CHAPTER 35: PLANT STRUCTURE, GROWTH, AND DEVEOPMENT
The Plant Body
1. Describe several examples of a plant’s structural responses to environmental change.
For example, underwater leaves are feathery, an adaptation that protects them from damage
by lessening their resistance to moving water. In contrast, the surface leaves are pads that aid
in flotation. Both leaves have genetically identical cells, but dissimilar environments result in
the turning on or off of different genes during leaf development. Another example is most
cactus species, regardless of local environment, have leaves that are so highly reduced to
spines that the stems are the primary photosynthetic organs, which enhances the reproductive
success of cacti because with the reduced leaf surface areas, they lose less water.
2. Describe the relationships between the evolution of multicellular plants and that of
animals.
Plants, like most animals, have organs composed of different tissues, which are composed of
cell of different types.
3. Describe and compare the three basic organs of plants. Explain how these basic organs
are interdependent.
 Roots: A root is a multicellular organ that anchors a vascular pant in the soil, absorbs
minerals and water, and often stores carbohydrates. A taproot system consists of one
main vertical root that gives rise to lateral roots or branch roots. Adventitious roots
arise from stems or leaves. Seedless vascular plants and monocots have a fibrous root
system characterized by thin lateral roots with no main root. In most plants,
absorption of water and minerals occurs near the root hairs, where vast numbers of
tiny root hairs increase the surface area. Many plants have modified roots.
 Stems: A stem is an organ consisting of an alternating system of nodes, the points at
which leaves are attached, and internodes, the stem segments between nodes. The
axillary bud, which is in the upper angle formed by each leaf and the stem, is a
structure that can form a lateral shoot or branch. Most axillary buds of a young shoot
are dormant (not growing). An apical/terminal bud is located near the shoot tip and
causes the elongation of a young shoot. Apical dominance helps maintain dormancy
in most nonapical buds. Many plants have modified stems.
 Leaves: In most vascular plants, the leaf is the main photosynthetic organ. (Green
stems also perform photosynthesis.) Leaves generally consist of a flattened blade and
a stalk called the petiole, which join the leaf to a node of the stem. Monocots and
eudicots differ in the arrangement of veins, the vascular tissue of leaves: most
monocots have parallel veins, and most eudicots have branching veins. In classifying
angiosperms, taxonomists may use morphology (external form) as criterion. Some
plant species have evolved leaves modified leaves that serve various functions.
These basic organs are interdependent because roots support stems and leaves and send
water and minerals, leaves make food, and stems support and exchange the two.
4. List the basic functions of roots. Describe and compare the structures and functions of
fibrous roots, taproots, root hairs, and adventitious roots.
The basic functions of roots are anchoring a vascular pant in the soil, absorbing minerals
and water, and often storing carbohydrates. Fibrous roots are thin, thread-like roots that
spread out below the soil surface. They don’t usually penetrate deeply and are therefore
best adapted to shallow soils or regions where rainfall is light and doesn’t moisten the
soil much below the surface layer. Taproots, which develop from embryonic roots, is one
main vertical root that stores sugars and starches that plants will consume during
flowering and fruit production. Root hairs, which are extensions of epidermal cells, are
located near the tips of roots, where the absorption of water and minerals primarily occur,
and increase the surface area of the root. Adventitious roots grow in unusual locations,
such as stems or leaves. Instead of giving rise to a main root when the embryonic root
dies in seedless vascular plants and in most monocots, many small roots grow from the
stem. Such roots are adventitious.
5. Describe the basic structure of plant stems.
Plant stems have leaves attached at nodes alternating with internodes. Apical buds
emerge just above the leaves. At the tip, the terminal bud contains the apical meristem.
6. Explain the phenomenon of apical dominance.
Apical dominance is the inhibition of axillary buds by an apical bud. The proximity of the
axillary buds to the apical bud is partly responsible for their dormancy. When axillary
buds break dormancy, they start growing. A growing axillary bud gives rise to a lateral
shoot, complete with its own apical bud, leaves, and axillary buds. Removing the apical
bud usually stimulates the growth of axillary buds, resulting in more lateral shoots.
