Download CHAPTER 31, PLANT STRUCTURE AND FUNCTION

Plant Structure and Growth
I. Introduction
The angiosperms or flowering plants comprise a diverse assemblage of
nearly a quarter million different species
 Angiosperms are vascular plants; they have stems, roots, and leaves.
 Unlike gymnosperms such as conifers, angiosperm's seeds are found
in a flower.
 Angiosperm eggs are fertilized and develop into a seed in an ovary
that is usually in a flower.
 The flowers of angiosperms have male or female reproductive organs
There are two subclasses of angiosperms, the Monocotyledoneae
(monocots) and the Dicotyledoneae (dicots).
 All monocots are herbaceous such as grasses (e.g. corn, wheat,
barley).
 The dicots include both woody and herbaceous members. Woody
trees such as oak and herbaceous plants such as beans
II. Plant systems
There are two major systems in the
typical plant:
Root system
 Underground (usually)
 Growth toward gravity
 Consists of roots
Shoot system
 Growth away from gravity along
axis
 Generally above ground
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 Consists of stems and leaves. Flowers are modified shoot systems
III. Basic types of plant cells
1. Meristems
 Meristematic cells are called undifferentiated cells
 Since meristem is not specialized, it’s not really a "tissue"; the cells
that are produced by meristems become the tissues of the plant body.
2. Parenchyma cells
 Least specialized plant cells
 Thin primary cell walls
 Living at maturity
 Generally have a large central vacuole
 Have many functions, including photosynthesis (called chlorenchyma
because cells contain chloroplasts) and food storage
2. Collenchyma cells
 Thicker primary cells walls (usually with uneven thickness)
 Living at maturity
 Lack secondary cell walls
 Role in flexible support of herbaceous plants and growing parts of
young plant
3. Schlerenchyma cells
 Thick secondary cell walls with lignin
 Dead at functional maturity
 There are two forms: fibers (long, slender, tapered cells occurring in
bundles) and sclerids (shorter, irregularly-shaped cells)
 Fibers from flax are used for making linen textiles
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 Sclerids are common in stone cells in pears and hard nut and seed
shells
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4. Vascular Tissues
Water-conducting cells of xylem: tracheids and vessel elements
Xylem consists of two cell types, both with secondary walls and both dead
at functional maturity.
 Tracheids are long, thin, tapered cells having lignin-hardened
secondary walls with pits
 Vessel elements are wider, shorter, thinner-walled, and less tapered
than tracheids.
 Perforation plates at each end of vessel elements
 Joined to form a continuous pipe, or vessel
Food-conducting cells of phloem: sieve-tube members
Sieve-tube members are chains of phloem cells that transport sucrose and
other organic compounds
 The cells are alive at functional maturity.
 Protoplasts lack a nucleus, ribosomes, and a distinct vacuole.
 In angiosperms, the end walls of sieve-tube members have pores and
are called sieve plates.
 At least one companion cell is connected to each sieve-tube member
by many plasmodesmata
 Companion cells, nucleated cells that control the sieve-tube element
and may aid in sucrose loading
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IV. Tissue of vascular plants
Each organ of a plant (leaf, stem, and root)
has three tissue systems: dermal, vascular,
and ground tissue systems.
1. Dermal tissue system (or epidermis)
 Single layer of tightly packed cells
covering and protecting the young parts
of the plant
 In addition to protection of the plant,
dermal tissue has different functions,
depending on its location on the plant
 Root hairs specialized for water and
mineral absorption (extensions of
epidermal cells near root tips).
 The waxy cuticle that helps the plant
retain water is secreted by epidermal cells of leaves and most stems.
2. Vascular tissue system
 The xylem and phloem that functions in transport and support
3. Ground tissue system
 Predominantly parenchyma, with some collenchyma and
sclerenchyma present
 Occupies the space between dermal and vascular tissue systems
 Functions in photosynthesis, storage, and support
V. Root System
Root Functions
 Anchor the plant in the soil
 Absorb (and conduct) water and nutrients
 Food storage
 Produce certain hormones (the gibberellins and cytokinins)
Types of roots
1. Tap root
 Seen in many dicots.
 One large, vertical root (the taproot) produces many smaller
secondary roots.
 Some taproots, such as carrots, turnips, and sweet potatoes, are
modified to store a large amount of reserve food.
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2. Fibrous roots
 Found primarily in monocots e.g., grasses.
 A mat of threadlike roots spreads out below the soil surface.
