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UNIT 5: Plants: Anatomy, Growth, and Function

Chapter 13: Plants: Uses, Form, and

Function

Chapter 14: Plants: Reproduction,

Growth, and Sustainability

How do plants grow and reproduce?

14: Plants: Reproduction, Growth, Sustainability

Seeds are the embryos of the next generation of plants. By saving

and exchanging seeds produced by plants with desirable traits,

farmers and gardeners ensure the plants’ genetic continuation.

UNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability

Plants reproduce by both asexual and sexual reproduction. Sexual

reproduction is by sporic reproduction (alternation of generations).

Haploid gametophyte cells (1n) produce gametes, while diploid

(2n) sporophyte cells produce spores. Male and female gametes

unite to form the sporophyte that continues the life cycle.

14.1 Plant Reproduction

UNIT 5 Chapter 14: Plants: Reproduction, Growth, and Sustainability Section 14.1

All plants have a life cycle

involving alternation of

generations. The cycle varies

among species. The variation

is mostly due to the type of

structure that releases the

spores.

Seedless plants include vascular ferns and non-vascular mosses.

Fertilization requires the sperm to swim from the male

gametophyte to the egg in the female gametophyte. Thus, water

must be present, and the sperm must have a flagellum.

In non-vascular plants, the gametophyte

is the dominant generation. In

vascular plants, the sporophyte

is the dominant generation.

Sexual Reproduction in Seedless Plants


Peat, or sphagnum, moss commonly

grows in boggy areas. Its antibacterial and

absorbent properties led to its historic use

by some Aboriginal peoples as dressing

for wounds.

Seed plants include gymnosperms and angiosperms. The

gametophytes are not free-living. The male gametophyte, called

a microspore, develops into a pollen grain and sperm.

The female gametophyte, called a macrospore, develops into an

egg cell. In order for pollination to occur, the male

gametophyte must be transferred to the female reproductive

structure.

Sexual Reproduction in Seed Plants


During pollination, the pollen grain develops a pollen tube to

reach the egg. Sperm develop in the tube and travel to the

egg. The zygote becomes an embryo with a small amount of

food protected by a tough seed coat. The seeds remain in the

female structure until maturity when they are released. Seeds

grow into sporophytes.

Sexual Reproduction in Gymnosperms


Angiosperm sexual reproduction involves the flower organ. Flowers

have:• sepals – protect the flower bud

• petals – attract pollinators

Pollination takes place on the stigma. Female gametophytes develop

in the ovules, where eggs are formed.

Sexual Reproduction in Angiosperms

UNIT 5 Section 14.1Chapter 14: Plants: Reproduction, Growth, and Sustainability

• stamens – male reproductive structure

• pistils – female reproductive structure

• Complete flowers have sepals, petals, stamens, and one or

more pistils (roses, tulips).

• Incomplete flowers are missing one or more flower parts

(grasses, wild ginger).

• Perfect flowers have both pistils and stamens on each flower.

• Imperfect monoecious plants have pistils and stamens found

on separate flowers on same plant (corn, oaks).

• Imperfect dioecious plants have pistils and stamens found on

different plants (willow, ginkgo).

Variations Among Flowers


Continued…

• Petal number can distinguish monocots from dicots:

o multiple of four or five: dicot (meadow beauty)

o multiple of three: monocot (trillium)

Variations Among Flowers


The meadow beauty (A) and sulfur cinquefoil (B) are

dicots. The trillium (C) is a monocot.

• Self-pollination: Plants pollinate themselves or another flower on

the same plant. This can lead to loss of genetic variation and

species vulnerability.

• Cross-pollination: Plants receive pollen from another plant,

ensuring genetic diversity.

• Animal Pollination: Insects and other small animals move from

flower to flower collecting nectar and moving pollen. Bright,

sweet-smelling flowers attract these pollinators.

• Wind Pollination: Some plants lack colourful reproductive organs

but produce large quantities of light pollen grains to increase the

chances of pollen landing on a receptive reproductive organ.

Pollination Mechanisms


As a pollen tube grows to

reach the ovule, the male

gametophyte performs

mitosis to create two

sperm cells. One fuses

with the egg, the other

fuses with the polar nuclei,

forming a triploid (3n) cell

that divides into nutrient-

rich endosperm tissue.

Follow the illustration for

the complete life cycle of a

peach.

Life Cycle of Flowering Plants


The zygote (2n) is a sporophyte that divides to produce a

monocot or dicot embryo with a 3n endosperm. The outside

layers of the ovule form a protective cover called the seed

coat. As the ovule develops into a seed, changes occur in the

ovary wall that lead to the formation of a fruit. The fruit may

also be made from other flower parts.