7. Describe the structures and functions of four types of modified shoots.
 Rhizomes: A horizontal shoot that grows just below the surface for storage and
dormancy. Vertical shoots emerge from axillary buds on the rhizome.
 Bulbs: Vertical underground shoots consisting mostly of enlarges bases of leaves that
store food. Have a short underground stem with leaves for storage and dormancy.
 Stolons: Horizontal shoots that grow along the surface. “Runners” that enable a plant
to reproduce asexually, as plantlets form at nodes along each runner. Grown across
the ground and may root.
 Tubers: The enlarged ends of rhizomes or stolons specialized for storing food.
8. Describe and distinguish between the leaves of monocots and eudicots.
Leaves on monocots lack petioles; instead the base of the leaf forms a sheath that
envelopes that stem. Monocots and eudicots differ in the arrangement of veins. Most
monocots have parallel major veins that run the length of the blade, and eudicots
generally have a branched network of major veins.
9. Describe and distinguish between the three tissue systems of plant organs.
 Dermal tissue system: The plant’s outer protective covering, forms the first line of
defense against physical damage and pathogens. In nonwoody plants, it’s usually a
single tissue called the epidermis, a layer of tightly packed cells. In leaves and most
stems, the cuticle, a waxy coating on the epidermal surface, helps prevent water loss.
In woody plants, protective tissue called the periderm replaces the epidermis in older


regions of stems and roots. Also, the epidermis has specialized characteristics in each
organ.
Vascular tissue system: It carries out long-distance transport of materials between the
root and shoot systems. The two types of vascular tissue are xylem and phloem.
Xylem conducts water and dissolved minerals upward from roots into the shoots.
Phloem transports sugars, the product of photosynthesis, from where they are made
(usually the leaves) to where they are needed (usually roots, sites of growth, such as
developing leaves and fruits). The vascular tissue of a root or stem is called the stele.
In angiosperms, the root stele is a solid central vascular cylinder of xylem and
phloem, whereas the stele of the stems and leaves consists of vascular bundles,
separate strands containing xylem and phloem.
Ground tissue system: Tissues that are neither dermal nor vascular. Ground tissue that
is internal to the vascular is known as pith, and ground tissue that is external to the
vascular tissue is called the cortex. The ground tissue system included various cells
specialized for functions such as storage, photosynthesis, and support.
10. Describe and distinguish between the three basic cell types of plant tissues.
 Parenchyma cells: Mature parenchyma cells have primary walls that are relatively
thin and flexible, and most lack secondary walls. Mature parenchyma cells generally
have a large central vacuole. Parenchyma cells perform most of the metabolic
functions of the plant, synthesizing and storing various organic products. Some
parenchyma cells in stems and roots have colorless plastids that store starch.
Parenchyma cells compose the fleshy tissue of many fruits. Most parenchyma cells
retain the ability to divide and differentiate into other types of plant cells under
particular conditions.
 Collenchyma cells: Collenchyma cells, grouped in strands or cylinders, help support
young parts of the plant shoot. Collenchyma cells have thicker primary walls than
parenchyma cells, though the walls are unevenly thickened. Young stems and petioles
have strands of collenchyma cells just below their epidermis. Collenchyma cells lack
secondary wall, and their primary walls don’t have the hardening agent lignin. These
cells, therefore, provide flexible support without restraining growth. When matured,
collenchyma cells are living and flexible, elongating with the stems and leaves they
support, unlike sclerenchyma cells.