 Provides extensive exposure to soil water and minerals
3. Adventitious roots
 These roots can grow from plant organs other than roots e.g. stems.
 Form in addition to the normal root system
 Some, such as prop roots of corn, help support the plant stem
Root structure- Dicot roots:
Epidermis
 Single layer of cells with very little cuticle or suberin
 Root hairs are found in the region of early maturation in the epidermis
 Root hair increases surface area for absorption of water and nutrients.
Cortex
 The root cortex is composed of loosely packed ground tissue cells
 Storage of photosynthetic products
Endodermis
 One cell thick that forms a boundary between the cortex and the stele
 The walls of the endodermis cells have a casparian strip, a layer or
band of suberin on the walls
 From this point, water and nutrients can no longer flow between cell
spaces (apoplastic movement) as they did in the root cortex.
 Water and minerals must now move through the endodermis cells and
are subject to membrane regulation.
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Stele (vascular cylinder)
 The stele includes all of the tissues inside of the endodermis: the
pericycle, the vascular tissues—xylem and phloem—and, in some
plants, a pith.
 Most dicot (eudicot) roots have a solid core of xylem in their center
whereas most monocots have a pith composed of parenchyma.
Pericycle
 The pericycle is a cylinder of parenchyma cells that lies just inside the
endodermis
 In dicots, it has the capacity to produce lateral roots.
Vascular tissue
 In monocot plants, the xylem and phloem cells are arranged in a circle
around a pith or center, whereas in dicots, the xylem cells form a
central column with radiating arms , and phloem cells fill in the
spaces between the lobes
Pith
 It consists of thin walled parenchyma cells. These cells are filled with
abundant starch grains.
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VI. The shoot system
 Above ground (usually)
 Many functions including: photosynthesis, reproduction and food and
water conduction
 In a few plants stems have additional functions, such as food
storage. Potatoes (tuber) are underground stems that store large
amounts of starch.
 The shoot system includes the leaves and the reproductive organs
a. Stems
Stem morphology includes
 Nodes – points where leaves attach
 Internodes – stem segment between nodes
 Axillary bud – at the point of attachment of each leaf
 Terminal bud – bud on shoot tip usually has developmental leaves
Primary tissues of stems
 The vascular tissue of the stem is organized into strands of vascular
bundles; each bundle is surrounded by ground tissue, including pith
and cortex.
In dicots, vascular bundles form a ring that divides the ground tissue into
cortex and pith. The pith is located in the center of the stem
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In monocots, vascular bundles are scattered throughout the ground tissue of
the stem. Ground tissue of stems is mostly parenchyma, strengthened in
many plants by collenchyma located beneath the epidermis
b. Leaves
 A leaf usually exists in the shape of a flattened blade which is joined
to the node of a stem by a petiole.
 Most monocots lack petioles; instead, the leaf base forms a sheath
surrounding the stem.
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Leaf venation patterns are an important distinction between monocots and
dicots. Monocots usually have parallel veins; dicots have netted veins,
generally
Leaf blades may be simple, dissected or compound with leaflets along the
petiole (or rachis) along one plane. A compound leaf can be distinguished
from a simple leaf by the location of buds. There are no buds in the axils of
leaflets.
Tissue organization of leaves
Leaves are cloaked by an epidermis of tightly interlocked cells
 A waxy cuticle prevents water loss.
 Stomata are pores flanked by guard cells which regulate gas
exchange with the surrounding air and photosynthetic cells inside the
leaf.
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 Stomata also allow transpiration (water loss from plant by
evaporation).
 Stomata are usually more numerous on the bottom surface of the leaf.
Leaf mesophyll
The ground tissue of a leaf is mesophyll.
 Consists mainly of parenchyma cells equipped with chloroplasts for
photosynthesis
Dicots usually have two distinct
regions of mesophyll:
 Palisade parenchyma: One
or more layers of columnar
cells of the upper half of a
leaf
 Spongy
parenchyma:
Irregularly shaped cells
surrounded by air spaces
through which oxygen and
carbon dioxide circulate.
Monocots usually do not have a
distinctive palisade and spongy
mesophyll.
They have parallel veins with a
general mesophyll of loosely
packed parenchyma cells on both
sides of the veins extending to the
epidermis layers
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Vascular tissue in leaves (Veins)
Leaf vascular tissue includes:
 Xylem (Toward upper epidermis) composed of vessels and fibers
 Phloem (Toward lower epidermis) composed of sieve tubesand
companion cells
In addition, many monocots have enlarged bundle sheath cells, surrounding
the veins, which have a function in C-4 photosynthesis for some plants.
