Plant Structure and Function

Plant Hierarchy

• Plants are composed of different tissues which are made from different types of cells.

• Tissue – a group of cells with a common function, structure, or both

• Organ – consists of several types of tissues that together carry out particular functions.

Basic plant organs

• Plants have three basic organs: roots, stems, and leave.

• These organs form a root system, and a shoot system (stem and leaves).

Roots

• Anchors a plant in soil, absorbs water, and often stores carbohydrates.

• Many roots are taproots, such as carrots having one main root and smaller branches outward

• Most of the absorption of water and minerals occurs primarily near the tips of roots where tiny root hairs increase the surface area.

Stems

• Organ consisting of alternating systems of notes (points where leaves attach) and internodes (stem segments).

• Axillary bud – a structure that can form a lateral root, or branch, mostly dormant in a young plant

• Apical bud, or terminal bud – near the shoot tip

• Inhibition of axillary buds by an apical bud is called apical dominance.

stems

• Apical dominance increases a plant’s exposure to light.

• Removal of the apical bud, usually stimulates the growth of axillary buds.

Leaves

• The leaf is the main photosynthetic organ (green stems as well)

• Leaves vary extensively but generally consist of a flattened blade, a stalk and a petiole (joins the leaf to the stem at a node)

Dermal, Vascular, and Ground Tissues

• Each plant organ has dermal, vascular, and ground tissues.

• The dermal tissue is the plant’s out protective covering (like skin) it forms the first line of defense against physical damage and pathogens

Fig. 35-8

Dermaltissue

Groundtissue Vascular

tissue

• In non-woody plants dermal tissue consists of the epidermis

• A waxy coating called the cuticle helps prevent water loss from the epidermis

• In woody plants, a protective tissue called periderm replaces the epidermis in older regions

Vascular tissue

• Vascular tissue carries out long-distance transport of materials between roots and shoots.

• The two vascular tissues are xylem and phloem

• Xylem moves water and dissolved minerals upward from roots into the shoots.

• Phloem transports organic nutrients from where they were made to where they are needed.

Ground Tissue

• Tissues that are neither dermal nor vascular are the ground tissue.

• Ground tissue internal to the vascular tissue is pith.

• Ground tissue external to the vascular tissue is cortex.

• Ground tissue includes specialized cells for storage, photosynthesis, and support.

Plant cell types: parenchyma cells

• Have thin flexible walls

• Are the least specialized

• Perform most metabolic functions

• Retain the ability to divide and differentiate

Fig. 35-10a

Parenchyma cells in Elodea leaf,

with chloroplasts (LM)60 µm

Collenchyma cells

• Are grouped into strands and help support the young parts of the plant shoot

• Have thicker uneven cell walls

• Provide flexible support without restraining growth

Fig. 35-10b

Collenchyma cells (in Helianthus stem) (LM)

5 µm

Schlerenchyma cells

• Are rigid because of thick secondary walls that are strengthened

• They are dead at functional maturity

Fig. 35-10c

5 µm

25 µm

Sclereid cells in pear (LM)

Fiber cells (cross section from ash tree) (LM)

Cell wall

Water conducting cells of the xylem

• The two types of water-conducting cells, tracheidsand vessel elements are dead at maturity

Fig. 35-10d1

Vessel Tracheids 100 µm

Tracheids and vessels(colorized SEM)

Sugar-conducting cells of the phloem

• Sieve-tube elements are alive at functional maturity, but lack organelles

• Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube

• Each sieve-tube element has a companion cell whose nucleus and ribosomes serve both cells

Fig. 35-10e

Sieve-tube element (left)and companion cell:cross section (TEM)

3 µmSieve-tube elements:longitudinal view (LM)

Sieve plate

Companioncells

Sieve-tubeelements

Plasmodesma

Sieveplate

Nucleus ofcompanioncells

Sieve-tube elements:longitudinal view Sieve plate with pores (SEM)

10 µm

30 µm

Fig. 35-10e3

Sieve-tubeelement

Plasmodesma

Sieveplate

Nucleus ofcompanioncells

Sieve-tube elements:longitudinal view Sieve plate with pores (SEM)

10 µm

meristems

• Meristems generate cells for new organs.

• A plant can grow throughout its life

• Annuals – complete their life cycle in a year or less

• Biennials – require two growing seasons

• Perennials – live for many years

Apical meristems

• Are located at the tips of roots and shoots and at the axillary buds of shoots

• Apical meristems elongate shoots and roots, called primary growth

Lateral meristems

• Lateral meristems add thickness to woody plants, a process called secondary growth.

