Plant Structure, Growth, and Development

Chapter 35 & 36

The Cells and Tissues of the Plant BodyCells of angiosperm embryos differentiate early in

development into 3 distinct tissues:

• A.  Dermal Tissue: forms the outside covering of plants– Epidermis– Cuticle– Cork– Bark– Stomata

• B.  Ground tissue: for storage, metabolism and support. Mostly parenchyma, with specialized support cells of collenchyma and sclerenchyma

• C. Vascular tissue: phloem and xylemconsists of special conducting cells, along with support fibers & parenchyma

The Three Tissue Systems: Dermal, Vascular, and Ground

Figure 35.8

Dermaltissue

Groundtissue Vascular

tissue

“Ground” tissue:

Includes various cells specialized for functions such as storage, photosynthesis, and support

•parenchyma: cells which occur in all 3 tissue systems, usually photosynthesis, elongated, loosely packed, thin, flexible cell walls•collenchyma: primary wall (in cells) thickened at corners, irregular shapes, provide support•sclerenchyma: 2 types, support and strengthen the plant, thick, even cell walls, dead cells provide framework for additional cells   1. fibers- elongated, elastic strands or bundles associated with the vascular tissue   2. sclereids- form hard outer covering of seeds, nuts, and fruit stones

Parenchyma, collenchyma, and sclerenchyma cells

Figure 35.9

Parenchyma cells 60 m

PARENCHYMA CELLS

80 m Cortical parenchyma cells

COLLENCHYMA CELLS

Collenchyma cells

SCLERENCHYMA CELLS

Cell wall

Sclereid cells in pear

25 m

Fiber cells

5 m

Vascular Tissue

• Xylem– Conveys water and dissolved

minerals upward from roots into the shoots

• Phloem– Transports organic nutrients from

where they are made to where they are needed

Water-conducting cells of the xylem and sugar-conducting

cells of the phloem

Figure. 35.9

WATER-CONDUCTING CELLS OF THE XYLEM

Vessel Tracheids 100 m

Tracheids and vessels

Vesselelement

Vessel elements withpartially perforated end walls

Pits

Tracheids

SUGAR-CONDUCTING CELLS OF THE PHLOEM

Companion cell

Sieve-tubemember

Sieve-tube members:longitudinal view

Sieveplate

Nucleus

CytoplasmCompanioncell

30 m

15 m

Vascular tissueTransports nutrients throughout a plant; such transport may occur over long distances

Figure 36.1

MineralsH2O CO2

O2

CO2 O2

H2O Sugar

Light

• A variety of physical processes– Are involved in the different types of

transport Sugars are produced byphotosynthesis in the leaves.5

Sugars are transported asphloem sap to roots and otherparts of the plant.

6

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

4

Transpiration, the loss of waterfrom leaves (mostly through

stomata), creates a force withinleaves that pulls xylem sap upward.

3

Water and minerals aretransported upward from

roots to shoots as xylem sap.

2

Roots absorb waterand dissolved minerals

from the soil.

1

Figure 36.2

Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.

7

Transpiration is the evaporation of water from plant leaves

• Turgor loss in plants causes wilting– Which can

be reversed when the plant is watered

Figure 36.7

• Plants lose a large amount of water by transpiration• If the lost water is not replaced by absorption

through the roots– The plant will lose water and wilt

XYLEM: Several factors are at work in the movement of water and

minerals up a plant stem• To survive

– Plants must balance water uptake and loss• Water is pulled upward by negative pressure in

the xylem, caused by losses by transpiration• Cohesion• Adhesion• Osmosis

– Determines the net uptake or water loss by a cell

– Is affected by solute concentration and pressure

• Water potential– Is a measurement that combines the effects of

solute concentration and pressure

PHLOEM• Organic nutrients are translocated through

the phloem• Translocation

– Is the transport of organic nutrients in the plant• Phloem sap

– Is an aqueous solution that is mostly sucrose– Travels from a sugar source to a sugar sink

• A sugar source– Is a plant organ that is a net producer of sugar,

such as mature leaves• A sugar sink

– Is an organ that is a net consumer or storer of sugar, such as a tuber or bulb

Phloem• The pressure flow hypothesis explains why

phloem sap always flows from source to sink• Experiments have built a strong case for

pressure flow as the mechanism of translocation in angiosperms

Aphid feeding Stylet in sieve-tubemember

Severed styletexuding sap

Sieve-Tubemember

EXPERIMENT

RESULTS

CONCLUSION

Sap dropletStylet

Sapdroplet

25 m

Sieve-tubemember

To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink.

The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.

The results of such experiments support the pressure flow hypothesis.Figure 36.19

The Plant Body

• Three basic organs evolved: roots, stems, and leaves

• They are organized into a root system and a shoot system

Figure 35.2

Reproductive shoot (flower)

Terminal bud

NodeInternode

Terminalbud

Vegetativeshoot

BladePetiole

Stem

Leaf

Taproot

Lateral roots Rootsystem

Shootsystem

Axillarybud

Growth in Meristems• When plants grow, they add new cells

(cells divide by mitosis) at the tips/ends of branches and roots

• Apical meristems– Are located at the tips of roots and in the

buds of shoots– Elongate shoots and roots through

primary growth

• Lateral meristems– Add thickness to woody plants through

secondary growth

The Root– Is an organ that anchors the vascular plant– Anchors the plant– Absorbs minerals and water– Often stores organic nutrients

Figure 35.3

In most plants:The absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root

• Many plants have modified roots

Figure 35.4a–e

(a) Prop roots (b) Storage roots (c) “Strangling” aerialroots

(d) Buttress roots (e) Pneumatophores

Primary Growth of RootsThe root tip is covered by a root cap, which protects

the delicate apical meristem as the root pushes through soil during primary growth

Figure 35.12

Dermal

Ground

Vascular

Key

Cortex Vascular cylinder

Epidermis

Root hair

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 m

Taproot and Fibrous Root Systems

dicot monocot

A stem is an organ consisting of An alternating system of nodes, the points at which leaves are attachedInternodes, the stem segments between nodes

Stems

1) hold leaves up and aloft for maximum sun exposure

2) transport nutrients/water up/down (connects leaves to roots)

3) some stems store food

Figure 35.11

This year’s growth(one year old)

Last year’s growth(two years old)

Growth of twoyears ago (threeyears old)

One-year-old sidebranch formedfrom axillary budnear shoot apex

Scars left by terminalbud scales of previouswinters

Leaf scar

Leaf scar

Stem

Leaf scar

Bud scale

Axillary buds

Internode

Node

Terminal bud

STEMS

Many plants have modified stems

Figure 35.5a–d

Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along thesurface.

(d)

Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specializedfor storing food. The “eyes” arranged in a spiral pattern around a potato are clusters of axillary buds that markthe nodes.

(c)

Bulbs. Bulbs are vertical,underground shoots consistingmostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise.

(b)

Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These “runners”enable a plant to reproduce asexually, as plantlets form at nodes along each runner.

(a)

Storage leaves

Stem

Root Node

Rhizome

Root

Tissue Organization of Stems

• In gymnosperms and most dicots– The vascular tissue consists of vascular bundles

arranged in a ring

Figure 35.16a

XylemPhloem

Sclerenchyma(fiber cells)

Ground tissueconnecting pith to cortex

Pith

Epidermis

Vascularbundle

Cortex

Key

Dermal

Ground

Vascular1 mm

(a) A eudicot stem. A eudicot stem (sunflower), withvascular bundles forming a ring. Ground tissue towardthe inside is called pith, and ground tissue toward theoutside is called cortex. (LM of transverse section)

Groundtissue

Epidermis

Vascularbundles

1 mm

(b) A monocot stem. A monocot stem (maize) with vascularbundles scattered throughout the ground tissue. In such anarrangement, ground tissue is not partitioned into pith andcortex. (LM of transverse section)

