Plant Diversity I

Cyanobacteria and protists likely existed on land by

1.2 billion years ago

Small plants, fungi, and animals emerged on land only within the last

500 million years

Since colonizing land, plants have diversified into more than ___ living species

290,000

Most present-day plants live on

land

Green algae called ___ are the closest relatives of plants

charophytes

Plants share the following traits with charophytes

• Rings of cellulose-synthesizing proteins

• Structure of flagellated sperm

• Formation of a phragmoplast – scaffold for cell plate assembly

Plants are separated from algae because plants form

embryos

Five key traits appear in nearly all plants but are absent in the charophytes

1) Alternation of generations

2) Multicellular, dependent embryos

3) Walled spores produced in sporangia

4) Multicellular gametangia

5) Apical meristems

1) Alternation of Generations

Plants alternate between two multicellular generations, a reproductive cycle called alternation of generations

gametophyte generation

haploid and produces haploid gametes by mitosis

Fusion of a sperm and egg gives rise to the ___

diploid sporophyte which produces haploid spores by meiosis that develop into gametophytes

2) Multicellular, Dependent Embryos

The diploid embryo is retained within the tissue of the female gametophyte

Nutrients are transferred from parent to embryo through

placental transfer cells

Plants are called ___ because of the dependency of the embryo on the parent

embryophytes

The sporophyte produces spores in organs called

sporangia

sporangia

called sporocytes undergo meiosis to generate haploid spores

Spore walls contain ___, a complex polymer which makes them resistant to harsh environments

sporopollenin

Gametes are produced within organs called

gametangia

Male gametangia, called ___

antheridia, produce and release sperm

Female gametangia, called

archegonia, produce a single nonmotile egg

Each egg is fertilized within an

archegonium

Plants sustain continual growth in length by repeated cell division within the

apical meristems

Fossilized spores and plant tissues have been extracted from

450-million-year-old rocks

Fossils of larger structures, such as a sporangium, date to

425 million years ago

Bryophytes and seedless vascular plants are each a grade organisms that are grouped based

on shared key biological features

Nonvascular plants are commonly called ___

bryophytes

Seedless vascular plants can be divided into clades

• Lycophytes (club mosses and their relatives)

• Monilophytes (ferns and their relatives)

Gymnosperms

produce seeds that are not enclosed in chambers

Angiosperms

produce seeds that develop inside chambers that originate within flowers

Bryophytes are represented today by three phyla of small, herbaceous (nonwoody) plants

• Liverworts, phylum Hepatophyta

• Mosses, phylum Bryophyta

• Hornworts, phylum Anthocerophyta

Mosses and other nonvascular plants have life cycles dominated by

gametophytes

A spore germinates into a gametophyte composed of a

protonema

protonema

one or more gamete-producing gametophores, and rhizoids anchoring gametophytes to the substrate

Bryophyte sporophytes never live independently of the __

gametophyte; they are the smallest and simplest sporophytes of all extant plant groups

sporophyte

consists of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristome

The earliest fossils of vascular plants date to

425 million years ago

In ferns, the familiar leafy plants are the __; the gametophytes are tiny plants that grow on or below the soil surface

sporophytes, gametophytes

Vascular plants have two types of vascular tissue:

xylem and phloem

Xylem conducts most of the water and minerals and includes tube-shaped cells called ___

tracheids

§Water-conducting cells are strengthened by ___ and provide structural support

lignin

Phloem

has cells arranged into tubes that distribute sugars, amino acids, and other organic products

Roots

Are organs that anchor vascular plants

• They enable vascular plants to absorb water and nutrients from the soil

• Roots may have evolved from subterranean stems

Leaves

organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

Leaves are categorized by two types:

• Microphylls, small, often spine-shaped leaves with a single vein

• Megaphylls, larger leaves with a highly branched vascular system

Lycophytes have

microphylls, monilophytes, gymnosperms

angiosperms have

megaphylls

Sporophylls

modified leaves with sporangia

Sori

clusters of sporangia on the undersides of sporophylls

Strobili

cone-like structures formed from groups of sporophylls

§Most seedless vascular plants are __, producing one type of spore that develops into a bisexual gametophyte

homosporous

All seed plants and some seedless vascular plants are

heterosporous

Heterosporous species produce

megaspores, which give rise to female gametophytes and microspores, which give rise to male gametophytes

There are two clades of seedless vascular plants

• Phylum Lycophyta includes club mosses, spike mosses, and quillworts

• Phylum Monilophyta includes ferns, horsetails, and whisk ferns and their relatives

Phylum Monilophyta

Ferns are the most widespread seedless vascular plants, with more than 12,000 species which are most diverse in the tropics but also thrive in temperate forests

• Horsetails were diverse during the Carboniferous period, but are now restricted to the genus Equisetum

• Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns