Inside Stems

Chapter 3

Tissue Patterns of Stems

Steles

central cylinder of vascular tissue

bounded by the cortex

We will learn patterns in three types:

herbaceous dicots

woody dicots

herbaceous monocots

 

Herbaceous Dicot Stems

Vascular bundles in discrete patches

Xylem oriented toward center of stem

Phloem oriented toward surface of stem

 

Herbaceous Dicot Stems

Woody Dicot Stem

Xylem

Ray cells: long-lived parenchyma cells involved in lateral conduction, produced by ray initials

Wood

Wood is secondary xylem tissue

Annual rings

Spring wood + summer wood

Tree age can be determined

 

Tree rings

Rings appear in wood because of change in size of xylem cells produced over the growing season

Spring time good growing (water and warmth) large cells, appear lighter, spring wood, early wood

Later in summer, drier conditions, more energy used for reproduction, cells smaller, summer wood, late wood

 

 

Dendrochronology (tree time)

Climate conditions during the tree's life can be reconstructed

wide ring = good growth year (plenty of moisture or warmth)

small ring = poor growth year (usually too dry or too cold)

Heartwood and sapwood

Heartwood found in center of stem, usually dark in color

Sapwood found closest to the vascular cambium, remains functional in water conduction

Bark

Inner bark: primary and secondary phloem tissues

Outer bark: periderm

Monocot Stems

No lateral meristems (no vascular or cork cambium)

Do not attain great sizes (grasses; lilies)

Vascular bundles scattered throughout stem

parenchyma between vascular bundles called ground or fundamental tissue

 

Monocot Stems

Vascular bundles

Xylem oriented toward center of stem

Phloem oriented toward surface of stem

In corn, concentration of bundles beneath the epidermis

strengthens stem against weight of leaves and ears of corn

 

Monocot stem

Monocot stem

Palm trees

Some attain considerable size without true cambium

Parenchyma cells divide and enlarge

Fiber cells give flexible strength

Fibers from leaves and stems of monocots

Manila hemp, sisal (individual vascular bundles)

Wood and Its Uses

Composition

Cellulose (60-75%)

found in all plant cell walls, makes them stiff

Lignin (15-25%)

only in woody plants, adds strength to wood

Other substances present

resins, gums, oils, dyes, tannins, starch

 

Physical Properties

Density

specific gravity is the comparative density of woods

Durability

hardness

Flexibility

Strength: Weight ratio

generally, a piece of wood is stronger than most other materials of the same weight, but varies among species

 

Knots

Bases of older branches covered over with newer growth of wood

In older parts of a log, knots are found in greater concentration towards the center. Why?

Wood and its Uses

Wood Products

About half of US and Canadian wood production is used as lumber, primarily for construction.

Veneer - Thin sheet of desirable wood glued to cheaper lumber.

Second most extensive use of wood is pulp for making paper.

In developing countries, approximately half of cut timber is used for fuel.

Less than 10% in US and Canada.