saline soil. Research on
the molecular aspects of salt tolerance suggests that mutations in
membrane transport proteins confer salt tolerance. Biologists genetically engineered a salt-tolerant variety of Arabidopsis to overexpress a single gene that codes for a sodium transport protein in
the vacuolar membrane. These plants can store large quantities of
sodium in their vacuoles, thereby tolerating saline soils.
Review
❚
What are the four components of soil, and how is each
important to plants?
What is cation exchange?
How do weathering processes convert rock into soil?
What are macronutrients and micronutrients?
How are minerals lost from the soil?
❚
❚
❚
❚
S UM M A RY WI T H KE Y TE RM S
Learning Objectives
1
Distinguish between taproot and fibrous root systems
(page 749).
❚ A taproot system has one main root (formed from the
radicle), from which many lateral roots extend.
❚ A fibrous root system has several to many adventitious
roots of the same size developing from the end of
the stem. Lateral roots branch from these adventitious
roots.
Explore the pathways of water and dissolved minerals in the root by clicking on the figures in
ThomsonNOW.
4
Learn more about root systems by clicking on
the figure in ThomsonNOW.
Label cross sections of a primary eudicot root and a monocot
root, and describe the functions of each tissue (page 749).
❚ Primary roots have an epidermis, ground tissues (cortex
and, in certain roots, pith), and vascular tissues (xylem and
phloem). Each root tip is covered by a root cap, a protective layer that covers the delicate root apical meristem
and may orient the root so that it grows downward.
❚ Epidermis protects the root. Root hairs, short-lived extensions of epidermal cells, aid in absorption of water and
dissolved minerals.
❚ Cortex consists of parenchyma cells that often store
starch. Endodermis, the innermost layer of the cortex,
regulates the movement of water and minerals into the
root xylem. Cells of the endodermis have a Casparian
strip around their radial and transverse walls that is impermeable to water and dissolved minerals. Minerals are
actively transported through carrier proteins in the plasma
membranes of endodermal cells.
❚ Pericycle, xylem, and phloem collectively make up the
root’s stele, or vascular cylinder. Pericycle gives rise to
lateral roots and lateral meristems. Xylem conducts water
and dissolved minerals; phloem conducts dissolved sugar.
❚ Xylem of a herbaceous eudicot root forms a solid core
in the center of the root. The center of a monocot root
often consists of pith surrounded by a ring of alternating bundles of xylem and phloem. Monocot roots lack a
vascular cambium and do not have secondary growth.
3 Trace the pathway of water and mineral ions from the soil
through the various root tissues, and distinguish between the
symplast and apoplast (page 749).
❚ As water and dissolved mineral ions move from the soil
into the root, they pass through the following tissues: root
hair/epidermis ¡ cortex ¡ endodermis ¡ pericycle
¡ root xylem.
❚ Water and dissolved minerals move through the epidermis and cortex along one of two pathways, the apoplast (along the interconnected porous cell walls) or the
symplast (from one cell’s cytoplasm to the next through
plasmodesmata).
2
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Chapter 35
5
6
7
8
Discuss the structure of roots with secondary growth
(page 749).
❚ Roots of gymnosperms and woody eudicots develop
secondary tissues (wood and bark). The production of
secondary tissues is the result of the activity of two lateral
meristems, the vascular cambium and cork cambium. The
vascular cambium produces secondary xylem (wood) and
secondary phloem (inner bark). The cork cambium produces periderm (outer bark).
Describe at least three roots that are modified to perform
uncommon functions (page 749).
❚ Prop roots develop from branches or from a vertical stem
and grow downward into the soil to help support certain
plants in an upright position. Buttress roots are swollen
bases or braces that support certain tropical rainforest
trees that have shallow root systems.
❚ Pneumatophores are aerial “breathing” roots that may
assist in getting oxygen to submerged roots.
❚ Certain epiphytes have roots modified to photosynthesize,
absorb moisture, or, if parasitic, penetrate host tissues.
List and describe two mutualistic relationships between roots
and other organisms (page 756).
❚ Mycorrhizae are mutually beneficial associations between
roots and soil fungi.
❚ Root nodules are swellings that develop on roots of leguminous plants and house millions of rhizobia (nitrogenfixing bacteria).
Describe the roles of weathering, organisms, climate, and
topography in soil formation (page 757).
❚ Factors influencing soil formation include parent material (type of rock), climate, organisms, the passage of
time, and topography. Most soils are formed from parent
material that is broken into smaller and smaller particles
by weathering processes. Climate and organisms work
together in weathering rock.
❚ Soil organisms such as plants, algae, fungi, worms, insects,
spiders, and bacteria are important not only in forming soil
but also in cycling minerals.
❚ Topography, a region’s surface features, affects soil formation. Steep slopes have little or no soil on them, whereas
moderate slopes often have deep soils.
List the four components of soil, and give the ecological
significance of each (page 757).
❚ Soil is composed of inorganic minerals, organic matter,
air, and water. Inorganic minerals provide anchorage and
essential minerals for plants. Organic matter increases
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the soil’s water-holding capacity and, as it decomposes,
releases essential minerals into the soil. Soil air provides
oxygen for soil organisms to use during aerobic respiration. Soil water provides water and dissolved minerals to
plants and other organisms.
9 Describe how roots absorb positively charged mineral ions by
the process of cation exchange (page 757).
❚ Cations, positively charged mineral ions, are attracted and
reversibly bound to clay particles, which have predominantly negative charges on their outer surfaces. In cation
exchange, roots secrete protons (H), which are exchanged for other positively charged mineral ions, freeing
them into the soil water to be absorbed by roots.
10 Distinguish between macronutrients and micronutrients
(page 757).
❚ Plants require 19 essential elements for normal growth.
