Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

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Chapter 25Plant Tissues

(Sections 25.7 - 25.9)

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

Fig. 25.17a, p. 408

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

cortexpith

corkcambium

Secondary Growth

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

Fig. 25.17b, p. 408

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

stem surface

vascular cambium

primary xylem primary phloem

secondaryphloem

Primary Growth at Terminal and Lateral Buds

Growth at a Vascular Cambium

Fig. 25.17c, p. 408

C Overall pattern of growth at vascular cambium.

outer surface of stem or root

The pattern of cell division and differentiation into xylem and phloem continues through growing season.

One of the two descendant cells differentiates into a phloem cell (pink); the other stays meristematic.

One of the two descendant cells differentiates into a xylem cell (blue); the other stays meristematic.

Vascular cambium cell as secondary growth starts

divisiondivision

Growth at a Vascular Cambium

The pattern of cell division and differentiation into xylem and phloem continues through growing season.

One of two daughter cells differentiates into a xylem cell (blue); the other stays meristematic.

division

One of two daughter cells differentiates into a phloem cell (pink); the other stays meristematic.

outer surface of stem or root

division

Vascular cambium cell as secondary growth starts

Fig. 25.17c, p. 408

Stepped Art

C Overall pattern of growth at vascular cambium.

Growth at a Vascular Cambium

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

Fig. 25.18a, p. 409

A Structure of a typical woody stem.

periderm (includes cork cambium, cork, some phloem, and new parenchyma)

secondary phloem

bark

vascular cambium

heartwood (old xylem)

sapwood (new xylem)

Structure of a Woody Stem

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 large-diameter xylem cells were made for water uptake

• In most temperate zone trees, one ring forms each year

Early Wood and Late Wood

Fig. 25.18b, p. 409

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.

direction of growthvessel in xylem

early early early early earlylatelatelatelate

Early Wood and Late Wood

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

p. 411

2year: 1 3Tree Rings and Old Secrets

Some Tree Rings

Fig. 25.19a, p. 411

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.

Some Tree Rings

Fig. 25.19b, p. 411

B The rings of this oak tree show dramatic differences in yearly growth patterns over its lifetime.

Some Tree Rings

Fig. 25.19c, p. 411

C An elm made this series between 1911 and 1950.

Some Tree Rings

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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