Plant Evolution and Reproduction

• Plants are multicellular, photosynthetic eukaryotes

Ancestry and Features of Land Plants

• Plants are thought to have evolved from freshwater green algae around 590 MYA
• Green algae contain chlorophylls and accessory pigments
• Green algae store excess carbohydrates as starch
• Green algae have cellulose in their cell walls
• Land plants protect and nourish the embryo, as well as protect the zygote, unlike algae
• Land plants are most closely related to freshwater green algae known as charophytes
• Adaptations are required for the land environment
• There is a constant threat of desiccation (drying out)
• Desiccation protects all phases of reproduction from drying out including sperm, egg and embryo
• The embryo of seed plants are dispersed within the seed giving it food in a protective coat
• The water environment provides plentiful water and support for the plant body

Plant Reproduction

• Seed plants reproduce by means of seeds
• Seedless plants reproduce by means of spores
• Gymnosperms have seeds in cones
• Angiosperms have seeds in fruits
• All land plants exhibit alternation of generations
• The plant has two alternating forms in the course of its life cycle
• Alternation is between haploid and diploid phases
• Multicellular 1n individuals (gametophytes) produce multicellular 2n individuals (sporophytes)
• Multicellular 2n individuals (sporophytes) produce multicellular 1n individuals (gametophytes)
• The sporophyte (2n) is a multicellular individual which produces spores by meiosis
• A haploid spore develops into a new organism without fusing with another reproductive cell
• Spores undergo mitosis to becoming a new multicellular gametophyte
• The gametophyte (1n) is a multicelluar individual that produces gametes
• Gametes fuse in fertilization to form the zygote
• The zygote is a diploid cell that will become the spore-forming sporophyte
• Land plants differ as to which generation is dominant
• Gametophyte is dominant in mosses
• Sporophyte is dominant in ferns, pine trees, and peach trees
• The shift to sporophyte dominance is an adaptation to land
• The gametophyte becomes reduced in size and dependent on sporophyte

Adaptations to Land

• To conserve water, the land plant body is covered by a waxy cuticle
• Cuticles are impervious to water
• Cuticles allow carbon dioxide to enter so that photosynthesis can continue
• Stomata are tiny openings/pores mostly on the undersides of leaves
• A vascular system transports water in the body of the land plant
• Tracheids can be present or absent in different land plants
• Tracheids facilitate the upward transport of water and minerals
• Land plants are exposed to higher UV intensity than aquatic algae, which can lead to mutations
• A diploid genome can hide effects of a single deleterious allele
• Terrestrial plants have a diploid and haploid generation and the diploid allows for more genetic variability in land plants

Bryophytes (Nonvascular Plants)

• Bryophytes are nonvascular plants
• Support for the plant body is provided by vascular tissue; these plants are low-lying
• They are the first plants to colonize land
• They lack specialized means of transporting water and organic nutrients
• Bryophytes do not have true roots, stems, and leaves
• In bryophytes, the gametophyte is the dominant generation and gametes are produced in gametangia (archegonia and antheridia)
• Archegonia produce eggs
• Antheridia produce flagellated sperm
• Sperm swim to egg in a film of water

Mosses

• Considered the largest phyla of nonvascular plants
• Thrive in damp environments but when mosses are in dry environments, they shrivel, turn brown, and look dead but resume metabolic activity and turn green when moist
• Usually exhibit a leafy shoot, but some are secondarily flattened
• The sporophyte is always attached to the gametophyte

Liverworts

• Liverworts can have either flattened thallus (thallose liverworts) or a leafy appearance (leafy liverworts)
• Rhizoids can be found in some liverworts
• Marchantia is an example of a liverwort with rhizoids
• Rhizoids are hairy extensions that anchor the plant in soil and aid in absorption
• Liverworts can reproduce either sexually or asexually
• Gemmae are groups of cells that detach from the thallus and start a new plant (asexual)
• Sexual reproduction depends on either antheridia or archegonia.

Hornworts

• Hornworts have small sporophytes rising out of thin gametophytes that perform photosynthesis
• Hornworts most live in moist, well-shaded areas
• Hornworts are photosynthetic
• Symbiotic relationships with cyanobacteria allow nitrogen fixation from the air for hornworts
• Hornworts can bypass alternation of generations by reproducing asexually through fragmentation

Vascular Plants

• Vascular plants dominate the natural landscape
• Vascular plants can achieve great heights because of roots and vascular and nutrient-conducting tissue
• Xylem conducts water and dissolved minerals up from roots
• Phloem conducts sucrose and other organic compounds throughout the plant
• Lignin strengthens cell walls of conducting cells in xylem
• Most seedless vascular plants are homosporous
• Windblown spores are dispersal agents
• Cooksonia were some of the first early vascular plants

Lycophytes

• Lycophytes roots come off of a branching, underground rhizome.
• Licophyte roots evolved as lower stem extensions
• Lycophyte leaves are microphylls with one strand of vascular tissue
  • Microphylls evolved as simple side extensions of the stem
• Sporophylls are microphylls bearing sporangia
  • Sporophylls are grouped into club-shaped strobili
• The sporophyte generation is dominant
• Some lycophytes have spores that grow into one type of gamete (homosporous), others have spores with two types of spores (heterosporous)
  • Microspores become the male gametophyte and megaspores become a female gametophyte
• Today's lycophytes are called club mosses
  • Examples: Ground pines (Lycopodium), spike mosses (Selaginella), and quillworts (Isoetes)

