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Chapter 5
Tissues and the Primary
Growth of Stems
Concepts
• Plants began living on land 420 million years ago
• Challenges of terrestrial life lead to evolution of
distinct, specialized tissues and organs
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Competition for sunlight
Need for water
Need for support
Need for protection
Concepts
• All flowering plants have
leaves, stems, and roots
• These parts can be modified
and may not be easily
recognizable
• No species exists for which
roots, stems, or leaves have
been completely lost
evolutionarily
Concepts
• Flowering plants are classified as the Division
Magnoliophyta
• They are known informally as angiosperms
• Several groups within this division:
• Basal angiosperms基礎被子植物
• Eudicots (broadleaf plants)(真)雙子葉植物
• Monocots (narrow-leaved)單子葉植物
Concepts
• There are 2 types of
plant bodies:
• The primary plant body is
herbaceous
• The secondary plant body
is woody
• An herb is a plant that
never becomes woody
and covered with bark
• It is often an annual
• Can be monocots or
eudicots.
Basic Types of Cells and Tissues
• Stems, leaves, and roots all share a basic,
simple organization
• All plant cells belong to just 3 classes based on
the nature of their cell walls:
• Parenchyma薄壁組織
• Collenchyma厚角組織
• Sclerenchyma厚壁組織
Parenchyma
• Parenchyma cells have
only thin primary walls
• In mass, form
parenchyma tissue.
• Most metabolically
• Most remain alive after
they mature
• Special parenchyma
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Chlorenchyma綠色組織
Glandular cells腺細胞
Transfer cells傳送細胞
Phloem韌皮部
Specialized Parenchyma
• Chlorenchyma cells are
photosynthetic
parenchyma cells
• Thin walls allow light and
carbon dioxide to pass
through to the chloroplast
• Pigmented cells are
parenchyma
• Thin walls of parenchyma
cells also allow pigments
to be seen
Specialized Parenchyma
• Glandular cells secrete:
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Nectar蜜液
Fragrances香味
Mucilage黏液
Resins松脂
Oils油
Specialized Parenchyma
• Transfer cells mediate short distance transport of
material
• They have a large, extensive plasma membrane with
numerous molecular pumps
Specialized Parenchyma
• Phloem is parenchyma tissue that conducts
nutrients over long distances
• Some parenchyma cells function by dying at
maturity to open areas
• Parenchyma cells are relatively inexpensive to
build
• Little glucose is expended in constructing such thin
walls
• Most leaves are soft, composed almost entirely of
parenchyma,
• They are not very expensive metabolically
Collenchyma
• Collenchyma cells have a thin primary wall that
becomes thickened in other areas
• This allows plastic support
• Collenchyma tends to exist:
• Beneath the epidermis
• Supporting vascular bundles
Sclerenchyma
• Sclerenchyma has a
primary wall and a thick
secondary wall that is
usually lignified木質化
次生壁
• These walls are elastic
• Sclerenchyma supports
the plant by its strength
• Usually dead at
maturity
• Two types:
• Conductive
• Mechanical
Sclerenchyma
• Mechanical Sclerenchyma
• Fibers are long and flexible
• Sclereids are short, isodiametric (cuboidal), inflexible,
and brittle
Sclerenchyma
• Small, plasmodesmatarich areas must remain
free of the secondary
wall
• These become narrow
pits in the secondary wall
• Adjacent pits are called
pit-pairs
• Conducting
sclerenchyma
transports water
• Tracheary elements管狀
細胞 of the Xylem木質部
External Organization of Stems
• The shoot is a stem
with included leaves
• Nodes are where
leaves are attached
• Internodes are the
regions between
nodes
External Organization of Stems
• The stem area just
above the point where
a leaf attaches is the
leaf axil
• It contains:
• an axillary bud
• a miniature shoot with a
dormant apical meristem 頂
端分生組織
• several young leaves
• At the extreme tip of
each stem is a terminal
bud
External Organization of Stems
• Phyllotaxy 葉序 is the
arrangement of leaves
on the stem
• It is important in
positioning leaves so
they do not shade each
other
• If only one leaf is present
at each node, the stem
has alternate phyllotaxy
• Two leaves per node is
opposite phyllotaxy
• Three or more per node is
whorled
Stem Modification
• The stems of some plants are modified
• Stolens走莖 have especially long and thin
internodes allow for dispersal of daughter plants
• Nutrient storage is important in some shoots:
• Bulbs鱗莖 are short shoots that have thick, fleshy
leaves
• Corms球莖 are vertical, thick stems that have thin,
papery leaves
• Rhizomes根莖 are fleshy horizontal stems that allow a
plant to spread underground
• Tubers塊莖 are horizontal like rhizomes, but they grow
for only a short period and are mainly a means of
storing nutrients
Stem Modification
Bulbs鱗莖
Corms球莖
Tubers塊莖
External Organization of Stems
• Although the axil of every leaf contains a bud,
only a few buds ever develop into a branch
• Others remain dormant
• Or produce flowers
• For most plants, most axillary buds are not
needed as long as the apical meristem is healthy
• A plant hormone produced at the apical meristem
enforces the dormancy of axillary buds (頂端優勢)
• If the apical meristem is killed, no hormone produced,
axillary buds become active and replace it
Internal Organization of Stems:
Primary Tissues
• Epidermis表皮 is the
outermost surface of an
herbaceous stem
• A single layer of
parenchyma cells
• All interchange of material
between a plant and its
environment occurs by
means of its epidermis
• Functions in protection
and preventing water loss
Primary Tissues: Epidermis
• Outer tangential walls are coated with waterproof
cutin角質
• It builds up as a layer called the cuticle角質層
• Under dry condition a wax layer maybe add
external to the cuticle
Primary Tissues: Epidermis
• The cuticle prevents
desiccation but it also
prevents gas exchange
• This critical function is
accomplished by stomata
氣孔
• Guard cells保衛細胞 (a
pair)
• Stomatal pore
Primary Tissues: Epidermis
• Guard cells swell by
absorbing water
• The pore between them
opens, permitting entry of
carbon dioxide and exit of
oxygen
• Water is lost through
stomata
• Guard cells regulate
when the pores are
open/closed
• Remain closed after
sunset or during periods
of water stress
Primary Tissues: Epidermis
• Some epidermal cells
elongate outward and
become trichomes毛狀
物 (hairs)
• Protective roles
• Deter herbivory
• Minimize water loss
• Protect for over expose to
sunlight
Primary Tissues: Cortex
• Cortex is interior to the
epidermis
• Often homogenous,
composed of photosynthetic
parenchyma and sometimes
collenchyma.
• Cells are typically tightly fitted,
but some plants have a cortex
of aerenchyma通氣組織,
loosely packed with large
intercellular air spaces.
星狀厚壁細胞
細胞間隙
Primary Tissues: Vascular Tissues
• Vascular Tissues維管束組織 are responsible for
the conduction of materials throughout the plant.
• Two types of vascular tissues occur in plants:
• Xylem conducts water and minerals
• Phloem distributes sugars and minerals
• Xylem木質部 is dead and hollow at maturity
• Phloem韌皮部 is alive at maturity
Vascular Tissues: Xylem
• Xylem consists of conductive cells, tracheids管
胞 and/or vessel elements導管細胞
• Both are types of sclerenchyma
• Collectively referred to as tracheary elements管狀細胞
• Strength of cells due to the 2nd cell walls
• Simplest organization produces a set of annular
thickenings on the inside of the primary wall
• In helical thickening, the secondary wall is one to three
helices inside the primary wall
• Scalariform thickening is stronger as the extensive
secondary wall underlies most of the inner surface of
the primary wall (ladder-like pattern)
• Reticulate thickening, the secondary wall is deposited in
the shape of a net
Tracheary Elements
helical thickening
螺旋狀加厚
Scalariform thickening
梯狀加厚
annular thickening
環狀加厚
helical thickening
螺旋狀加厚
Pitted 2nd wall
紋孔壁
Reticulate thickening
網狀加厚
annular thickening
環狀加厚
Vascular Tissues: Xylem
• The most derived and strongest tracheary
elements have circular bordered pits緣孔
• The primary wall is underlain by secondary wall
• The pits are weak points in the wall
• The weakness is reduced by a border of extra wall
material
Vascular Tissues: Xylem
• Water moves between
tracheids through pit
membranes
• Vessel elements
provide a way to move
water with less friction
• Perforations穿孔 form
between vertically
stacked vessel elements
• A stack of vessel
elements is called a
vessel
Vascular Tissues: Xylem
• All plants with vascular
tissue have tracheids
• Conifers have only
tracheids
• Also found in leaf veins of
flowering plants
• Only flowering plants
have vessel elements
• Perform long-distance
water conduction in roots
and stems
Figure 05.28B: Tracheary elements. (b and e) Vessel elements tend to be wider
and shorter than tracheids, but the most important feature is the perforation.