Ten elements are macronutrients: carbon, hydrogen, oxy-
gen, nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and silicon. Macronutrients are required in fairly
large quantities.
❚ Nine elements are micronutrients: chlorine, iron, boron,
manganese, sodium, zinc, copper, nickel, and molybdenum. Micronutrients are needed in trace amounts.
11 Explain the impacts of mineral depletion and soil erosion on
plant growth (page 757).
❚ Mineral depletion may occur in soils that are farmed
because the natural pattern of nutrient cycling is disrupted
when crops are harvested (and not allowed to decompose
into the soil).
❚ Soil erosion is the removal of soil from the land by the
actions of agents such as water and wind. Erosion causes a
loss in soil fertility, because essential minerals and organic
matter that are a part of the soil are removed.
T E ST Y OU R UN D E RS TA ND ING
1. One main root, formed from the enlarging embryonic root,
with many smaller lateral roots branching off it is a(an)
(a) fibrous root system (b) adventitious root system (c) taproot system (d) storage root system (e) prop root system
2. Roots produced at unusual places on the plant are (a) fibrous
(b) adventitious (c) taproots (d) rhizobial (e) mycorrhizae
3. Root hairs (a) cover and protect the delicate root apical meristem (b) increase the absorptive capacity of roots (c) secrete
a waxy cuticle (d) orient the root so it grows downward
(e) store excess sugars produced in the leaves
4. Certain plants adapted to flooded soil produce aerial “breathing” roots known as (a) fibrous roots (b) pneumatophores
(c) mycorrhizae (d) nodules (e) prop roots
5. Unlike stems, roots produce (a) nodes and internodes
(b) root caps and internodes (c) axillary buds and root hairs
(d) terminal buds and axillary buds (e) root caps and root
hairs
6. The waterproof region around the radial and transverse walls
of endodermal cells is the (a) Casparian strip (b) pericycle
(c) apoplast (d) symplast (e) pneumatophore
7. The apoplast is (a) a layer of cells that surround the vascular
region in roots (b) the layer of cells just inside the endodermis
(c) a system of interconnected plant cell walls through which
water moves (d) the central cylinder of the root that comprises the vascular tissues (e) a continuum of cytoplasm of
many cells, all connected by plasmodesmata
8. Plants obtain positively charged mineral ions from clay
particles in the soil by cation exchange, in which (a) roots
passively absorb the positively charged mineral ions (b) mineral ions flow freely along porous cell walls (c) roots secrete
protons, which free other positively charged mineral ions to
be absorbed by roots (d) the Casparian strip effectively blocks
the passage of water and mineral ions along the endodermal
cell wall (e) a well-developed system of internal air spaces in
the root allows both gas exchange and cation exchange
9. The cell layer from which lateral roots originate is the (a) epidermis (b) cortex (c) endodermis (d) pericycle (e) vascular
cambium
10. The center of a herbaceous eudicot root is composed of
, whereas the center of a monocot root is
composed of
. (a) pith; cortex (b) xylem;
phloem (c) phloem; xylem (d) xylem; pith (e) pith; xylem
11. Mutually beneficial associations between certain soil fungi
and the roots of most plant species are called (a) mycorrhizae
(b) pneumatophores (c) nodules (d) rhizobia (e) humus
12. Which of the following statements about soil is true? (a) pore
spaces are always filled with about 50% air and 50% water
(b) a single teaspoon of fertile agricultural soil may contain
up to several hundred living microorganisms (c) the texture
of a soil is determined by the soil’s pH (d) a soil’s organic matter includes litter, droppings, and the dead remains of plants,
animals, and microorganisms (e) soil formation is unaffected
by a region’s climate or topography
13. The technique of growing plants in aerated water containing
dissolved mineral salts is known as (a) hydration (b) hydroponics (c) hydrophilic (d) hydrostatic (e) hydrolysis
14. Carbon, hydrogen, oxygen, nitrogen, potassium, calcium,
magnesium, phosphorus, sulfur, and silicon are collectively
known as (a) micronutrients (b) microvilli (c) micronuclei
(d) macronuclei (e) macronutrients
15. In roots of woody plants, (a) xylem does not form the central
tissue of the root (b) the cortex composes the bulk of the root
(c) the vascular cambium gives rise to secondary xylem and
secondary phloem (d) the pericycle gives rise to the apical
meristem (e) secondary growth occurs despite the lack of
a vascular cambium
16. Corn, sorghum, red mangrove, and banyan are plants that
have (a) prop roots (b) buttress roots (c) pneumatophores
(d) storage roots (e) root nodules
Roots and Mineral Nutrition
765
C R I TI C AL TH I N KI N G
1. A mesquite root is found penetrating a mine shaft about 46 m
(150 ft) below the surface of the soil. How could you determine when the root first grew into the shaft? (Hint: Mesquite
is a woody plant.)
6. Explain why, once secondary growth has occurred, that portion of the root is no longer involved in absorption. Where
does absorption of water and dissolved minerals occur in
plants that have roots with secondary growth?
2. How would you distinguish between a root hair and a small
lateral root?
7. Evolution Link. A barrel cactus that is 60 cm tall and 30 cm
in diameter has roots more than 3 m long. However, all the
plant’s roots are found in the soil at a depth of 5 to 15 cm.
What possible adaptive value does such a shallow root system
confer on a desert plant?
3. You are given a plant part that was found growing in the soil
and are asked to determine whether it is a root or an underground stem. How would you identify the structure without
a microscope? With a microscope?
4. How would you design an experiment to determine whether
gold is essential for plant growth? What would you use for an
experimental control?
Additional questions are available in
ThomsonNOW at www.thomsonedu.com/
login
5. Why does overwatering a plant often kill it?
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Chapter 35
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