 Sclerenchyma cells: Sclerenchyma cells function as supporting elements in the plants
but with thick secondary walls that are usually strengthened by lignin. They are much
more rigid than collenchyma cells. Mature sclerenchyma cells can’t elongate, and
they occur in regions of the plant that have stopped growing in length. Many
sclerenchyma cells are dead at functional maturity, but they produce secondary walls
before the protoplast (the living part of a cell) dies. The rigid walls remain as a
“skeleton” that supports the plant. The two types of sclerenchyma cells are sclereids
and fibers; they are specialized entirely for support and strengthening. Sclereids are
shorter than fibers and irregular in shape, have thick, lignified secondary walls,
impart hardness to nutshells and seed coasts and gritty texture to pear fruits. Fibers
are long, slender, and tapered and usually arranged in threads.
The Process of Plant Growth and Development
11. Distinguish between plant growth and plant development.
Plant growth is the process by which a plant increases in the number and size of leaves
and stems. Growth occurs throughout the plant’s life, known as indeterminate growth.
Plants are capable of indeterminate growth because they have meristems (apical or
lateral). Apical meristems cause plant to grow in length, known as primary growth.
Lateral meristems cause growth in thickness, known as secondary growth. Most plants
grow continuously, but do die. Plant development is the process of a plant changing from
one growth stage to another. The process of plant development includes the formation of
a complete embryo from a zygote; seed germination; the elaboration of a mature
vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many
of the plant’s responses to its environment.
12. Distinguish between annual, biennial, and perennial plants.
 Annual: Annuals complete their life cycle from germination to flowering seed
production to death (in a year or less). Many wildflowers and most staple food crops,
including legumes and cereal grains, such as wheat and rice are annuals.
 Biennial: Biennials generally require two growing seasons to complete their life
cycle, flowering and fruiting only in their second year. Radishes and carrots, even
though harvested after the first year, are biennials.
 Perennial: Perennials live many years and include trees, shrubs, and some grasses.
When perennials die, it is usually not from old age but from an infection or some
environmental trauma, such as fire or sever drought.
13. Explain how plants are capable of indeterminate growth.
Plants are capable of indeterminate growth because they have perpetually embryonic
tissues called meristems. There are two main types: apical meristems and lateral
meristems. Apical meristems provide additional cells that enable the plant to grow in
length—primary growth, which allows the root to extend throughout the soil and shoots
to increase their exposure to light. Lateral meristems’ activity called the vascular
cambium and cork cambium causes the growth in thickness or secondary growth. The
vascular cambium adds layers to vascular tissue called secondary xylem (wood) and
secondary phloem. The cork cambium replaces the epidermis with the thicker, tougher
periderm.
14. Define and distinguish between primary and secondary growth.
Primary growth is caused by apical meristems that provide additional cells that enable the
plant to grow in length. Secondary growth, or growth in thickness, is caused by the
activity of lateral meristems called the vascular cambium and cork cambium. The
vascular cambium adds layers to vascular tissue called secondary xylem (wood) and
secondary phloem. The cork cambium replaces the epidermis with the thicker, tougher
periderm.
15. Describe in detail the primary growth of tissues of roots and shoots. Describe the specific
tissue organization of leaves.
Primary growth of tissues of roots occurs behind the tip of a root, which is covered by a
root cap, in three zones: cell division, elongation, and differentiation. The zone of cell
division includes the apical meristem and its derivatives. New root cells are produced in
this region, including the root cap. A millimeter behind the tip of the root is the zone of
elongation, where root cells elongate, sometimes to more than ten times their original
length. Cell elongation in this zone pushes the tip further into the soil. Meanwhile, the
root apical meristem keeps adding cells to the younger end of the zone of elongation.
Before the root cells finish lengthening, many begin specializing in structure and
function. In the zone of differentiation (zone of maturation) cells complete their
differentiation and become distinct cell types. The primary growth of roots produces the
epidermis, ground tissue, and vascular tissue. In most roots, the stele is the vascular
cylinder. The ground tissue fills the cortex, the region between the vascular cylinder and
epidermis. The innermost layer of the cortex is called the endodermis Lateral roots arise
from within the pericycle, the outermost cell layer in the vascular cylinder. As for
primary growth of shoots, a shoot apical meristem is a dome-shaped mass of dividing
cells at the shoot tip. Leaves develop from leaf primordia along the sides of apical
meristem. Axillary buds develop from meristematic cells left at the bases of leaf
primordia.
