george b. johnson copyright ©the mcgraw-hill companies, inc. permission required for reproduction...

GEORGE B. JOHNSON

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

PowerPoint® Lectures prepared by Johnny El-Rady

27 Plant Reproductionand Growth

Essentials of

The Living World

First Edition


27.1 Angiosperm Reproduction

In asexual reproduction, an individual inherits all of its chromosomes from a single parent

Offspring and parent are genetically identical

In a stable environment, asexual reproduction is more advantageous than sexual reproduction

It has a lower investment of energy


RunnersLong, slender stems that grow along the soil surface

RhizomesUnderground horizontal stems that create a network underground

Vegetative reproduction occurs when new individuals are cloned from parts of the parent

Fig. 27.1

“Maternity plant”

SuckersSprouts produced by roots give rise to new plants

Adventitious plantletsNew plants arise from notches along the leaves


Sexual reproduction in plants is characterized by an alteration of generations

Diploid sporophyte haploid gametophyte

Male gametophytes are pollen grainsDevelop from microspores

Female gametophyte is the embryo sacDevelops from a megaspore

Angiosperms have different structures for producing male and female gametes

These are not permanent parts of the adult individual


Flowers contain male parts (stamens) and female parts (carpels)

Dioecious plantsContain flowers that produce only ovules or only pollen

Monoecious plantsContain male and female parts in separate flowers, but in the same plant

Often flowers contain both parts, but there are exceptions

Fig. 27.17a


Pollen grains develop from microspores formed in pollen sacs located in the antherEggs develop in ovules, each of which contains a megaspore mother cell

Fig. 20.2Fig. 27.2


The process by which pollen is transferred to stigma

Self-pollination occurs when a flower’s anther pollinates the same flower’s stigma

This can lead to self-fertilization

Other plants are adapted to outcrossing

Crossing between two different plants

Some plants exhibit self-incompatibility

Genetic relatedness blocks flower fertilization

Pollination


In angiosperms, pollen is carried from flower to flower by insects and other animals

These pollinators are drawn to the flower’s nectar

In certain angiosperms and all gymnosperms, pollen is wind-blown and reaches the stigma passively

Wind-pollinated plants grow in dense strands

Fig. 27.3

Yellow flowers attract bees

Long proboscis gets deep nectar supply


Pollen adheres to stigma and begins to grow a pollen tube

Pollen tube pierces the style and eventually reaches the ovule

Two sperm cells are released

Fertilization

One fertilizes the egg cell to form the zygote (2n)

The other fuses with two polar nuclei to form the endosperm (3n)

Double fertilization


27.2 Seeds

Development is the entire series of events that occur between fertilization and maturity

The first stage is the formation of the embryo

Early in development, the angiosperm embryo stops developing and becomes dormant because of drying

Outermost covering of ovule develops into seed coat enclosing dormant embryo and a stored food source

Most of embryo’s metabolic activities cease

Germination is the resumption of metabolic activities leading to growth of the mature plant

Occurs when conditions are favorable for plant’s survival


Fig. 27.4 Development in an angiosperm embryo

Polarnuclei

MicropyleSperm

Pollentube

Egg(n)

(n)

Triploid endospermmother cell

Zygote(2n)

First celldivision

Endosperm

Suspensor

Basalcell

(3n)

Globularproembryo Cotyledon

Groundmeristem

Procambium

Cotyledons

Protoderm

Root apex (radicle)

Endosperm

Shootapicalmeristem

Hypocotyl

Root apicalmeristem

Cotyledons


27.3 Fruit

During seed formation, the flower ovary begins to develop into fruit

Fruits can be fleshy or dry and hard

There are three main kinds of fleshy fruit

Berries Drupes Pomes

Fig. 27.5


Fleshy fruit are normally dispersed by animals

Animals eat the fruit and excrete the seeds as solid wastes

Dry fruits are dispersed by several mechanisms

Mangroves

Coconuts

By wind By water By attaching to animals

Fig. 27.5e

Dandelion

Fig. 27.5f

Burgrass


27.4 Germination

Germination is the resumption of a seed’s growth and reproduction

It is triggered by water

The seed coat ruptures and the plant begins to send out roots and shoots

Oxygen is required for active growth

Endosperm or cotyledons provide the food source


Fig. 27.6 Development of angiosperms

Dicot: Soybean

Cotyledons emerge from the underground

Monocot: Corn

Cotyledon stays

underground


27.5 Plant Hormones

Differentiation in plants, unlike that in animals, is largely reversible

In the 1950s, F.C. Steward was able to regenerate a fertile carrot plant from bits of phloem tissue

