Chapter 35 Plant Structure, Growth, and Development
Overview: No Two Plants Are Alike
• The fanwort, an aquatic weed, demonstrates the great developmental
plasticity that is characteristic of plants. The fanwort has feathery
underwater leaves and large, flat, floating surface leaves. Both leaf
types have genetically identical cells, but the dissimilar environments
in which they develop cause different genes involved in leaf formation
to be turned on or off.
• The form of any plant is controlled by environmental and genetic
factors. As a result, no two plants are identical.
• In addition to plastic structural responses of individual plants to
specific environments, plant species have adaptive features that
benefit them in their specific environments.
• For example, cacti have leaves that are reduced as spines and a
stem that serves as the primary site of photosynthesis. These
adaptations reduce water loss in desert environments.
• Angiosperms comprise 90% of plant species and are at the base of
the food web of nearly every terrestrial ecosystem.
• Most land animals, including humans, depend on angiosperms
directly or indirectly for sustenance.
Concept 35.1 The plant body has a hierarchy of organs, tissues, and
• Plants, like multicellular animals, have organs that are composed of
different tissues, and tissues are composed of different cell types.
• A tissue is a group of cells with a common structure and
• An organ consists of several types of tissues that work together
to carry out particular functions.
Vascular plants have three basic organs: roots, stems, and leaves. • The basic morphology of vascular plants reflects their evolutionary
history as terrestrial organisms that inhabit and draw resources from
two very different environments.
• Plants obtain water and minerals from the soil.
• They obtain CO2 and light above ground.
• To obtain the resources they need, vascular plants have evolved two
systems: a subterranean root system and an aerial shoot system of
stems and leaves.
• Each system depends on the other.
• Lacking chloroplasts and living in the dark, roots would starve
without the sugar and other organic nutrients imported from the
photosynthetic tissues of the shoot system.
• Conversely, the shoot system (and its reproductive tissues,
flowers) depends on water and minerals absorbed from the soil
by the roots.
• A root is an organ that anchors a vascular plant in the soil, absorbs
minerals and water, and stores food.
• Most eudicots and gymnosperms have a taproot system,
consisting of one large vertical root (the taproot) that produces
many small lateral, or branch, roots.
• In angiosperms, taproots often store food that supports
flowering and fruit production later.
• Seedless vascular plants and most monocots, including
grasses, have fibrous root systems consisting of a mat of thin
roots that spread out below the soil surface.
• A fibrous root system is usually shallower than a taproot
• Grass roots are concentrated in the upper few
centimeters of soil. As a result, grasses make excellent
ground cover for preventing erosion. • Sturdy, horizontal, underground stems called rhizomes
anchor large monocots such as palms and bamboo.
• The root system helps anchor a plant.
• In both taproot and fibrous root systems, absorption of water
and minerals occurs near the root tips, where vast numbers of
tiny root hairs enormously increase the surface area.
• Root hairs are extensions of individual epidermal cells on the
• Absorption of water and minerals is also increased by
mutualistic relationships between plant roots and bacteria
• Some plants have modified roots. Some arise from roots while
adventitious roots arise aboveground from stems or even from
• Some modified roots provide additional support and
anchorage. Others store water and nutrients or absorb
oxygen or water from the air.
• A stem is an organ consisting of alternating nodes, the points at
which leaves are attached, and internodes, the stem segments
• At the angle formed by each leaf and the stem is an axillary bud with
the potential to form a lateral shoot or branch.
• Growth of a young shoot is usually concentrated at its apex, where
there is a terminal bud with developing leaves and a compact series
of nodes and internodes.
• The presence of a terminal bud is partly responsible for inhibiting the
growth of axillary buds, a phenomenon called apical dominance.
• By concentrating resources on growing taller, apical dominance
is an evolutionary adaptation that increases the plant’s
exposure to light. • In the absence of a terminal bud, the axillary buds break
dominance and give rise to a vegetative branch complete with
its own terminal bud, leaves, and axillary buds.
• Modified shoots with diverse functions have evolved in many plants.
• These shoots, which include stolons, rhizomes, tubers, and
bulbs, are often mistaken for roots.
• Stolons, such as the “runners” of strawberry plants, are
horizontal stems that grow on the surface and enable a
plant to colonize large areas asexually as plantlets form at
nodes along each runner.
• Rhizomes, like those of ginger, are horizontal stems that
• Tubers, including potatoes, are the swollen ends of
rhizomes specialized for food storage.