16. Describe in detail the secondary growth of the tissues of shoots and roots.
Secondary growth occurs in stems and roots of woody plants, but rarely in leaves. The
secondary plant body consists of the tissues produced by the vascular cambium and cork
cambium. The vascular cambium is a cylinder of meristematic cells one cell layer thick.
It develops from parenchyma cells. In cross section, the vascular cambium appears as a
ring of initials. The initials increase the vascular cambium’s circumference and add
secondary xylem to the inside of the secondary phloem to the outside. Secondary xylem
accumulates as wood and consists of tracheids, vessel elements (only in angiosperms),
and fibers. Early wood, formed in the spring, has thin wall cells and maximize water
delivery. Late wood, formed in late summer, has thick-walled cells and contributes more
to stem support. In temperate regions, the vascular cambium of perennials is dormant
through the winter. As a tree or woody shrub ages, the older layers of secondary xylem,
or heartwood, no longer transport water and minerals. The outer layers, or sapwood, still
transport minerals through xylem. Older secondary phloem sloughs off and doesn’t
accumulate. The cork cambium gives rises to the secondary plant body’s protective
covering, or periderm, which consists of the cork cambium and layers of cork cells that it
produces. Bark consists of all the tissues external to the vascular cambium, including
secondary phloem and periderm. Lenticels in the periderm allow for gas exchange
between living stem or root cells and the outside air.
Mechanisms of Plant Growth and Development
17. Explain why Arabidopsis is an excellent model for the study of plant development.
Describe the result of recent research on this plant’s development.
Arabidopsis, a weed of the mustard family that is small enough to allow researchers to
cultivate thousands of plants in a few square meters of lab space. Arabidopsis was the
first plant to have its entire genome sequenced. Determining the function of many
Arabidopsis genes has greatly expanded our understanding of plant development. To fill
the gaps in our knowledge, plant biologists have launched a quest to determine the
function of every one of the plant’s genes.
18. Describe and distinguish between morphogenesis, differentiation, and growth.
 Morphogenesis: The development of body form and organization. Cells must be
organized into multicellular arrangements of tissues and organs for development to
proceed properly. Pattern formation is the development of specific structures in
specific locations. It’s determined by positional information in the form of signals
indicating to each cell its location. Positional information may be provided by
gradients of molecules. Polarity provides one type of positional information.
Polarization is initiated by an asymmetrical first division of the plant zygote.
Homeotic genes control morphogenesis in plants and other multicellular organisms.
 Differentiation: In differentiation, cells of developing organism synthesize different
proteins and diverge in structure and function even though they have a common
genome. Cellular differentiation to a large extent depends on the positional
information and is affected by homeotic genes.
 Growth: Plants pass through developmental phases, called phase changes, developing
from a juvenile phase to an adult phase. Phase changes occur within the shoot apical
meristem. The most obvious morphological changes typically occur in leaf size and
shape. Flower formation involves the phase change from vegetative growth to
reproductive growth. Transition from vegetative growth is associated with the
switching on of floral meristem identity genes (plant homeotic genes).
19. Explain why (a) the plane and symmetry of cell division, (b) the orientation of cell
expansion, and (c) cortical microtubules are important determinants of plant growth and
development.
a) If the planes of division are parallel to the plane of the first division, a single file of
cells is produced. If the planes of division vary randomly, asymmetrical cell division
occurs.
b) Plant cells grow rapidly and “cheaply” by intake and storage of water in vacuoles.
Plant cells expand primarily along the plant’s main axis. Cellulose microfibrils in the
cell wall restrict the direction of cell elongation.
c) The immature epidermal cells that are in contact with two underlying cells of the root
cortex differentiate into root hair cells, whereas immature epidermal cells in contact
with only one cortical cell differentiate into mature hairless cells. Differential
expression of a homeotic gene called GLABRA-2 is required for appropriate hair
distribution.
