NOTES: CH 35 - Plant Structure & Growth

In their evolutionary journey, plants adapted to the problems of a

terrestrial existence as they moved from water to land

ANGIOSPERMS (flowering plants)

-most diverse and widespread of plants (about 275,000 species!)

-characterized by FLOWERS and FRUITS (adaptations for reproduction and seed dispersal)

*Angiosperms are divided into two taxonomic classes:

 

1) MONOCOTS

2) DICOTS

                    

Terrestrial plants faced a divided habitat:

air is the source of CO2 for

photosynthesis (not dissolved in water)

sunlight cannot penetrate

soil

soil provides water and dissolved minerals to plant

To accommodate this divided habitat, plants have two basic systems:

1) ROOT SYSTEM

(subterranean)

2) SHOOT SYSTEM

(aerial)

Roots and Shoots are interdependent!!!

*Roots depend on shoots

for sugar and other

organic nutrients

*Shoots depend on

roots for minerals,

water and support

THE ROOT SYSTEM:

Root structure is well adapted to:

> Anchor plants

> Absorb and conduct water and nutrients

> Store food

Two major types of root systems:

1) TAPROOT SYSTEM:

-one large, vertical root

-provides firm

anchorage in soil

-modified taproots:

carrots, turnips,

sweet potatoes

2) FIBROUS ROOT SYSTEM:

-mat of threadlike roots spread out below soil

-provides exposure to soil water and minerals

-roots concentrated in upper

few cm of soil

-helps to prevent soil

erosion

*Absorption of water is greatly enhanced by ROOT HAIRS

THE SHOOT SYSTEM:

• Stem morphology includes:

-axillary buds = embryonic side shoots in angle of leaf and stem; usually dormant

-terminal bud = bud on a shoot tip

• Growth of a shoot is concentrated at the apex of the shoot (TERMINAL BUD)

**the terminal bud inhibits development of axillary buds – this is called

APICAL DOMINANCE

LEAVES *the main photosynthetic organs of a plant

*leaves of monocots and dicots differ:

-monocot leaves have parallel veins

-dicot leaves have a branched network of veins

PLANT CELL OVERVIEW: *Recall: in addition to the structures in

other eukaryotic animal cells, PLANT CELLS have unique organelles:

 -CELL WALL-CHLOROPLASTS-LARGE, CENTRAL VACUOLE-TONOPLAST (membrane around vacuole)

TypicalPlantCell

Specialized Plant Cells: 1) Parenchyma cells:

-least specialized of plant cells-function in synthesizing and storing organic products

Specialized Plant Cells:

2) Collenchyma cells:-usually grouped in strands to support young parts of plants-elongate as the stems and leaves grow-e.g.: make up the strings in celery

Specialized Plant Cells: 3) Sclerenchyma cells:

-function in support-FIBERS: long, slender, tapered cells-SCLERIDS: shorter, irregularly-shaped cells

Each organ of a plant (leaf, stem, root) has three tissue systems:

1) DERMAL

2) VASCULAR

3) GROUND

• DERMAL TISSUE SYSTEM (epidermis)

-covers and protects plant

-root hairs are extensions of epidermal cells near root tips

-waxy CUTICLE secreted by epidermal cells of the leaf and stem

• VASCULAR TISSUE SYTEM

-xylem and phloem

-functions in transport and support; continuous throughout plant

Specialized Plant Tissues: ● XYLEM: water-conducting vessels

● PHLOEM: sugar / sap-conducting vessels

Specialized Plant Tissues (continued):

Food-conducting cells of PHLOEM include:

● at least one COMPANION CELL is connected to

each phloem segment by openings called

PLASMODESMATA

• GROUND TISSUE SYSTEM -predominantly parenchyma

-occupies space between dermal and vascular tissues

-functions in: photosynthesis, storage, support

PLANT GROWTH!