• Bulbs, such as onions, are vertical, underground shoots
consisting mostly of the swollen bases of leaves that store
• Leaves are the main photosynthetic organs of most plants, although
green stems are also photosynthetic.
• While leaves vary extensively in form, they generally consist of
a flattened blade and a stalk, the petiole, which joins the leaf to
a stem node.
• Grasses and other monocots lack petioles. In these plants, the
base of the leaf forms a sheath that envelops the stem.
• Most monocots have parallel major veins that run the length of the
blade, while eudicot leaves have a multibranched network of major
• Plant taxonomists use floral morphology, leaf shape, spatial
arrangement of leaves, and the pattern of veins to help identify and
classify plants. • For example, simple leaves have a single, undivided blade,
while compound leaves have several leaflets attached to the
• The leaflet of a compound leaf has no axillary bud at its
• In a doubly compound leaf, each leaflet is divided into smaller
• Most leaves are specialized for photosynthesis.
• Some plants have leaves that have become adapted for other
• These include tendrils that cling to supports, spines of cacti for
defense, leaves modified for water storage, and brightly colored
leaves that attract pollinators.
Plant organs are composed of three tissue systems: dermal, vascular,
• Each organ of a plant has three tissue systems: dermal, vascular,
• Each system is continuous throughout the plant body.
• The dermal tissue is the outer covering.
• In nonwoody plants, it is a single layer of tightly packed cells, or
epidermis, that covers and protects all young parts of the plant.
• The epidermis has other specialized characteristics consistent with
the function of the organ it covers.
• For example, the root hairs are extensions of epidermal cells
near the tips of the roots.
• The epidermis of leaves and most stems secretes a waxy
coating, the cuticle, which helps the aerial parts of the plant
• In woody plants, protective tissues called periderm replace the
epidermis in older regions of stems and roots. • Vascular tissue, continuous throughout the plant, is involved in the
transport of materials between roots and shoots.
• Xylem conveys water and dissolved minerals upward from roots
into the shoots.
• Phloem transports food made in mature leaves to the roots; to
nonphotosynthetic parts of the shoot system; and to sites of
growth, such as developing leaves and fruits.
• The vascular tissue of a root or stem is called the stele.
• In angiosperms, the vascular tissue of the root forms a
solid central vascular cylinder, while stems and leaves
have vascular bundles, strands consisting of xylem and
• Ground tissue is tissue that is neither dermal tissue nor vascular
• In eudicot stems, ground tissue is divided into pith, internal to
vascular tissue, and cortex, external to the vascular tissue.
• The functions of ground tissue include photosynthesis, storage,
• For example, the cortex of a eudicot stem typically consists of
both fleshy storage cells and thick-walled support cells.
Plant tissues are composed of three basic cell types: parenchyma,
collenchyma, and sclerenchyma.
• Plant cells are differentiated, with each type of plant cell possessing
structural adaptations that make specific functions possible.
• Cell differentiation may be evident within the protoplast, the cell
contents exclusive of the cell wall.
• Modifications of cell walls also play a role in plant cell
• We will consider the major types of differentiated plant cells:
parenchyma, collenchyma, sclerenchyma, water-conducting cells of
the xylem and sugar-conducting cells of the phloem. • Mature parenchyma cells have primary walls that are relatively thin
and flexible, and most lack secondary walls.
• The protoplast of a parenchyma cell usually has a large central
• Parenchyma cells are often depicted as “typical” plant cells
because they generally are the least specialized, but there are
• For example, the highly specialized sieve-tube members of the
phloem are parenchyma cells.
• Parenchyma cells perform most of the metabolic functions of the
plant, synthesizing and storing various organic products.
• For example, photosynthesis occurs within the chloroplasts of
parenchyma cells in the leaf.
• Some parenchyma cells in the stems and roots have colorless
plastids that store starch.
• The fleshy tissue of most fruit is composed of parenchyma
• Most parenchyma cells retain the ability to divide and
differentiate into other cell types under special conditions, such
as the repair and replacement of organs after injury to the plant.
• In the laboratory, it is possible to regenerate an entire plant
from a single parenchyma cell.
• Collenchyma cells have thicker primary walls than parenchyma cells,
though the walls are unevenly thickened.
• Grouped into strands or cylinders, collenchyma cells help
support young parts of the plant shoot.