Pteridophytes

• Pteridophytes are seedless vascular plants
• Ferns along with their allies: horsetails and whisk ferns belong to the pteridophytes
• Pteridophytes have megaphylls (broad leaves)
• Plants are able to efficiently harvest solar energy allowing them to produce more food and possibly create more offspring Resulting in increased fitness

Horsetails

• Consist of one genus, Equisetum, and 25 species
• Mostly grow in wet, marshy environments
• Most have a rhizome that produces aerial stems
• They contain whorls of slender, green branches
• Their leaves are small and are formed with whorls at the joints

Whisk Ferns

• Can exist as epiphytes (plants that live in or on trees)
• Have two genera: Psilotum and Tmesipteris
• Psilotum species lack leaves
  • Horizontal rhizomes rise to an aerial stem and forks repeatedly
  • Sporangia are on short side branches
• Tmesipteris species have reduced megaphylls

Ferns

• Are most abundant in tropical regions
• Range from 1 cm - 20 m
• Large, conspicuous megaphylls are called fronds and are divided into leaflets
• Clusters of sporangia on the undersides of fronds are called sori
• Dominant sporophyte produces windblown spores
• The windblown spores eventually germinate into gametophytes which make sperm
• The sperm then swim to the egg for fertilization
• When the gametophyte disappears the sporophyte becomes independent

The Uses of Ferns

• Used as a source of food
• Ferns are used as decoration
• Azolla harbors nitrogen-fixing cyanobacteria and grows in rice paddies, fertilizing rice plants
• Used as medicines in China
• Extracts from ferns have also been used to kill insects

Seed Plants

• Seed plants are vascular plants using seeds during the dispersal phase of their life cycle
• Most plentiful plants in the biosphere
• The seed coat and stored food allow the embryo to survive harsh conditions for a long period of time
• Heterosporous
• Microspores develop into drought-resistant pollen grains
• Megaspore develops into an ovule, becoming a seed after fertilization
• External water is not needed for fertilization
• Seeds allow plants to radiate onto drier land and become more abundant

Gymnosperms

• Seeds are not enclosed by fruit and are "naked", encompassing about 1,000 species in four groups
• Conifers, the most plentiful out of the four gymnosperm species
• Cycads
• Ginkos
• Gnetophytes

Conifers

• Produce cones
• Tough, needlelike leaves conserve water with a thick cuticle and recessed stomata
• Sporophyte is dominant
• Pollen grains are windblown
• Seed is the dispersal stage
• Monoecious: A single plant produces both pollen cones (male reproductive structure) and seed cones (female reproductive structure).

Cycads

• Includes 10 genera and 320 species of distinctive gymnosperms
• They showcase large finely divided leaves
• Cones can be more than 1 meter long
• Pollen/seed cones are on separate plants
• The species are pollinated by insects rather than wind
• Multiflaggelated sperm swim to reach an egg
• Existed during the dinosaurs' Mesozoic era
• Dinosaurs fed on cycad seeds

Ginkgoes

• Dioecious
• Some trees produce seeds.
• Other trees produce pollen.
• Have one surviving species: Ginkgo biloba
• Fleshy seeds ripen in fall and have a foul oder.

Gnetophytes

• There are three living genera with 70 diverse species
• Gnetophytes contain similarly structured xylem.
• They don't have the archegonia
• Have strobili with similar construction
• Gnetum consists of trees or climbing vines with broad, leathery leaves arranged in pairs
• Ephedrine is extracted from Ephedra, and is found in many medicines but has serious side effects

Angiosperms

• Angiosperms are flowering plants

Cotyledons

• Cotyledons are the seed leaves carrying nutrients that feed the embryo
• Two flower classes: monocots and eudicots
• Monocots contain one cotyledon in a seed
• Edicots contain two cotyledons in a seed

Flower Structure

• Flowers have a common structures
• Sepals (collectively, calyx) protect flower bud before it opens
• Petals (collectively, corolla) are used to attract a pollinator
• Stamens consist of a filament (stalk) and anther
• Carpel is a vaselike structure with three regions
• Stigma: Sticky receptor of pollen grains
• Style: Elevates stigma
• Ovary: Becomes fruit
  • Enlarged base that encloses one or two ovules (seeds)
• A complete flower has all four parts; otherwise, it is incomplete

Flower Attributes

• Each stamen has an anther and a filament (stalk)
• Carpel has three major regions
• Ovary: A base enclosing ovules
  • Ovules develop into seeds
  • The ovary develops into a fruit
• The Style Elevates stigma
• The Stigma is a sticky receptor attracting pollination
• Wind-pollinated flowers are not showy
• Bird/insect-pollinated flowers are colorful
• Night-blooming flowers attract nocturnal mammals/insects
  • Are typically be white or cream-colored and give off an aroma
• Fruits of flowers protect and aid in seed dispersal Utilizes wind, gravity, water, and animals for dispersal

Flower Terminology

• Complete: All four parts (sepals, petals, stamens, and carpels) present
• Incomplete: Lacks one or more of the four parts
• Perfect: Has both stamens and (a) carpel(s)
• Imperfect: Has stamens or (a) carpel(s), but not both
• Inflorescence: A cluster of flowers
• Composite: Appears to be a single flower but consists of a group of tiny flowers