20. Explain how pattern formation is determined in plants.
In plants, pattern formation is determined by positional information in the form of signals
that continuously indicate to each cell its location within a developing structure.
21. Explain how cellular differentiation is controlled by gene expression.
Cellular differentiation is controlled by gene expression because cells of developing
organism can synthesize different proteins and diverge in structure and function even
though they share a common genome. If a mature cell removed from a root or leaf can
dedifferentiate in tissue culture and give rise to the diverse cell types of a plant, then it
must process all the genes necessary to make any kind of plant cell.
22. Explain how a cell’s location influences its developmental fate.
The activation or inactivation of specific genes involved in cellular differentiation
depends largely on positional information (where a particular cell is located relative to
other cells). The cell’s final position in an emerging organ determines the kind of cell it
will become.
23. Define phase change and explain its importance to the development of plants.
Phase change is the morphological changes that arise from the transitions of plants from
developing from a juvenile phase to an adult vegetative phase to an adult reproductive
phase in shoot apical meristem activity. During the transition from a juvenile phase to an
adult phase, the most obvious morphological changes typically occur in leaf size and
shape.
24. Explain how a vegetative shoot tip changes into a floral meristem.
The change from a vegetative shoot tip to a floral meristem is triggered by a combination
of environmental cues and internal signals. Transition from vegetative to flowering is
associated with the switching on of floral meristem identity genes. Several organ identity
genes (plant homeotic genes) regulate the development of floral pattern.
Vocabulary
Root system: All of a plant’s roots, which anchor it in the soil, absorb and transport minerals in
water, and store food
Shoot system: The aerial portion of a plant body, consisting of stems, leaves, and (in
angiosperms) flowers.
Fibrous root: Thin, moderately branching roots growing from the stem.
Taproot: A main vertical root that develops from an embryonic root and gives rise to later
(branch) roots.
Root hair: An extension of a root epidermal cell, growing just behind the root top and increasing
surface area for absorption of water and minerals.
Adventitious: A plant organ that grows in an unusual location, such as roots arising from stems
or leaves.
Node: A point along the stem of plant at which leaves are attached.
Internode: A segment of a plant stem between the points where leaves are attached.
Axillary bud: A structure that has the potential to form a lateral shoot, or branch. The bud
appears in the angle formed between a leaf and a stem.
Terminal bud: a bud at the tip of a plant stem, also called an apical bud.
Apical dominance: Concentration of growth the tip of a plant shoot, where a terminal bud
partially inhibits axillary bud growth.
Blade: (1) A leaflike structure of a seaweed that provides most of the surface area for
photosynthesis. (2) The flattened portion of a typical leaf.
Petiole: The stalk of a leaf, which joins the leaf to a node of a stem.
Dermal tissue: The outer protective covering of plants
Epidermis: The dermal tissue system of nonwoody plants, usually consisting of single layer of
tightly packed cells.
Cuticle: A waxy covering on the surface of stems and leaves that acts as an adaptation that
prevents desiccation in terrestrial plants.
Vascular tissue: Plant tissue consisting of cells joined into tubes that transport water and
nutrients throughout the plant body.
Xylem: Vascular plant tissue consisting mainly of tubular dead cells that conduct most of the
water and minerals upward from the roots to the rest of the plant.
Phloem: Vascular plant tissue consisting of living cells arranged into elongated tubes that
transport sugar and other organic nutrients through the system.
Tracheid: A long, tapered water-conducting cell found in the xylem of nearly all vascular plants.
Function tracheids are no longer living.
Vessel elements: A short, wide, water-conducting cell in the xylem of most angiosperms and
after nonflowering vascular plants. Dead at maturity, vessel elements are aligned end to end to
form micropipes called vessels.
