Lecture 7: Plant Structure and Function

I. Background

A. Challenges for terrestrial plants

1. Habitat is divided

a. Air is the source of CO2 for photosynthesis

i. Sunlight cannot penetrate soil

b. Soil provides water and dissolved minerals

B. Plants have two interdependent systems

1. Root system

2. Aerial shoot system

3. Roots depend on shoots for sugar and other organic nutrients

4. Shoots depend on roots for minerals, water and structural support

II. Plant Body

A. Root system

1. Function

a. Anchor

b. Absorb and conduct water and nutrients

c. Store food

2. Types

a. Taproot

i. One large, vertical root with many smaller secondary roots

ii. Firm anchorage

b. Fibrous root

i. Mat of threadlike roots

ii. Extensive exposure to soil water and minerals

3. Root hairs

a. Increase surface area

b. Increase absorption of water

c. Mycorrhizae increase mineral and water absorption

i. Symbiotic association between roots and fungi

4. Adventitious roots

a. Roots arising above the ground from stems or leaves

B. Shoot system

1. Vegetative shoots and floral shoots

2. Include stems and leaves

3. Stems

a. Nodes

i. Point where leaves attach to the stem

b. Internodes

i. Stem segments between nodes

c. Axillary bud

i. Embryonic side shoot in the angle between leaf and stem

ii. Usually dormant

iii. Grow when terminal bud is damaged

iv. May form floral elements or other vegetative shoots

d. Terminal bud

i. Bud on shoot tip

ii. Developing leaves and a compact series of nodes and internodes

e. Modified stems

i. Stolons are horizontal stems growing along the surface of the ground

ii. Rhizomes are horizontal stems growing underground

iii. Bulbs are vertical underground shoots modified for food storage

4. Apical dominance

a. Presence of terminal bud inhibits the development of axillary buds

b. Growth of shoot is concentrated at the apex

5. Leaves

a. Photosynthetic organs of a plant

b. Flattened blade which is joined to the node of a stem by a petiole

i. Most monocots lack petioles

c. Leaves vary in the arrangement of their veins

i. Monocots have parallel veins running the length of the leaf

ii. Dicots have a branched network of major veins

iii. Veins may be organized in a pinnate or palmate fashion

d. Classification includes the special organization of the leaves on a stem

III. Plant Cells

A. Types

1. Parenchyma

2. Collenchyma

3. Sclerenchyma

4. Tracheids and vessel elements

5. Sieve-tube members

B. Parenchyma

1. Characteristics

a. Least specialized

b. Primary cell wall is thin and flexible

c. Lack secondary cell wall

d. Protoplast has a large central vacuole

e. Function in synthesizing and storing organic products

i. Photosynthesis occurs in the chloroplasts of mesophyll cells

f. Some cells in stems and roots have plastids for starch storage

g. Most mature cells do not divide

C. Collenchyma

1. Characteristics

a. Primary cell wall is thicker than parenchyma

b. Lack secondary cell wall

c. Living cells that elongate as the stem grows

d. Grouped in strands to support young plants

D. Sclerenchyma

1. Characteristics

a. Function in support

b. Thick secondary cell wall

c. Lack protoplasts with maturity

i. May be dead

d. Two forms

i. Fibers

ii. Sclerids

E. Water conducting cells of xylem: tracheids and vessel elements

1. Both have secondary cell walls and are dead at maturity

2. Tracheids

a. Long, thin, tapered cells

i. Lignin-hardened secondary cell walls

b. Have pits that allows water to flow cell to cell

c. Also function in support

3. Vessel elements

a. Wider, shorter, thin-walled, and less tapered than tracheids

b. Aligned end to end

c. Walls are perforated to permit free flow of water through long chains of vessel elements

F. Food conducting cells of phloem: sieve tube members

1. Sieve tube members are chains of phloem cells that transport organic compound and some minerals

a. Alive at functional maturity

b. Protoplasts lack a nucleus, ribosomes and a distinct vacuole

2. Angiosperms have end walls called sieve plates

3. Companion cells are connected to each sieve-tube member by multiple plasmodesmata

a. Load sugar into sieve-tube members

b. Perform cellular functions sieve-tube members cannot perform due to the absence of nuclei,

etc.

G. Organization of plant cells

1. Three tissue systems present in each plant organ (leaf, stem, root)

a. Dermal

b. Vascular

c. Ground tissue

2. Tissue systems are continuous throughout the plant

3. Dermal tissue systems (epidermis)

a. Single layer of tightly packed cells covering and protecting young plant parts

b. Unique characteristics required for the functioning of the organs it covers

i. Root hairs are extensions of epidermal cells

ii. The waxy cuticle is secreted by epidermal cells

4. Vascular tissue system

a. Xylem and phloem that function in transport and support

b. Continuous throughout the plant

5. Ground tissue system

a. Predominantly parenchyma

b. Occupies space between dermal and vascular tissue systems

c. Functions in photosynthesis, storage and support

IV. Plant Growth

A. Background

1. Meristems generate cells for new organ growth throughout the life of the plant

a. Growth begins with germination and continues for the lifespan of a plant

2. Plants have finite life spans

a. Most are determined genetically

i. Annuals complete their life cycle in on growing season

ii. Biennials typically have life spans of two growing seasons

iii. Perennials life many years

b. Some are determined environmentally

3. Indeterminate growth results from meristems

a. Meristems are perpetually embryonic tissues

b. Indeterminate growth refers to continued growth as long as the plant lives

i. Not seen in most animals

4. Meristematic cells are unspecialized cells and divide to generate new cells near the growth point

a. Initials

i. Cells formed by division that remain in the region to make new cells

b. Derivatives

i. Displaced new cells that become incorporated and specialized into tissues

5. Types of meristems

a. Apical

i. Located at tips of roots and stems

ii. Provide cells for plants to grow in length

b. Lateral

i. Cylinders of dividing cells extending along the length of roots and shoots

ii. Produce secondary dermal tissues which are thicker and more durable than the

epidermis it replaces

6. Types of growth

a. Primary

i. Initiated by apical meristem and forms primary tissues organized into the three tissue

types

b. Secondary

i. Thickening of roots and shoots

ii. Results from lateral meristems

B. Primary growth of roots

1. Growth occurs near the root tip

2. Root tip is covered by the root cap

a. Protects the meristem

b. Lubricates the soil to permit the growing root to advance in the soil

3. Root tip has three zones of cells in successive stage of primary growth

a. Zone of cell division

i. Apical meristem produces the primary meristems that produce the three tissue

systems

b. Zone of elongation

i. Cells elongate pushing the root through the soil

c. Zone of maturation

i. Farthest from the tip

ii. Region where cells become specialized in structure and function

iii. Three tissue systems complete their differentiation

C. Primary tissues of roots

1. Protoderm is the outmost primary meristem and gives rise to epidermis

a. Water and minerals must cross this layer of cells

2. Procambium forms the stele where xylem and phloem develop

a. In dicots the xylem radiates from the stele’s center in two or more spokes

b. In monocots a central pith is ringed by vascular tissue

i. Pith is a core of parenchyma cells

3. Ground meristem is located between the protoderm and the procambium and gives rise to ground

tissue

a. Mostly parenchyma and fills the cortex

b. Stores food

c. Active in mineral uptake

d. Has endodermis

i. Single-cell thick, innermost layer of the cortex that selectively regulates the passage

of substance from the soil to the stele

4. Pericycle is a layer of cells just inside the endodermis that may become meristematic and divide to

form a lateral root

a. Lateral root forms as a clump of cells in the pericycle

i. It elongates and pushes through the cortex

b. The lateral root maintains its vascular connection to the stele

C. Primary growth of shoots

1. Apical meristem of the shoot

a. Mass of dividing cells at the tip of a terminal bud

b. Produces the primary meristems that produce the three tissue systems

2. Leaf primordial flank the apical meristem and give rise to leaves

3. Meristematic cells left by the apical meristem at the base of the leaf primordial develop into axillary

buds

a. Bud primordium

4. Most elongation results from growth of internodes

a. Cell division and elongation of cells within the internode

5. Primary tissues of the stem

a. Vascular tissue is organized into vascular bundles that run the length of the stem

b. Each bundle is surrounded by ground tissue

c. In dicots, bundles are arranges with pith inside and cortex outside

i. Xylem faces the pith

ii. Phloem faces the cortex

d. In Monocots, vascular bundles are scattered throughout the ground substance

i. Ground substance is mostly parenchyma

D. Tissue organization of leaves

1. Outer layer is comprised of a tightly interlocked layer of epidermal cells

a. Protects against damage

b. Waxy cuticle reduces water loss

c. Stomata regulate gas exchange

i. Permit transpiration

ii. More numerous in lower epidermis

2. Mesophyll

a. Ground tissue

b. Mostly parenchyma cells

i. Include chloroplasts

c. Dicots have two types of mesophyll

i. Palisade parenchyma in upper half of a leaf

ii. Spongy parenchyma in the lower half

d. Spongy parenchyma has irregular shaped cells with air spaces for gas circulation

3. Vascular tissue is continuous with that of the stem

a. Leaf traces are branches off the stem vascular bundles

b. Continue in the petiole as veins

i. Veins branch repeated through the mesophyll of the leaf blade

V. Secondary Growth

A. Background

1. Lateral meristems add girth by producing secondary vascular tissue and periderm

2. Secondary plant body results from secondary growth

3. Secondary plant body is comprised of secondary tissues

4. Secondary growth results from two lateral meristems

a. Vascular cambium

b. Cork cambium

5. Vascular cambium produces secondary xylem and phloem

6. Cork cambium produces a tough, thick covering for roots and stems that replaces epidermis

7. Secondary growth occurs in gymnosperms and most dicot angiosperms

a. Secondary growth is rare in monocots

B. Secondary growth of stems

1. Vascular cambium forms when meristematic cells develop between the primary xylem and primary

phloem of each vascular bundle in the rays of ground tissue between the bundles

2. Meristematic cells form a continuous ring of dividing cells around the xylem and pith of the stem

3. New vascular tissue is produced

a. Secondary xylem to the inside of the vascular cambium

b. Secondary phloem to the outside

4. Cork cambium

a. Cylinder of meristematic tissue that forms the protective layers of the secondary plant body

b. Cork cells form to the outside of the cork cambium

c. Cork cells mature and deposit suberin in their walls

i. Cork cells die

d. Dead cork tissues protect the stem

5. Periderm

a. Protective coat of the secondry plant body

b. Replaces the primary epidermis

c. Combination of cork cambium, layers of cork, and phelloderm

i. Phelloderm is parenchyma cells that forms to the inside of the cork cambium

C. Secondary growth of roots

1. Involves both vascular cambium and cork cambium

2. Vascular cambium produces secondary xylem to the inside and secondary phloem to the outside

a. It is first located between xylem and phloem of the stele

b. The cortex and epidermis split and are shed as the stele grows in diameter

3. Cork cambium forms the pericycle of the stele and produces the periderm

a. Periderm becomes secondary dermal tissue

i. Impermeable to water

ii. Roots with secondary growth function to anchor the plant and transport water and

solutes between younger roots and the shoot system