VII. Plant Growth
Plants do not live indefinitely, they have finite life spans.
 Anuals: Plants that live one year or less. Example, legumens
 Biennials: Plants that live two years. Example, carrots
 Perenials: Plants that live many years. Example, trees
Primary growth (elongation) is initiated by apical meristems and produces
the primary plant body, which consists of three primary tissue systems.
The youngest portions of woody plants and herbaceous plants are examples
of primary plant bodies.
Primary growth of roots
 Root growth is concentrated near its tip and results in roots extending
through the soil.
 The root tip is covered by a root cap, which protects the meristem
and secretes a polysaccharide coating (mucigel) that lubricates the
soil ahead of the growing root.
 The root tip contains three zones of cells
1. Zone of cell division
 Located near the tip of the root; includes the apical meristem and its
derivatives, the primary meristems.
 As roots mature the tip region of meristem becomes inactive and is
called the quiescent zone.
 The primary meristems form as three concentric cylinders of cells (the
protoderm, procambium, and ground meristem) that will produce the
tissue systems of the roots.
2. Zone of cell elongation
 In this region, cells elongate to at least ten times their original length.
 The elongation of cells in this region pushes the root tip through the
soil.
3. Zone of maturation
 Is located farthest from the root tip
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 Region where the new cells become specialized in structure and
function and where the three tissue systems complete their
differentiation.
Primary tissues of roots
The apical meristem produces three primary meristems, which in turn give
rise to the three primary tissues of roots
 Protoderm - outermost primary meristem - produces cells which will
become dermal tissue
 Ground meristem - central primary meristem - produces cells which
will become ground tissue
 Procambium - innermost primary meristem - produces cells which
will become vascular tissue
Lateral roots may initiate from the pericycle
Primary growth of shoots
 A shoot’s apical meristem is a dome-shaped mass of dividing cells at
the tip of the terminal bud
 Forms the primary meristems that differentiate into the three tissue
systems.
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 On the flanks of the apical meristem dome are leaf primordia which
form leaves.
 Meristematic cells left by the apical meristem at the base of the leaf
primordia develop into axillary buds.
 Axillary buds may form the lateral branches of the stem system
Secondary growth
Secondary growth (increased girth) is the thickening of roots and shoots
which occurs in woody plants due to development of two lateral meristems:
the vascular cambium and the cork cambium.
 Vascular cambium produces secondary xylem and phloem.
 Cork cambium produces a tough, thick covering for roots and stems
that replaces the epidermis.
Secondary growth occurs in all gymnosperms and most dicot angiosperms;
it is rare in monocots.
Secondary growth of stems
1. Vascular cambium and the production of secondary vascular tissue
 The cambium cells give rise to secondary xylem on the inside and
secondary phloem on the outside
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 The Cambium layers in woody secondary growth plants have seasonal
dormancy. Growth is most active in spring and tapers in summer,
ceasing in fall.
 Spring (or early) wood typically has larger vessels that are more
porous and fewer, smaller rays.
 Summer (or late) wood is comprised of denser, smaller cells with
thicker walls. Annual growth rings are the result of the alternating
pattern of spring and summer wood.
The annual growth rings that are evident in cross sections of most tree
trunks can be used to estimate the age of the tree.
Another difference in wood appearance is caused by aging of wood.
Heartwood contains the older xylem that is non-functioning. The
functioning xylem toward the exterior is the sapwood.
2. Cork cambium and the production of periderm
Cork cambium is a cylinder of meristematic tissue that forms protective
layers of the secondary plant body.
 Cork cambium produces cork cells, which form exterior to the cork
cambium. As cork cells mature, they secrete suberin (a waxy
substance) in their cell walls and then die
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 Cork cells function as a barrier to protect the stem from physical
damage and from pathogens
 The combination of cork cambium and layers of cork form the
periderm (the protective coat of the secondary plant body that replaces
primary epidermis).
 The term bark refers to all the tissues external to the vascular
cambium (phloem, cork cambium and cork)
Lenticel: Spongy region in the bark which permits gas exchange by living
cells within the trunk
2. Secondary growth of roots
Vascular cambium and cork cambium also function in secondary growth
of roots.
 The vascular cambium produces secondary xylem to its inside and
secondary phloem to its outside.
 Cork cambium forms from the pericycle of the stele and produces the
periderm, which becomes secondary dermal tissue.
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