• Young stems and petioles often have strands of collenchyma
just below the epidermis, providing support without restraining
• Mature collenchyma cells are living and flexible and elongate
with the stems and leaves they support. • Sclerenchyma cells have thick secondary walls usually strengthened
by lignin and function as supporting elements of the plant.
• They are much more rigid than collenchyma cells.
• Unlike parenchyma cells, they cannot elongate.
• Sclerenchyma cells occur in plant regions that have stopped
• Many sclerenchyma cells are dead at functional maturity, but they
produce rigid secondary cells walls before the protoplast dies.
• In parts of the plant that are still elongating, secondary walls are
deposited in a spiral or ring pattern, enabling the cell wall to
stretch like a spring as the cell grows.
• Two types of sclerenchyma cells, fibers and sclereids, are specialized
entirely for support.
• Fibers are long, slender, and tapered, and usually occur in
• Those from hemp fibers are used for making rope, and
those from flax are woven into linen.
• Sclereids are irregular in shape and are shorter than fibers.
• They have very thick, lignified secondary walls.
• Sclereids impart hardness to nutshells and seed coats
and the gritty texture to pear fruits.
• The water-conducting elements of xylem, the tracheids and vessel
elements, are elongated cells that are dead at functional maturity.
• The thickened cell walls remain as a nonliving conduit through
which water can flow.
• Both tracheids and vessels have secondary walls interrupted by pits,
thinner regions where only primary walls are present.
• Tracheids are long, thin cells with tapered ends.
• Water moves from cell to cell mainly through pits. • Because their secondary walls are hardened with lignin,
tracheids function in support as well as transport.
• Vessel elements are generally wider, shorter, thinner walled, and less
tapered than tracheids.
• Vessel elements are aligned end to end, forming long
micropipes or xylem vessels.
• The ends are perforated, enabling water to flow freely.
• In the phloem, sucrose, other organic compounds, and some mineral
ions move through tubes formed by chains of cells called sieve-tube
• These are alive at functional maturity, although a sieve-tube
member lacks a nucleus, ribosomes, and a distinct vacuole.
• The end walls, the sieve plates, have pores that facilitate the
flow of fluid between cells.
• Each sieve-tube member has a nonconducting nucleated
companion cell, which is connected to the sieve-tube member
by numerous plasmodesmata.
• The nucleus and ribosomes of the companion cell serve both
that cell and the adjacent sieve-tube member.
• In some plants, companion cells in leaves help load sugar into
the sieve-tube members, which transport the sugars to other
parts of the plant.
Concept 35.2 Meristems generate cells for new organs
• A major difference between plants and most animals is that plant
growth is not limited to an embryonic period.
• Most plants demonstrate indeterminate growth, growing as long as
the plant lives.
• In contrast, most animals and certain plant organs, such as flowers
and leaves, undergo determinate growth, ceasing to grow after they
reach a certain size. • Indeterminate growth does not mean immortality.
• Annual plants complete their life cycle—from germination through
flowering and seed production to death—in a single year or less.
• Many wildflowers and important food crops, such as cereals
and legumes, are annuals.
• The life of a biennial plant spans two years.
• Often, there is an intervening cold period between the
vegetative growth season and the flowering season.
• Plants such as trees, shrubs, and some grasses that live many years
• Perennials do not usually die from old age, but from an infection
or some environmental trauma.
• A plant is capable of indeterminate growth because it has perpetually
embryonic tissues called meristems in its regions of growth.
• These cells divide to generate additional cells, some of which
remain in the meristematic region, while others become
specialized and are incorporated into the tissues and organs of
the growing plant.
• Cells that remain as wellsprings of new cells in the meristem
are called initials.
• Those that are displaced from the meristem, derivatives,
continue to divide for some time until the cells they produce
differentiate within developing tissues.
• The pattern of plant growth depends on the location of meristems.
• Apical meristems, located at the tips of roots and in the buds of
shoots, supply cells for the plant to grow in length.
• This elongation, primary growth, enables roots to extend
through the soil and shoots to increase their exposure to light
and carbon dioxide. • In herbaceous plants, primary growth produces almost all of the
• Woody plants also show secondary growth, progressive
thickening of roots and shoots where primary growth has
• Secondary growth is produced by lateral meristems,
cylinders of dividing cells that extend along the length of
roots and shoots.
• The vascular cambium adds layers of vascular tissue
called secondary xylem and phloem.
• The cork cambium replaces the epidermis with thicker,
• In woody plants, primary growth produces young extensions of roots
and shoots each growing season, while secondary growth thickens
and strengthens the older parts of the plant.