Pits: Region of the plant cell wall in which the secondary well is interrupted, exposing the
underlying primary cell wall.
Xylem vessels: Vessels in plants that transport water and minerals from the roots to parts of the
plant where it’s needed, through transpiration and turgor pressure in roots.
Sieve-tube member: A living cell that conducts sugars and other organic nutrients in the phloem
of angiosperms; also called a sieve-tube member. Connected end to end, they form sieve lands.
Sieve plates: An end wall in a sieve-tube element, which facilitates the glow of phloem sap in
angiosperm sieve tubes.
Companion cell: A type of plant cell that is connected to a sieve-tube element by many
plasmodesmata and whose nucleus and ribosomes may serve one or more adjacent sieve-tube
elements.
Ground tissue: Plant tissues that are neither vascular nor derma, fulfilling a variety of function,
such as storage, photosynthesis, and support.
Pith: Ground tissue that is internal to the vascular tissue in a stem; in many monocot roots,
parenchyma cells that form the central core of the vascular cylinder.
Cortex: Ground tissue that is between the vascular tissue and dermal tissue in a root or eudicot
stem
Protoplast: A plant, bacterial, or fungal cell that had its cell wall completely or partially removed
using either mechanical or enzymatic means.
Parenchyma cell: A relatively unspecialized plant cell type that carries out most of the
metabolism, synthesizes and stores organic products, and develops into a more differentiated cell
type.
Collenchyma cell: A flexible plant cell type that occurs in strands of cylinders that support young
parts of the plant without restraining growth.
Sclerenchyma cell: A rigid, supportive plant cell type usually lacking a protoplast and possessing
thick secondary walks strengthened by lignin and maturity.
Fiber: A lignified cell type that reinforces the xylem of angiosperms and functions in mechanical
support, a slender, tapered sclerenchyma cell that usually occurs in bundles.
Sclereid: A short, irregular sclerenchyma cell in nutshells and seed coats. Schereids are scattered
throughout the parenchyma of some plants.
Growth: The process by which a plant increases in the number and size of leaves and stems.
Growth occurs throughout the plant’s life, known as indeterminate growth. Plants are capable of
indeterminate growth because they have meristems (apical or lateral). Apical meristems cause
plant to grow in length, known as primary growth. Lateral meristems cause growth in thickness,
known as secondary growth. Most plants grow continuously, but do die.
Development: The process of a plant changing from one growth stage to another. The process of
plant development includes the formation of a complete embryo from a zygote; seed
germination; the elaboration of a mature vegetative plant from the embryo; the formation of
flowers, fruits, and seeds; and many of the plant’s responses to its environment.
Annual: A flowering plant that completes its entire life cycle in a single year or growing season.
Biennial: A flowering plant that requires two years to complete its life cycle.
Parennial: A flowering plant that lives for many years.
Meristem: Plant tissue that remains embryonic as long as the plants lives, allowing for
intermediate growth.
Apical meristem: Embryonic plants tissue in the tips of roots and the buds of shoots. The
dividing cells of an apical meristem enable the plant to grow in length.
Primary growth: Growth produced by apical meristems, lengthening stems and roots.
Secondary truth: Growth produced by lateral meristems, thickening the roots and shoots of
woody plants.
Lateral meristem: A meristem that thickens the roots and shoots of woody plants. The vascular
cambium and cork cambium are lateral meristems.
Primary plant body: The tissues produced by apical meristems, which lengthen stems and roots.
Root cap: A cone of cells at the tip of a plant root that protects that apical meristem.
Zone of cell division: Produces the cells that give rise to the primary body of the plant. Most cell
divisions occur along the edges of the area between active division and the cap (quiescent
center).
Quiescent center: The area where cell divide more slowly.
Protoderm: The primary meristem in vascular plants that gives rise to epidermis.
Procambium: The part of the primary meristem of a plant that forms cambium and primary
vascular tissues.