• There are two lateral meristems: the vascular cambium and the cork cambium

• The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem

• The cork cambium replaces the epidermis with periderm, which is thicker and tougher

Fig. 35-11

Shoot tip (shootapical meristemand young leaves)

Lateral meristems:

Axillary budmeristem

Vascular cambium

Cork cambium

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

Periderm

Corkcambium

Cortex

Primaryphloem

Secondaryphloem

Pith

Primaryxylem

Secondaryxylem

Vascular cambium

Fig. 35-12Apical bud

This year’s growth

(one year old)

Bud scale

Axillary buds

Leaf

scar

Bud

scar

Node

Internode

One-year-old side

branch formed

from axillary bud

near shoot tip

Last year’s growth

(two years old) Leaf scar

Stem

Bud scar left by apical

bud scales of previous

winters

Leaf scar

Growth of two

years ago

(three years old)

Fig. 35-13

Ground

Dermal

Key

to labels

Vascular

Root hair

Epidermis

Cortex Vascular cylinder

Zone of

differentiation

Zone of

elongation

Zone of cell

division

Apical

meristem

Root cap

100 µm

Roots and Root Growth

Fig. 35-14Epidermis

Cortex

Endodermis

Vascular

cylinder

Pericycle

Core of

parenchyma

cells

Xylem

Phloem100 µm

Root with xylem and phloem in the center

(typical of eudicots)

(a)

Root with parenchyma in the center (typical of

monocots)

(b)

100 µm

Endodermis

Pericycle

Xylem

Phloem

50 µm

Key

to labels

Dermal

Ground

Vascular

Fig. 35-14a1

Root with xylem and phloem in the center

(typical of eudicots)

(a)

100 µm

Epidermis

Cortex

Endodermis

Vascular

cylinder

Pericycle

Xylem

Phloem

Dermal

Ground

Vascular

Key

to labels

Fig. 35-14a2

Vascular

Ground

Dermal

Key

to labels

Root with xylem and phloem in the center

(typical of eudicots)

(a)

Endodermis

Pericycle

Xylem

Phloem

50 µm

Arrangement of stem tissues

• In plants called monocots, the vascular bundles are spread out throughout the ground tissue.

• In plants called dicots, the vascular bundles are in a ring around the inner ground tissue, with a thin layer of ground tissue surrounding.

Fig. 35-17a

Sclerenchyma

(fiber cells)

Phloem Xylem

Ground tissue

connecting

pith to cortex

Pith

CortexEpidermis

Vascular

bundle

1 mm

Cross section of stem with vascular bundles forming

a ring (typical of eudicots)

(a)

Dermal

Ground

Vascular

Key

to labels

Fig. 35-17b

Ground

tissue

Epidermis

Key

to labels

Cross section of stem with scattered vascular bundles

(typical of monocots)

Dermal

Ground

Vascular

(b)

Vascular

bundles

1 mm

Tissue organization of leaves

• The epidermis in leaves has breaks in it called stomata, which allow CO2 exchange between the air and the photosynthetic cells in a leaf.

• Each stomatal pore is surrounded by two guard cells, which regulate its opening and closing.

• The ground tissue in a leaf, called mesophyll, is sandwiched between the upper and lower epidermis

Tissue organization in leaves

• The upper layer of mesophyll is called palisade mesophyll where photosynthesis mostly occurs.

• Below this layer is a loosely arranged layer called the spongy mesophyll, where gas exchange occurs

• The veins or vascular tissue connect with those of the stem.

• Each vein is surrounded by a protective bundle sheath.

Fig. 35-18a

Key

to labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-

sheath

cell

Xylem

Phloem

(a) Cutaway drawing of leaf tissues

Guard

cells

Vein

Cuticle

Lower

epidermis

Spongy

mesophyll

Palisade

mesophyll

Upper

epidermis

Fig. 35-18b

Guard

cells

Stomatal

pore

Surface view of a spiderwort

(Tradescantia) leaf (LM)

Epidermal

cell

(b)

50

µm

Fig. 35-18c

Upperepidermis

Palisademesophyll

Keyto labels

Dermal

Ground

Vascular

Spongymesophyll

Lowerepidermis

Vein Air spaces Guard cells

Cross section of a lilac(Syringa) leaf (LM)

(c)

10

0 µ

m

Secondary Growth

• Secondary growth adds girth to stems and roots in woody plants, (it doesn’t occur in leaves)

• The secondary tissues are produced by the vascular cambium and cork cambium

Fig. 35-19a1

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Fig. 35-19a2

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Cork

Bark

Most recent corkcambium

Layers ofperiderm