Fig. 27.7


Following germination, further plant development depends on the activity of meristematic tissues

And the interaction with the environment

Fig. 27.8 Stages of plant differentiation


Differentiation results from the activation or suppression of key genes

This gene expression is controlled by hormones

Plant hormones are produced in non-specialized tissues

Five major types of hormones

Auxins

Gibberellins

Cytokinins

Ethylene

Abscisic acid


TABLE 27.1


27.6 Auxin

Charles Darwin and his son Francis published a book called The Power of Movement in Plants (1881)

In it, they describe the phenomenon of phototropism

Growing plants bending toward light

The Darwins concluded that, in response to light, an “influence” arises at the tip of the shoot

It is then transmitted downward causing the shoot to bend


Fig. 27.9 The Darwins’ experiment with phototropism


In 1926, Frits Went identified the hormone involved in phototropism

Fig. 27.10

He called it auxin (from the Gr. word, “to increase”)


Phototropism can be explained as such

Auxin contents on the two sides of shoot differ

The side that is in the shade has more auxin

Cells elongate more than those on the lighted side

Auxin appears to act by increasing the stretchability of the plant cell wall

Fig. 27.11


Synthetic auxins are routinely used to control weeds

When treated, the weeds literally “grow to death”

2,4-dichlorophenoxyaceticacidKnown as 2,4-D

Affects only broad-leaved dicots

2,4,5-trichlorophenoxyaceticacidKnown as 2,4,5-T

Kills woody seedlings and weeds

Notorious as the Agent Orange of the Vietnam War

Easily contaminated with dioxin

An endocrine disruptor


27.7 Other Plant Hormones

Gibberellins

Also hasten seed germination

A large class of over 100 hormones

Play major role in stem elongation

Promote elongation between the node regions

Fig. 27.12

Defective in gibberellin production



Cytokinins

Stimulate cell division

Determine the course of differentiation

Promote growth of lateral buds

Inhibit formation of lateral roots

Fig. 27.13

Apical meristem intact

Auxin inhibits lateral buds Cytokinins stimulate

lateral buds



Ethylene

Gas that hastens fruit ripening

Fig. 27.14

Accelerates abscission of leaves or fruits damaged by various stress agents

Holly twig



Abscisic Acid (ABA)

Fig. 27.15

May cause synthesis of ethylene

Plays a role in the dormancy of seeds

May also function in transpiration

Opening/closing of stomata ABA causes

efflux of K+ out of guard cells


27.8 Photoperiodism and Dormancy

Photoperiodism is the physiological response of organisms to changes in the length of day and night

Angiosperm flower productionLong-day plants

Initiate flowers when nights become shorter than a certain length

Short-day plantsInitiate flowers when nights become longer than a certain length

Day-neutral plantsProduce flowers without regard to day length


Fig. 27.16 How photoperiodism works in plants


Plants contain a pigment called phytochrome It exists in two forms converted by darkness

Pr (inactive)

Pfr (active)

Fig. 27.17

To this day, the existence of a flowering hormone remains strictly hypothetical


Plants respond to their external environment largely by changes in growth rate

When conditions are not favorable, they become dormant

They stop growing altogether

In temperate regions, dormancy is generally associated with winter

Dormancy


27.9 Tropisms

Tropisms are directional and irreversible growth responses of plants to external stimuli

PhototropismGrowth toward sources of light

GravitropismGrowth in response to gravity

Stems grow upward and roots downward

ThigmotropismGrowth in response to touch


Fig. 27.18 Tropism guides plant growth

Phototropism

Gravitropism

Thigmotropism

george b. johnson copyright ©the mcgraw-hill companies, inc. permission required for reproduction...

Documents

mcgrawhill companies

plant reproduction

asexual reproduction

vegetative reproduction

angiosperm reproduction

different plants

flowers stigma

monoecious plants