Ground meristem: The primary meristem in vascular plants that gives rise to the nonvascular
tissues, such as cortex, pericycle, and pith. Within the seeds of angiosperms, it surrounds the
procambium.
Zone of elongation: Cells in zone stretch and lengthen as small vacuoles within the sytoplasm
coalesce and fill with water. One or two large vacuoles occupy almost all of the cell volume in
fully elongated cells. Cellular expansion in this zone is responsible for pushing the root cap and
apical tip forward through the soil.
Zone of maturation: The elongating cells complete their differentiation into the tissues of the
primary body in this zone. They greatly increase the absorptive surface of roots during the
growth period when large amounts of water and nutrients are needed. An individual root hair
lives for only a day or two, but new ones form constantly nearer the tip as old ones die in the
upper part of the zone.
Stele: The vascular tissue of a stem or root.
Endodermis: The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms
that boundary between the cortex and the vascular cylinder.
Lateral root: A root that arises from the pericycle of an established root.
Pericycle: The outermost layer in the vascular cylinder from which lateral roots arise.
Vascular bundles: A strand of primary conductive plant tissue consisting essentially of xylem
and phloem that forms the fluid-conducting channels of vascular tissue in the rhizomes, stems,
and leaf veins of vascular plants, the arrangement varying with the type of plant.
Stomata: Microscopic pores surrounded by guard cells in the epidermis of leaves and stems that
allow gas exchange between the environment and the interior of the plant.
Guard cells: The two cells that flank the stomatal pore and regulate the opening and closing of
the pore.
Mesophyll: The ground tissue of a leaf, sandwiched between the upper and lower epidermis and
specialized for photosynthesized.
Secondary plant body: The tissues produced by the vascular cambium and cork cambium, which
thicken the stems and roots of woody plants.
Vascular cambium: A cylinder of meristematic tissue in woody plants that adds layers of
secondary vascular tissue called secondary xylem (wood) and secondary phloem.
Cork cambium: A cylinder of meristematic tissue in woody plants that replaces the epidermi with
thicker, tougher cork cells.
Ray initials: Cubical initials in the vascular cambium that give rise to the components of the ray
(radial) system of the secondary xylem and secondary phloem, such as ray parenchyma cells.
Fusiform initials: Elongate initials in the vascular cambium that give rise to the components of
the axial (longitudinal) system of the secondary xylem and secondary phloem, such as vessel
elements, tracheids, and sieve tube elements. They also give rise to the ray initials.
Periderm: The protective coat that replaces the epidermis in woody plant during secondary
growth, formed of the cork and cork cambium.
Lenticel: A small raised area in the bark of stems and roots that enables gas exchange between
living cells and the outside air.
Bark: Al tissues external to the vascular cambium, consisting mainly of the secondary phloem
and layers of periderm.
Morphogenesis: The development of body shape and organization.
Differentiation: Differentiation in plants refers to the processes by which distinct cell types arise
from precursor cells and become different from each other.
Asymmetrical cell division: Produces two daughter cells with different cellular fates. Stem cells
divide asymmetrically to give rise to two distinct daughter cells: one copy of original stem cell as
well as a second daughter, it must rely on intrinsic asymmetry.
Preprophase band: Microtubules in the cortex (outer cytoplasm) of a cell that are concentrated
into a ring.
Pattern formation: The development of a multicellular organism’s spatial organization, the
arrangement of organs and tissues in their characteristic places in three-dimensional space.
Positional information: Molecular cues that control pattern formation in an animal or pant
embryonic structure by indicating a cell’s location relative to the organism’s body axes. These
cues elicit a response by genes that regulate development.
Polarity: A lack of symmetry; structural differences in opposite ends of an organism or structure,
such as the root end and shoot end of a plant.
Phase change: A shift from one developmental phase to another.
Meristem identity gene(s): A plant gene that promotes the switch from vegetative growth to
flowering.
Organ identity gene(s): A plant homeotic gene that uses positional information to determine
which emerging leaves develop into which types of floral organs.