*Plant growth begins with germination of the seed and continues for the lifespan of

the plants

MERISTEMS:

-perpetually embryonic tissues

-unspecialized and constantly dividing

Two types of plant growth:

1) PRIMARY GROWTH: elongation

2) SECONDARY GROWTH: thickening of roots and shoots

-APICAL MERISTEMS: located in root tips and shoot buds; involved in primary growth

-LATERAL MERISTEMS: cylinders of dividing cells extending along the lengths of roots and shoots; involved in secondary growth

Primary Growth of Roots:

• root growth is concentrated near its tip and results in roots extending through the soil

• the root is covered by a

ROOT CAP which

protects the meristem

and secretes a polysaccharide

coating that lubricates

the soil ahead of the

growing root

• the root tip contains 3 zones of cells:

*ZONE OF CELL DIVISION:

near tip of root;

-includes apical meristem

*ZONE OF CELL ELONGATION: above the zone of cell division

-cells elongate to at least 10x their original length

-elongation of cells here pushes the root tip through the soil

*ZONE OF MATURATION:

located farthest from the

root tip

-where new cells become

specialized

Primary Tissues of Roots:

1) PROTODERM:

gives rise to epidermis

Primary Tissues of Roots:

2) PROCAMBIUM: forms a STELE (central cylinder) where xylem and phloem develop

*difference between monocots and dicots

Primary Tissues of Roots:3) GROUND MERISTEM:

-gives rise to ground tissue system;

-fills the CORTEX (root area between stele and epidermis);

-stores food;

-has ENDODERMIS (innermost layer of cortex; forms boundary between cortex and stele)

*LATERAL ROOTS may sprout from the outermost layer of the stele (the PERICYCLE) as a result, the lateral root maintains its vascular connection to the stele of the main root

TISSUE ORGANIZATION OF STEMS:• vascular tissue is organized into strands of

VASCULAR BUNDLES that run the length of the stem;

• these converge at the shoot root transition to join the root stele

• each vascular bundle is surrounded by ground tissue

DICOTS:

-ground tissue includes PITH (inside) and CORTEX (outside)

-bundles arranged in a RING with:

XYLEM facing PITH

PHLOEM facing CORTEX

MONOCOTS:-vascular bundles scattered throughout the

ground tissue of stem

TISSUE ORGANIZATION OF LEAVES:

• EPIDERMIS: protects against water loss and pathogens

• STOMATA: pores flanked by GUARD CELLS which regulate gas exchange and allow for TRANSPIRATION (water loss)

stomata are more numerous on the bottom surface of the leaf…WHY?

MONOCOT =parallel veins

DICOT =networkveins

TISSUE ORGANIZATION OF LEAVES:

• the ground tissue inside the leaf is called: MESOPHYLL

-consists mainly of parenchyma cells equipped to perform photosynthesis

Mesophyll

TISSUE ORGANIZATION OF LEAVES:

*Dicots have two mesophyll regions:1) Palisade parenchyma: upper half of leaf; cells are column-shaped2) Spongy parenchyma: lower half of leaf; irregularly shaped cells surrounded by air spaces

TISSUE ORGANIZATION OF LEAVES:

**Vascular bundles from stem branch repeatedly throughout leaf blade forming VEINS

this brings the photosynthetic tissue of the leaf in close contact with the xylem and phloem

also forms a skeleton to support the shape of the leaf

SECONDARY GROWTH:• results in thickening of stems / roots

(seen in gymnosperms & some dicots)

• results from two lateral meristems:

1) VASCULAR CAMBIUM: produces secondary xylem and phloem

2) CORK CAMBIUM: produces a tough, thick covering for roots and stems that replaces the epidermis

VASCULAR CAMBIUM:

• forms between the 1º xylem and phloem of each vascular bundle

• a continuous cylinder of dividing cells:

-2º xylem cells form towards the inside

-2º phloem cells form

towards the outside

*accumulated layers of 2º xylem produces WOOD

VASCULAR CAMBIUM:**forms annual growth rings due to yearly

activity:

1) cambium dormancy (winter)

2) spring wood production

3) summer wood production

*2º phloem does not accumulate extensively; it becomes part

of bark and eventually

sloughs off the tree trunk

CORK CAMBIUM:• cylinder of meristematic tissue that forms in

the outer cortex of the stem• cork cells form to the outside; as these cells

mature, they deposit a waxy material (SUBERIN) in their walls and die

• these dead cork tissues protect the stem from damage and pathogens and reduce water loss

BARK = all tissues external to the vascular cambium phloem, cork cambium and cork

Secondary Growth in Roots:-similar to stems

-in roots, the outer layer is impermeable to water…consequently, the roots with 2º growth function to anchor the plant and to transport water and nutrients between the younger roots and the shoot system