Figure 35.16b

In most monocot stemsThe vascular bundles are scattered throughout the

ground tissue, rather than forming a ring

Secondary growth adds girth to stems and roots in woody

plants

Secondary phloemVascular cambiumLate wood

Early woodSecondaryxylem

Corkcambium

CorkPeriderm

(b) Transverse sectionof a three-year-old stem (LM)

Xylem ray

Bark

0.5 mm0.5 mmFigure 35.18b

As a tree or woody shrub agesThe older layers of secondary xylem, the heartwood, no longer transport water and mineralsThe outer layers, known as sapwoodStill transport materials through the xylem

Growth ring

Vascularray

Heartwood

Sapwood

Vascular cambium

Secondary phloem

Layers of periderm

Secondaryxylem

Bark

Leaves

The main photosynthetic organs of most vascular plants

• Leaves generally consist of– A flattened blade and a stalk– The petiole, which joins the leaf to a

node of the stem

In classifying angiosperms

– Taxonomists may use leaf morphology as a criterion

Figure 35.6a–c

Petiole

(a) Simple leaf. A simple leafis a single, undivided blade.Some simple leaves are deeply lobed, as in anoak leaf.

(b) Compound leaf. In acompound leaf, theblade consists of multiple leaflets.Notice that a leaflethas no axillary budat its base.

(c) Doubly compound leaf. In a doubly compound leaf, each leaflet is divided into smaller leaflets.

Axillary bud

Leaflet

Petiole

Axillary bud

Axillary bud

LeafletPetiole

Differ in the arrangement of veins, the vascular tissue of leaves

Monocots and dicots

Most dicotsHave branching vein “network”

Most monocotsHave parallel veins

Some plant species

Have evolved modified leaves that serve various functions

Figure 35.6a–e

(a) Tendrils. The tendrils by which thispea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines.

(b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems.

(c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water.

(d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators.

(e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil.

Keyto labels

DermalGround

Vascular

Guardcells

Stomatal pore

Epidermalcell

50 µmSurface view of a spiderwort(Tradescantia) leaf (LM)

(b)Cuticle

Sclerenchymafibers

Stoma

Upperepidermis

Palisademesophyll

Spongymesophyll

Lowerepidermis

Cuticle

VeinGuard cells

Xylem

Phloem

Guard cells

Bundle-sheathcell

Cutaway drawing of leaf tissues(a)

Vein Air spaces Guard cells

100 µmTransverse section of a lilac(Syringa) leaf (LM)

(c)Figure 35.17a–c

Leaf anatomy

Leaf anatomy• The outer surface of the leaf has a thin waxy covering called the

cuticle. This layer's primary function is to prevent water loss within the leaf. (Plants that leave entirely within water do not have a cuticle).

• Directly underneath the cuticle is a layer of cells called the epidermis.

• The vascular tissue, xylem and phloem are found within the veins of the leaf. Veins are actually extensions that run from to tips of the roots all the way up to the edges of the leaves. The outer layer of the vein is made of cells called bundle sheath cells, and they create a circle around the xylem and the phloem. In most veins, xylem is the upper layer of cells and the lower layer of cells is phloem. Recall that xylem transports water and phloem transports sugar (food).

• Within the leaf, there is a layer of cells called the mesophyll. The word mesophyll is Greek and means "middle" (meso) "leaf" (phyllon). Mesophyll can then be divided into two layers, the palisade layer and the spongy layer.

• Palisade cells are more column-like, and lie just under the epidermis,

• the spongy cells are more loosely packed and lie between the palisade layer and the lower epidermis. The air spaces between the spongy cells allow for gas exchange.

• Mesophyll cells (both palisade and spongy) are packed with chloroplasts, and this is where photosynthesis actually occurs.

stomata• Stomata are microscopic pores found

on the under side of leaves. You will find the stomata in the epidermal tissue. The stomata is bounded by two half moon shaped guard cells that function to vary the width of the pore.

Stomata help regulate the rate of transpiration

• About 90% of the water a plant loses escapes through stomata• open

– Increase photosynthesis– Increase water loss through stomata

• closed– Decrease water loss through transpiration– Decrease gas exchange and reduce photosynthesis

20 µm

Figure 36.14