• At the tip of a winter twig of a deciduous tree is the dormant terminal
bud, enclosed by bud scales that protect its apical meristem.
• In the spring, the bud will shed its scales and begin a new spurt
of primary growth.
• Along each growth segment, nodes are marked by scars left
when leaves fell in autumn.
• Above each leaf scar is either an axillary bud or a branch twig.
• Farther down the twig are whorls of scars left by the scales that
enclosed the terminal bud during the previous winter.
• Each spring and summer, as the primary growth extends the shoot,
secondary growth thickens the parts of the shoot that formed in
Concept 35.3 Primary growth lengthens roots and shoots
• Primary growth produces the primary plant body, the parts of the root
and shoot systems produced by apical meristems. • An herbaceous plant and the youngest parts of a woody plant
represent the primary plant body.
• Apical meristems lengthen both roots and shoots. However, there are
important differences in the primary growth of these two systems.
• The root tip is covered by a thimblelike root cap, which protects the
meristem as the root pushes through the abrasive soil during primary
• The cap also secretes a polysaccharide slime that lubricates
the soil around the growing root tip.
• Growth in length is concentrated just behind the root tip, where three
zones of cells at successive stages of primary growth are located.
• These zones—the zone of cell division, the zone of elongation,
and the zone of maturation—grade together.
• The zone of cell division includes the root apical meristem and its
• New root cells are produced in this region, including the cells of
the root cap.
• The zone of cell division blends into the zone of elongation where
cells elongate, sometimes to more than ten times their original length.
• It is this elongation of cells that is mainly responsible for
pushing the root tip, including the meristem, ahead.
• The meristem sustains growth by continuously adding cells to
the youngest end of the zone of elongation.
• In the zone of maturation, cells become differentiated and
become functionally mature.
• The primary growth of roots consists of the epidermis, ground tissue,
and vascular tissue.
• Water and minerals absorbed from the soil must enter through the
epidermis, a single layer of cells covering the root.
• Root hairs greatly increase the surface area of epidermal cells. • Most roots have a solid core of xylem and phloem. The xylem
radiates from the center in two or more spokes, with phloem
developing in the wedges between the spokes.
• In monocot roots, the vascular tissue consists of a central core
of parenchyma surrounded by alternating patterns of xylem and
• The ground tissue of roots consists of parenchyma cells that fill the
cortex, the region between the vascular cylinder and the epidermis.
• Cells within the ground tissue store food and are active in the
uptake of minerals that enter the root with the soil solution.
• The innermost layer of the cortex, the endodermis, is a cylinder one
cell thick that forms a selective barrier between the cortex and the
• An established root may sprout lateral roots from the outermost layer
of the vascular cylinder, the pericycle.
• The vascular tissue of the lateral root maintains its connection
to the vascular tissue of the primary root.
• The apical meristem of a shoot is a dome-shaped mass of dividing
cells at the terminal bud.
• Leaves arise as leaf primordia on the flanks of the apical
• Axillary buds develop from islands of meristematic cells left by
apical meristems at the bases of the leaf primordia.
• Within a bud, leaf primordia are crowded close together because
internodes are very short.
• Most of the elongation of the shoot occurs by growth in length
of slightly older internodes below the shoot apex.
• This growth is due to cell division and cell elongation within the
internode. • In some plants, including grasses, internodes continue to
elongate all along the length of the shoot over a prolonged
• These plants have meristematic regions called intercalary
meristems at the base of each leaf.
• This explains why grass continues to grow after being
• Unlike their central position in a root, vascular tissue runs the length
of a stem in strands called vascular bundles.
• Because the vascular system of the stem is near the surface,
branches can develop with connections to the vascular tissue
without having to originate from deep within the main shoot.
• In gymnosperms and most eudicots, the vascular bundles are
arranged in a ring, with pith inside and cortex outside the ring.
• The vascular bundles have xylem facing the pith and phloem
facing the cortex.
• In the stems of most monocots, the vascular bundles are scattered
throughout the ground tissue rather than arranged in a ring.
• In both monocots and eudicots, the stem’s ground tissue is mostly
• Many stems are strengthened by collenchyma just beneath the
• Sclerenchyma fiber cells within vascular bundles also help
• The leaf epidermis is composed of cells tightly locked together like
pieces of a puzzle.
• The leaf epidermis is the first line of defense against physical
damage and pa