Seed and Fruit Development


Seeds of monocots

differ in structure

and function from

those of dicots.

Seed Dispersal


If the seed lands in a location with sufficient resources, it can

develop into a seedling. If there are not enough resources, the seed

may lose water slowly and enter dormancy for many, many years.

Seeds of

different species

are adapted to

be dispersed in

different ways.

The survival rate of a species is increased when seeds are dispersed

away from the parent plant(s) to reduce competition for resources.

Fruits attract animals and can be transported great distances, aiding

wide dispersal.

Water, animals, and wind are means of dispersal.

Germination is the resumption of growth after dormancy. The seed

absorbs water, and the seed coat breaks. The stored food supports

the growth of the embryo. The radicle emerges first and becomes

roots. The hypocotyl then emerges as an early stem. It may have

the cotyledons and the embryonic leaves on it. Monocots leave their

cotyledon below ground.

Seed Germination


Photosynthesis begins when chloroplast-containing cells emerge.

Asexual reproduction (cloning) can be an advantage when plants

are well-adapted to their environment. Farmers and gardeners

have studied and perfected techniques of artificial propagation

that involve asexual reproduction from a plant’s roots, stems, or

leaves (vegetative propagation).

How does this method of reproduction affect genetic

diversity? Justify your answer.

Asexual Reproduction


• split the plant into two or more; each piece contains roots

and shoots

• simple and inexpensive way to propagate or thin out plants

• examples: bulbs (tulips), plants with more than one stem

(peonies, hostas)

Asexual Reproduction: Division (splitting)


• shoot or root part is cut and

joined to the vascular

cambium of another

• allows combination of

characteristics of two

varieties of plants; helps

repair damaged trees;

quickens fruit production

• examples: fruit trees (apple),

nut trees (almond),

grapevines

Asexual Reproduction: Grafting


• a part of a leaf or entire leaf is cut and placed in growth

medium (water, soil, or vermiculite) so that meristem cells

can grow shoots and roots

• faster than propagating from seed; can be done out of

season

• examples: African violets, snake plants, aloe vera

Asexual Reproduction: Leaf Cutting


• A stem or shoot tip is cut and

placed in growth medium to

grow roots from meristem cells

• Faster than propagating from

seed; can be done out of season

• examples: herbs (basil),

gymnosperm and angiosperm

trees (pine, willow), flowering

bushes (roses), grapevines

Asexual Reproduction: Stem Cutting


• A root is cut and placed in a growth medium in the same

orientation as the original plant; meristem cells form a new

root and shoot system

• can be done when plant is in spring or autumn dormancy

• examples: trilliums, mint, irises

Asexual Reproduction: Root Cutting


• A long, vine-type stem is bent to touch the ground, slightly cut

to promote growth of roots, and buried until a plant develops

that can be cut from the parent to grow independently

• Large clone produced quickly; water and nutrients from parent

will support rooting process

• examples: honeysuckle, willow, hydrangea

Asexual Reproduction: Simple Layering


• A strip of outer bark is removed from

a woody stem; moist sphagnum moss

is packed around the wound and

plastic-wrapped until roots develop;

the rooted stem can then be cut from

the parent plant and planted

• Large woody plant clone produced

quickly

• examples: tropical plants (rubber

trees), lilacs, magnolias, fruit and nut

trees

Asexual Reproduction: Air Layering


• A cell or small piece of tissue is placed in a sterile nutrient

medium that promotes shoot and root growth; a tiny plantlet

develops

• Can be used to produce millions of plantlets for genetic

modification

• examples: most ferns,

gymnosperms, angiosperms

Asexual Reproduction: Cell Culturing


Summary of Plant Reproduction


Section 14.1 Review

UNIT 5 Section 14.1Chapter 14: Plants: Reproduction, Growth, and Sustainability

Plant hormones are chemical compounds. They:

14.2 Plant Growth and Development


• play a role in determining cellular differentiation and gene

expression in any cell as a plant grows from shoot and root

apical meristem

• regulate the differentiation of plant cells, plant growth, and the

plant’s response to a given stimulus (gravity, light, touch)

• act as chemical signals between cells and tissues

• include auxins, cytokinins, gibberellins, ethylene, and abscisic

acid

Stimulatory hormones:

Stimulatory vs. Inhibitory Hormones


• include auxins, cytokinins, and gibberellins

• can stimulate cell division, cell differentiation, early flowering

and cell elongation that develops into apical dominance (growth

is upward with little lateral growth)

• used in industry to increase fruit and cluster size

• examples: indoleacetic acid (auxin)

Continued…

(A) Auxin stimulates apical

meristem growth and inhibits

the growth of side branches.

