The Evolution of Seeded Plants

Question 1

What changed the course of plant evolution?

Answer 1

Seeds - enabled their bearers to become the dominant producers in most terrestrial ecosystems
- Originated about 360 mya

Question 2

What does a typical seed consist of?

Answer 2

An embryo and nutrients surrounded by a protective coat

Question 3

What are common to all seed plants?

Answer 3

• Reduced gametophyte
• Heterospory
• Ovules
• Pollen

Question 4

Gametophytes and Sporophytes in Mosses and other nonvascular plants

Answer 4

Gametophyte - Dominant
Sporophyte - Reduced, dependent on gametophyte for nutrition

Question 5

Gametophytes and Sporophytes in Ferns and other seedless vascular plants

Answer 5

Gametophyte - Reduced, independent (photosynthetic and free-living)
Sporophyte - Dominant

Question 6

Gametophytes and Sporophytes in Seed plants (gymnosperms and angiosperms)

Answer 6

Gametophytes - Reduced (usually microscopic), dependent of surrounding sporophyte tissue for nutrition
Sporophyte - Dominant

Question 7

What are the advantages of a reduced gametophyte?

Answer 7

• The gametophytes of seed plants are mostly microscopic
• The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte
• Advantage is protection (desiccation, UV)
• Obtain energy from sporophyte

Question 8

Ovule

Answer 8

Develops within ovary of plant
- consists of a megasporangium, megaspore, and one or more protective integuments (layers of sporophyte tissue that contributes to the structure of an ovule of a seed plant)

Question 9

Pollen grains

Answer 9

Develop from microspores, contain the male gametophytes

Question 10

What does pollen eliminate the need for?

Answer 10

Pollen eliminates the need for a film of water and can be dispersed great distances by air or animal

Question 11

Pollination

Answer 11

The transfer of pollen to the part of a seed plant containing the ovules

Question 12

What does the germination of a pollen grain give rise to?

Answer 12

It gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule
- sperm doesn’t swim to the egg, the pollen tube carries sperm to the egg

Question 13

What are the evolutionary advantages of seeds?

Answer 13

• A seed develops from the whole ovule
• A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat
• They remain dormant for days to years, until conditions are favorable for germination
• They may be transported long distances by wind or animals

Question 14

Spores vs. Seeds fpr sidpersal

Answer 14

Seedless plants
- Spores are protective and only dispersal stage of life cycle
- Simple single-celled structure that gives rise to gametophytes
- Spores can often survive conditions that kill plants
- Spores are tiny and can disperse
- Spores were the main route for dispersal for early terrestrial plants

Seeded vascular plants
-See represents and alternative solution for resistance and dispersal
- Complex multi-cellular structure consisting of embryo protected by seed coat
- Seed can remain dormant for long time
- Seeds have own supply of stored food enhancing ability to get established
- In meric (very moist) or xeric (dry) habitats, seeds are often larger
- Seeds germinate under favorable conditions

Question 15

What kind of seeds do gymnosperms have?

Answer 15

They have “naked” seeds, not enclosed by ovaries and are exposed on modified leaves that usually form cones
- Appear in fossil record 305 mya

Question 16

What are the four phyla of Gymnosperms?

Answer 16

• Coniferophyta (conifers, such as pine trees)
• Cycadophyta (cycads)
• Gingkophyta (one living species: Ginkgo biloba)
• Gnetophyta (three genera)

Question 17

Phylum Coniferophyta

Answer 17

• By far the largest of the gymnosperm phyla
• Most conifers are evergreens and can carry out photosynthesis year round

Question 18

What are three key features of the gymnosperm life cycle?

Answer 18

• Increasing dominance of the sporophyte generation
• Advent of seed as a resistant, dispersible stage of life cycle
• The evolution of pollen as an airborne agent bringing gametes together

Question 19

Phylum Cycadophyta

Answer 19

• 300 extant species of Cycads
• Consensus that they have remained pretty much unchanged for more than 300 million years
• Researchers sequenced the DNA of many species
• Constructed a phylogenetic tree

Question 19

What did scientists find when they constructed a phylogenetic tree for Cycads?

Answer 19

• Major branches of the family tree may reach back 300 million years
• Tips of the tree (today’s species) arose about 12 million years ago
• Similar bursts of new species around that time, including cacti, ice plants and agave
• Coincides with a period of global cooling and drying

Question 20

Angiosperms

Answer 20

Flowering plants
- have reproductive structures called flowers and fruits
- most widespread and diverse of all plants
- classified in a single phylum
- Anthophyta - from the Greek anthos, flower

Question 21

Flowers

Answer 21

• The flower is an angiosperm structure specialized for sexual reproduction
• Specialized shoot with up to four types of modified leaves

Question 22

Sepal

Answer 22

Leaves that enclose the flower

Question 23

Petals

Answer 23

Brightly colored and attract pollinators

Question 24

Stamens

Answer 24

Produce pollen
- made up two parts anthers and filaments

Question 25

Carpels

Answer 25

Produce ovules
- three parts; stigma, style, and ovary

Question 26

What tends to transfer pollen for many angiosperms?

Answer 26

Insects or other animals transfer pollen from one flower to the female sexual organs of another
- makes pollination more directed than the wind-dependent pollination of gymnosperms
- some species that live in dense populations, like grasses and temperate trees, are wind-pollinated

Question 27

What kind of structures is the flower of the sporophyte composed of?

Answer 27

Both male and female structures

Question 28

Male gametophytes

Answer 28

Contained within pollen grains produced by the microsporangia of anthers

Question 29

Female gametophyte

Answer 29

Also known as the embryo sack, develops within an ovule contained within an ovary at the base of a stigma

Question 30

The pollination cycle

Answer 30

• A pollen grain that has landed of a stigma germinates and the pollen tube of the male gametophyte grows down the ovary
• The ovule is entered by a pore called the micropyle
• One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperms
• The triploid endosperm nourishes the developing embryo
• Within a seed, the embryo consists of a root and one of two seed leaves

Question 31

Double fertilization

Answer 31

Occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule

Question 32

Fruits

Answer 32

• Formed when the ovary wall thickens and matures
• Protect seeds and aid in their dispersal
• Wall of the ovary, the pericarp, can be either fleshy or dry or both

Question 33

Angiosperm evolution

Answer 33

• Gymnosperms and angiosperms diverged 305 mya
• Angiosperms originated at least 140 mya
• During the late Mesozoic, the major branches of the clade diverged from their common ancestor
• Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits

Question 34

What does the formation of amborella eggs suggest?

Answer 34

Amborella eggs form in a similar manner to those of gymnosperms suggesting the existence of a common ancestor

Question 35

Monocot characteristics

Answer 35

Embryos - one cotyledon
Leaf venation - veins usually parallel
Stems - vascular tissue scattered
Roots - root system usually fibrous (no main root)
Pollen - pollen grain with one opening
Flowers - floral organs usually in multiples of three

Question 36

Eudicot characteristics

Answer 36

Embryos - two cotyledons
Leaf venation - veins usually netlike
Stems - vascular tissues usually arranged in ring
Roots - taproot (main root) usually present
Pollen - pollen grain with three openings
Flowers - floral organs usually in multiples of four or five