Post on 28-Dec-2015
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