Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 25
Plant Tissues
(Sections 25.7 - 25.9)
Albia Dugger • Miami Dade College
25.7 Secondary Growth
• Woody plants thicken (add secondary growth) by cell
divisions in lateral meristems
• Secondary growth occurs at two types of lateral meristem,
vascular cambium and cork cambium – both arise from
pericycle
Key Terms
• lateral meristem
• Vascular cambium or cork cambium; sheetlike cylinder of
meristem that gives rise to plant secondary growth
• vascular cambium
• Ring of meristematic tissue that produces secondary
xylem (wood) and phloem
• cork cambium
• In plants, a lateral meristem that gives rise to periderm
Secondary Growth
• Vascular cambium
gives rise to secondary
tissues
• Cork cambium gives
rise to periderm
Secondary Growth
cork
cambium
A Secondary growth (thickening
of older stems and roots)
occurs at two lateral meristems,
vascular cambium and cork
cambium. Vascular cambium
gives rise to secondary tissues;
cork cambium, to periderm.
vascular cambium
pith
cortex
Fig. 25.17a, p. 408
Vascular Cambium
• Divisions of vascular cambium cells produce secondary xylem
on the cylinder’s inner surface, and secondary phloem on its
outer surface
• Displaced cells of the vascular cambium divide in a widening
circle, so the tissue’s cylindrical form is maintained
Primary Growth at
Terminal and Lateral Buds
Primary Growth at Terminal and Lateral Buds
stem
surface
primary xylem
primary phloem
vascular cambium
B In spring, primary growth
resumes at terminal and lateral
buds. Secondary growth resumes
at vascular cambium. Divisions of
meristem cells in the vascular
cambium expand the inner core of
xylem, displacing the vascular
cambium (orange) toward the
surface of the stem or root.
secondary
xylem
secondary
phloem
Fig. 25.17b, p. 408
Growth at a Vascular Cambium
Growth at a Vascular Cambium
outer surface of
stem or root
division
Vascular
cambium
cell as
secondary
growth
starts
division
One of the two
descendant cells
differentiates
into a xylem
cell (blue); the
other stays
meristematic.
One of the two
descendant cells
differentiates
into a phloem
cell (pink); the
other stays
meristematic.
The pattern of
cell division and
differentiation into
xylem and phloem
continues through
growing season.
C Overall pattern of growth at vascular cambium.
Fig. 25.17c, p. 408
Growth at a Vascular Cambium
outer surface of
stem or root
division
division
Vascular One of two
cambium daughter cells
differentiates
cell as
secondary into a xylem
cell (blue); the
growth
other stays
starts
meristematic.
One of two
daughter cells
differentiates
into a phloem
cell (pink); the
other stays
meristematic.
The pattern of
cell division and
differentiation into
xylem and phloem
continues through
growing season.
C Overall pattern of growth at vascular cambium.
Stepped Art
Fig. 25.17c, p. 408
ANIMATION: Secondary growth
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Cork Cambium
• Cork cambium forms and gives rise to periderm
• Periderm consists of parenchyma and cork, and the cork
cambium that produces them
• Bark consists of all of the living and dead tissues outside the
vascular cambium
• The cork component of bark protects, insulates, and
waterproofs a stem or root surface
Key Terms
• bark
• Secondary phloem and periderm of woody plants
• cork
• Component of bark
• Protects the surfaces of woody stems and roots
• Has densely packed rows of dead cells with walls
thickened by a waxy substance (suberin)
Wood
• Secondary xylem (wood) is classified by its location and
function, as in heartwood or sapwood
• wood
• Accumulated secondary xylem
• heartwood
• Dense, dark accumulation of nonfunctional xylem at the
core of older tree stems and roots
• sapwood
• Functional secondary xylem between the vascular
cambium and heartwood in an older stem or root
Structure of a Woody Stem
Structure of a
Woody Stem
sapwood
(new xylem)
heartwood
(old xylem)
bark
secondary phloem
vascular
cambium
periderm (includes
cork cambium, cork,
some phloem, and
new parenchyma)
A Structure
of a typical
woody stem.
