Chapter 23

The overall category of “plants” are 1) eukaryotic 2) multicellular 3)organisms capable of photosynthesis 4)built with cellulose 5) and have alternation of generations.

Plants most likely first emerged during the Paleozoic era as an offshoot of green algae • Green algae and plants both are the only

organisms to contain chlorophyll a and b

• They also are highly similar based on RNA sequence analysis

• Unlike algae, plants have a protected embryo

All plants have a unique life cycle called the

alternation of generations

In the AofG life cycle, two multicellular

individuals alternate and produce each

other.

The two individuals are • 1) the sporophyte, which represents the diploid

generation

• 2) the gametophyte, which represents the haploid

generation

The sporophyte (2n) generation gets its name because of the spores it produces through meiosis • Spores are haploid reproductive cells, but they don’t

need to fuse with another haploid cell to reproduce

• Instead, they go through mitosis and produce gametophytes

The gametophyte (n) generation is named because of the gametes it produces • Two gametes (egg and sperm) form a diploid

zygote.

• The zygote undergoes a mitotic division to produce a sporophyte

Spore (n)

Gametophyte (n)

Gamete (n)

Sporophyte (2n)

Zygote (2n)

“Phyte” means “Growth of”

In any given plant, only one of the two generations is

dominant, or conspicuous

• In bryophytes, the gametophyte is dominant

• In seedless vascular plants, gymnoperms, & angiosperms,

the sporophyte is dominant

Flowering plants are so heavily dependent on

sporophytes, the gametophyte is microscopic

Plant evolution can be marked by the four major

events which broke them into their current 4 phyla

The first event, which separated plant cells from

algal cells, was formation of a protected embryo

The embryo is held within the body of a female

plant or female part of a plant • Seed, petal, cones, etc

The embryo is nourished and protected from the

environment until it is capable of growth and

development on its own

The first plant phyla are nonvascular plants, also

called Bryophytes (mosses, liverworts, hornworts)

Nonvascular plants have a “leafy”

appearance, but they are not leaves because

they don’t transport any material. • Nonvascular plants have difficulty with transporting

water and nutrients, and therefore are highly reduced

in size

Gametophytes contain two reproductive

structures • Antheridia for producing sperm

• Archegonia for producing eggs

• A liquid film is produced (or the plant relies on water

from the environment) for sperm to reach eggs

Hornworts have

sporophytes that look

like small green

broom handles

Broom handles carry

on photosynthesis,

but gain their

nourishment from a

filmy gametophyte at

the base

(I’ll provide details about

each class of organisms

today, but you should be

paying more attention to

phyla characteristics)

Liverworts are characterized by flattened thallus

(body) or a leafy appearance

The lower surface contains hairlike extensions

called rhizoids

• Rhizoids are root-like structures that dig into the soil

The thallus also contains gemma cups

• Gemma cups are groups of cells that contain eggs, which

detach in wind or rain and form a new plant

Mosses are found in nearly every ecosystem

from the arctic to antarctic

In ideal environments, they form large mats

that cover the ground, rocks, or other plants

Some only grow in rare, specific

environments, such as copper moss or

glowing cave moss

Many plants are given the name “moss”

even though they are not • Irish moss, spanish moss, club moss

Peat moss is the most commercial form of

moss

Peat is one of the best plants in terms of it’s

ability to hold water.

Peat can also be used as a fuel source

The next major event occurred 430 mya, and

was the appearance of the first Seedless

Vascular Plants • Vascular tissue is specialized tissue for transporting

nutrients and water throughout the plant

• Plants still have not formed seeds

Xylem: conducts water and dissolved minerals

upward from the roots

• Xylem are surrounded by a strong, waterproof

compound called lignin

Phloem: conducts sucrose and organic material

throughout the plant

Club mosses are common in moist woodlands of

the temperate zone

They are characterized by a horizontal stem called

a rhizome, which has multiple smaller aerial stems

Club moss leaves also contain microphylls.

• A microphyll is only a single strand of vascular tissue

Most club mosses are epiphytes, meaning they live

on trees but are not parasites.

Ferns are the only members of the horsetail

class that still exist today.

Ferns have rhizomes like club mosses, but their

branches form a whorl that looks like a horse’s

tail

The plants are so rough and strong, Native

Americans used them as scouring pads

Fern leaves (called fronds) are large and

conspicuous

Ferns are also the only seedless vascular plant

to have megaphylls (multiple strands of

vascular tissue).

Ferns are harvested by florists and flower

companies for bouquets and home and

garden decorations.

Tropical tree ferns are harvested because

their wood is highly resistent to decay and

termite infestation

Ferns also have some medicinal value, such

as astringents to stop bleeding and

expectorants

The gymnosperms appeared relatively quickly

next, about 400 mya.

Gymnosperms are characterized by the presence

of a seed.

Seeds are a combination of an embryo, nutrients

for the embryo, and a protective coat

Seeds are highly resistive to drought, heat (to a

point), rough terrain, and predators

• Even if they pass through a digestive system, they’re still

just as functional

Sylene Stenophylla

Seed plants are heterosporous, meaning

they have two kinds of gametophytes • Male gametophytes: drought-resistant pollen grains,

carried by wind or animal pollinator to female

gametophytes

Unlike earlier plant phyla, no external water is needed for

fertilization

• Female gametophytes: ovules which collect sperm

and eventually form a seed

Will be enclosed by a cone (or fruit in later generations)

until ready for release

Conifers are plants that bear cones

Conifers can grow to enormous heights

(more than 300 ft) and live for incredible

periods of time • General Sherman (Sequoia): 2,500 years old

• Old Tjikko (Spruce): roots are 9,550 years old

Conifers have thin leaves called needles

that are covered in drought-resistant cuticle

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Cycads grow naturally in the tropics, but are

often used for landscaping as well

Cycad trunks are unbranched, no matter

their height

The only surviving member of ginkgoes are

the Ginkgo biloba, or the maidenhair tree • Ginkgoes are highly resistant to pollution and are

often found in cities and parks

• Female fruits often give off a pretty ugly odor, so

most of the trees seen in public are male

Gnetophytes are the final phyla of

gymnosperms. The 70 species are all

extremely unique • They are the closest relatives of angiosperms

The final major evolution of plants are the

angiosperms

Angiosperms emerged 200 mya and are

characterized by the presence of flower structures. • Flowers are reproductive structures that attract pollinators such as

insects or birds, or use wind to pollinate

• Angiosperms were also the first to develop fruits for reproduction

They contain 240,000 species, six times more than

any other groups of plants

Angiosperms live in all ecosystems (except arctic)

and range from microscopic to 100 m in length

Flowering plants have two classes • Monocots have one cotyledon in their seeds

• Eudicots have two cotyledons in their seeds

Cotyledons are the seed leaves that nourish

the developing embryos until they emerge

from the ground and can begin their own

photosynthesis

Angiosperms need creative ways of

dispersing their seeds, which has resulted

in many different flower types

Wind-pollinated flowers • Plain, unshowy

Insect and bird-pollinated flowers • Colorful and bright petals

Night-blooming • White and highly aromatic

Fruit dispersal • Flavorful; eaten by animals and deposited as waste

NIGHT

INSECT

HUMMINGBIRD

ANIMAL

WIND