Courtesy of N.C. Brown Center for Ultrastructure Studies, SUNY-Environmental Science and Forestry, Syracuse, NY 13210
Vascular Tissues: Phloem
• Phloem has two types of conducting cells:
• sieve cells篩細胞
• sieve tube members篩管細胞
• The term “sieve element” refers to either
• Develop from parenchyma cells that remain alive
• Plasmodesmata enlarge to become sieve pores
篩孔
• The sieve pores aggregate in sieve areas篩域
Vascular Tissues: Phloem
• A sieve cell is similar in
shaped to a tracheid.
• It is elongated and
tapered.
• Sieve areas are
distributed over its
surface
• This type is found in
older fossils and in nonflowering vascular
plants.
Vascular Tissues: Phloem
• Sieve tube members
are similar to the vessel
members of the xylem
• Sieve plates篩板on each
end-wall.
• Align vertically to form a
sieve tube
• More effective flow of sap
• All angiosperms have
sieve tubes
• None of the
nonangiosperms have
them
Vascular Tissues: Phloem
• Nuclei of sieve
elements degenerate
• No complex metabolism
with out the nucleus
• They are associated with
neighboring cells that
exert nuclear control
• Sieve cells are
associated with
albuminous cells營養細胞
• Sieve tube members
are controlled by
companion cells伴細胞
Vascular Tissues: Phloem
• Companion cells are
also involved in loading
sugars
• They are often smaller
than the accompanying
conducting cell
• They have a prominent
nucleus and dense
cytoplasm filled with
ribosomes
Organization of Vascular Tissues
• Xylem and phloem
occur together in
vascular bundles
interior to the cortex
• Arrangement of bundles
differs between monocots
and other angiosperms
• Each bundle includes
xylem and phloem
running parallel to each
other.
射髓
Organization of Vascular Tissues
• In eudicots, vascular
bundles are arranged in
one ring surrounding
the pith
• Pith is a central region of
parenchyma similar to the
cortex
• In monocots, they are
distributed as a complex
network throughout the
inner part of the stem
Organization of Vascular Tissues
• The xylem of a vascular bundle is primary xylem
• In addition to the tracheary elements there are:
• A large proportion of xylem parenchyma
• Xylem fibers
• The phloem of a vascular bundle is primary
phloem
• In addition to the sieve elements and their
associate cells, there are:
• Storage parenchyma
• Phloem fibers or sclerids石細胞.
Stem Growth and Differentiation
• Stems grow longer by
creating new cells at
their tips,
• Regions known as apical
meristems
• Cells here retain their
ability to divide
• Expanding daughter cells
push the apical meristem
upward
• Subapical meristems
contains cells dividing
and growing, producing
cells for the region below
Stem Growth and Differentiation
• In the subapical meristem, differentiation begins
as some cells stop dividing
• Some differentiate in protoxylem原生木質部 – the
first xylem to appear
• Protoxylem must remain flexible while cells around
them continue to elongate
• This limits their 2nd wall thickenings to annular and
helical.
• Some of the elongating cells will differentiate and
mature into metaxylem後生木質部
• Since the cells around it have stopped elongating
producing one of the stronger 2nd wall types is not a
problem.
Tracheid Development.
次生壁形成中
次生壁形成中、
細胞展延中
次生壁形成中、
細胞展延中
Stem Growth and Differentiation
• A similar process occurs in the outer part of each
vascular bundle
• Exterior cells mature as protophloem
• Cells closest to the metaxylem become metaphloem
• Other tissues also differentiate in the subapical
region
• Epidermis
• Pith
• Cortex
Stem Growth and Differentiation
• Protoderm is epidermal cells that are in the early
stages of differentiation
• Young xylem and phloem cells are provascular
tissues
• The equivalent stages of pith and cortex are
ground meristem
• Primary tissues are produced by apical
meristems
• Primary growth is the growth and tissue
formation that results from apical meristem
activity
Stem Growth and Differentiation
Stem Growth and Differentiation