(B) Removing the apical

meristem decreases the

amount of auxin. As a result,

side branches grow.

Inhibitory hormones:

Stimulatory vs. Inhibitory Hormones


• include ethylene and abscisic acid

• can inhibit growth by weakening cell walls, promoting

breakdown of starch, blocking stimulatory hormones and blocking

the intake of carbon dioxide

• commercial use: In order to protect fruit in transit, it is shipped

long distances in an unripened state; ethylene gas is then used to

ripen it when it reaches its market.

Bananas and other fruits ripen due to ethylene.

Plant Hormones Summary


Plants can respond to a stimulus and grow toward it, or they can

perform a simple nastic response, which is a movement of the

plant that is reversible, repeatable, and does not include growth.

An example of a nastic response is the opening and closing of

flower petals as light conditions change.

Plant Responses to Environmental Stimuli


Nastic movements in the leaves of this sensitive plant (Mimosa pudica)

are caused by changes in water pressure in the leaf cells. When the

stimulus ends, the leaves return to their original orientation.

Tropic responses are growth responses to external stimulation

coming from one direction in the environment. They include:

Phototropism

• a growth response to light produced by an unequal distribution

of auxin. More auxin on the side with less light causes those cells

to elongate and bend the plant toward the light.

Gravitropism

• a “positive” growth response of the roots downward or a

“negative” growth response of stems upward

Thigmotropism

Tropic Responses


• a growth response to mechanical

(contact) stimuli

What tropism(s) are evident here?

A plant’s ability to grow is affected by:

• sunlight

• carbon dioxide

• water

• soil pH

• macro- and micronutrients

(dissolved in water)

Each plant species grows best within a narrow pH

range. Plants that thrive in acid soil include pine,

blueberry, and potatoes. Plants that thrive in alkaline

soil include lawn grass, beans, pears, and lettuce.

Macronutrients are nutrients that are needed in

amounts greater than 1% of a plant’s dry weight and

include nitrogen, potassium, calcium, magnesium,

phosphorus, and sulfur. Micronutrients are needed in

amounts less than 1% of a plant’s dry weight.

Other Plant Growth Factors


Nutrients


14.2 Review


14.3 Succession and Sustainability


In addition to the other human and ecosystem services plants

provide, they also play a role in establishing and developing

communities. This is called ecological succession.

Ecological succession is the change in an ecosystem when

one community replaces another; it results from changes in

abiotic and biotic factors.

Events that change the structure of a biological community and

sometimes destroy all actively growing organisms are called

ecological disturbances. They include:

• forest fires

• floods

Within months of most disturbances, new vegetation sprouts and

animals repopulate until, over time, the area is again thick with

growth. Ecologists believe that disturbances, both large and small,

are the norm rather than the exception in communities.

Understanding how disturbances help and hinder plant and animal

species is critical for the preservation of natural communities.

Ecological Disturbances


• volcanic eruptions

• retreating glaciers

Primary Succession


Primary succession is the establishment of a community in

an area after an ecological disturbance has left exposed rock

that does not have any topsoil.

Often, liverworts are the first species (pioneer species) to

colonize a barren area. With other organisms, such as

bacteria, algae, and lichens, they form a pioneer community

that can survive in harsh conditions.

What traits do these pioneer species have in common

that make them suited to living in barren areas?

Primary Succession


As pioneer species continue to grow in a barren area, early

organisms die and begin to form soil. As soil accumulates,

nutrient and moisture content builds and pH changes. This allows

larger species to grow in the area.

Scientists measured changes in the nitrogen content and the number

of plant species during primary succession in Glacier Bay, Alaska.

Primary Succession


The area continues to change as larger plants move in. Many early

species are vascular and non-vascular seed species; their seeds have

dispersed and germinated quickly. They compete for space and light.

Successive surviving species colonize the area. Different types of

plants provide habitat for different birds and animals. Changes

continually occur until a stable climax community of plants and

animals forms.

It will remain

stable until a

major

disturbance

occurs.

Secondary Succession


Secondary succession is the recolonization of an

area after an ecological disturbance that has left the

soil intact. It also includes changes in the

composition and number of species over time.

Biodiversity and Ecosystem Resilience


Research has shown that ecosystems (climax

communities) with high diversity are better able to

withstand disease, competition from invasive

species, and extreme weather events.

Maintaining sustainable

and diverse natural and

human-made ecosystems

is critical to the health of

the biosphere.

14.3 Review


UNIT 5 STSE Feature

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