Fig. 25.18a, p. 409
Tree Rings
• Rings visible in heartwood and sapwood are regions of early
and late wood
• Early wood forms during wet springs
• Late wood indicates a dry summer or drought when no largediameter xylem cells were made for water uptake
• In most temperate zone trees, one ring forms each year
Early Wood and Late Wood
Early Wood and Late Wood
vessel in xylem
early late early late
early
direction of growth
late
early late
early
B Early and late wood in an ash tree. Early wood forms
during wet springs. Late wood indicates that a tree did not
waste energy making large-diameter xylem cells for water
uptake during a dry summer or drought.
Fig. 25.18b, p. 409
Key Concepts
• Secondary Growth
• In many plants, secondary growth thickens branches and
roots during successive growing seasons
• Extensive secondary growth of eudicots and conifers
produces wood
• Tree rings can be used to study past environmental
conditions
ANIMATION: Growth in a Walnut Twig
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ANIMATION: Secondary Growth in a Root
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25.8 Variations on a Stem
• Specialized stems allow some plants to store nutrients, to
reproduce asexually, or both
• Specializations include stolons, rhizomes, bulbs, corms,
tubers, and cladodes
Stolons
• Stolons (runners) are
stems that branch from
the main plant stem
• Adventitious roots and
leafy shoots sprout from
nodes and develop into
new plants
• Example: strawberry
Rhizomes
• Rhizomes are fleshy,
primary stems that grow
under the soil, parallel
to its surface
• They are the plant’s
primary storage tissue
• Example: turmeric
Bulbs
• A bulb is a short
underground stem with
overlapping layers of
thick, modified leaves
(scales)
• Contains starch and
other stored products
• Example: onion
Corms
• A corm is a thickened
underground stem that
stores nutrients
• Unlike a bulb, a corm is
solid rather than layered
• Example: taro
Tubers
• Tubers are thickened
portions of underground
stolons
• They are the plant’s
primary storage tissue
• Example: potato
Cladodes
• Cladodes are flattened,
fleshy, photosynthetic
stems that store water
• Example: cactuses
Key Concepts
• Modified Stems
• Certain types of stem specializations are adaptations for
storing water or nutrients, and for reproduction
25.9 Tree Rings and Old Secrets
• Many trees form one ring each year
• Tree rings hold information about environmental conditions
that prevailed while the rings were forming
• Example: Relative thicknesses of rings reflect the availability
of water – rings show that settlers who arrived at Roanoke
Island in 1587 suffered a major drought
A Record of Rainfall
• A section of a bald cypress tree that was living near English
colonists when they first settled in North America – narrower
annual rings mark years of severe drought
Tree Rings and Old Secrets
• Tree rings are used to
date archaeological
ruins; gather evidence
of wildfires, floods,
landslides, and glacier
movements; and study
the ecology and effects
of parasitic insect
populations
Tree Rings
and Old
Secrets
year:
1
2
3
p. 411
Some Tree Rings
Some Tree Rings
A Pine is a softwood. It grows fast, so it tends to have wider rings than
slower-growing species. Note the difference between the appearance
of heartwood and sapwood.
Fig. 25.19a, p. 411
Some Tree Rings
B The rings of this oak tree show dramatic differences in yearly
growth patterns over its lifetime.
Fig. 25.19b, p. 411
Some Tree Rings
C An elm made this series between 1911 and 1950.
Fig. 25.19c, p. 411
Sequestering Carbon in Forests (revisited)
• Compared to other organic materials, decomposition of plant
matter is relatively slow, because molecules that waterproof
and reinforce plant cells are relatively stable
• A forest stops accumulating carbon as its trees mature and its
soil becomes saturated with organic matter
ANIMATION: Annual Rings
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