Bio 140: Chapters 29-31

Origin of land plants

all green algae and land plants shared a common ancestor a little over a billion years ago

Origin of land plant support

DNA sequences data

Cuticle

helped overcome desiccation (water loss) and protect from harmful effects of the sun

Stomata

helped maintain gas exchange

Land plants and fungi

fungi helped plants colonize land
made nutrients available

Two clades of green algae

chlorophytes and charophytes

Chlorophytes

never made it to land

Charophytes

sister to all land plants

Haplodiplontic

all land plants have multicellular haploid and diploid stages
trend toward more embryo protection and smaller haploid stage

Tracheids

how water is moved, consists of xylem and phloem

Sporophyte

multicellular diploid stage
produces spores by meiosis

Sporangia

diploid spore mother cells (sporocytes) undergo meiosis in this form
produces four haploid spores of gametophyte generation

Gametophyte

multicellular haploid stage
spores divide by mitosis
produces gametes which fuse to form the diploid zygote of the sporophyte generation

Moss sizes

large gametophyte, small dependent sporophyte

Angiosperm sizes

small dependent gametophyte, large sporophyte

Bryophytes

closest living descendants of the first land plants
called non-tracheophytes, lack tracheids or other conducting spells

Bryophyte characteristics

simple but highly adapted to diverse terrain
16,000 species in 3 clades
conspicuous gametophytes and small sporophytes

Bryophyte clades

liverworts, mosses, and hornworts

Liverworts

flattened gametophytes with liverlike lobes (80% look like mosses)

Liverwort gametangia

umbrella shaped structures

Mosses

small leaflike (not leaves because of no vascular tissue) structures around a stemlike axis --> gametophytes

Rhizoids

how mosses are anchored to the substrate

Archegonia

female gametangia of mosses

Antheridia

male gametangia of mosses
sperm must swim in water

Hornworts

puzzling origins due to no fossils until cretaceous
photosynthetic sporophytes
cells have one large chloroplast

Cooksonia

first vascular land plant, appeared 410 MYA
short with no roots or leaves and homosporous

Homosporous

only one type of spore produced

Xylem

conducts water and dissolved minerals upward from the roots

Phloem

conducts sucrose and hormones throughout the plant

Vascular tissues

xylem and phloem, enable enhanced height and size
develop sporophyta but not gametophyte

Tracheophyte clades

lycophytes, pterophytes, and seed plants
reduced gametophyte size and gametangia

Stems

early fossils reveal stems but no roots or leaves
contain food supply for young plants

Roots

provide transportation and support, lycophytes diverged before true roots appeared
lack of roots limited early tracheophytes

Leaves

increase surface area for photosynthesis
evolved twice

Lycophylls

found in seed plants with a singular vein

Euphylls

true leaves, found in ferns and seed plants with branched veins

Lycophytes

worldwide distribution but abundant in tropics
lack seeds (resemble mosses)
sporophyte dominant

Pterophytes

phylogenetic relationships among ferns and relatives is still unknown
form antheridia and archegonia, require free water for flagellated sperm

Pterophyte clades

whisk ferns and horsetails

Pterophyte: Whisk Fern

found in tropics, sporophyte is evenly forking green stems without true leaves or roots
some gametophytes develop elements of vascular tissues

Pterophyte: Horsetails

all 15 species are homosporous
one single genus, Equisetum
sporophyte is ribbed, jointed photosynthetic stems arising from branching rhizomes with roots at nodes

Scouring rush

silica deposits in cells

Ferns

most abundant group of seedless vascular plants (11,000 species)
coal formed from ferns
conspicuous sporophyte and smaller gametophyte (photosynthetic)

Fern life cycle

different from mosses, greater development and independence in sporophytes, gametophyte lacks vascular tissue

Fern morphology

sporophytes have rhizomes and not true roots
leaves (fronds) develop at the tip of the rhizome as fiddleheads

Sori

distinctive fern sporangia in clusters on the back of leaves

Seed plant evolution

began to diversify from seedless ancestors 319 MYA

Evolution of the seed

protects and provides food for the embryo
allows the clock to be stopped to survive harsh environments before germination
later fruit development enhanced dispersal

Integument

an extra layer or two of sporophyte tissue that hardens into the seed coat and protects the embryo

Megasporangium

divides meiotically inside ovule to produce haploid megaspore, which then produces the egg that combines with sperm

Male gametophytes

pollen grains dispersed by wind or a pollinator, no need for water

Female gametophytes

develop within an ovule, enclosed within diploid sporophyte tissues in angiosperms

Female ovary

ovule and protective tissue that develops into fruit

Gymnosperms

plants with "naked seeds"
lack flowers and fruits
ovule exposed on a scale

Four living groups of gymnosperms

coniferophytes, cycadophytes, gnetophytes, ginkgophytes

Gymnosperms: Conifers

pines, spores, firs, cedars, etc.
found in colder and sometimes drier regions
sources of important products like timber, paper, resin

Costal redwood

tallest living vascular plant

Bristlecone pine

oldest living tree

Gymnosperms: Pines

more than 100 species in the northern hemisphere
produce needlelike leaves in clusters
leaves with thick cuticle and recessed stomata to slow water loss

Resin

pines have leaves with this to deter insects and fungal attacks

Pine reproduction

male gametophytes are pollen grains formed from microspores by meiosis
female pine cones form on the upper branches (larger with woody scales)

Nucellus

each ovule of a pine contains a megasporangium with a specific name, and is then surrounded by the integument

Micropyle

small opening at the end of the integument

Pine fertilization

1. During the first spring, pollen grains drift down between open scales
2. Pollen grains are drawn down into the micropyle and scales close
3. 12 months later, female gametophyte matures as the pollen tube is digesting its way through and mature male gametophyte has 2 sperm
4. 15 months after pollination, pollen tube reaches the archegonium and discharges content
5. One sperm unites with the egg to form a zygote and the other sperm degenerates

Gymnosperms: Cycads

slow growing gymnosperms of tropical and subtropical regions, sporophytes of this clade resemble palm trees and female cones can weigh up to 45 lbs

Gymnosperms: Gnetophytes

only gymnosperms with vessels in their xylem, composed of linking vessel element cells rather than tracheids
cells join end to end in xylem tissue and are common in flowering plants

Gymnosperms: Ginkgophytes

only one living species remains
dioecious species

Dioecious

male and female reproductive structures form on different trees

Angiosperms

flowering plants that have ovules enclosed in diploid tissue at the time of pollination

Carpel

a modified leaf that covers seeds and develops into fruit

Angiosperm abundance

changed earth's terrain that was previously dominated by ferns, cycads, and conifers
unique features include flower production, insect pollination, and broad leaves with thick veins

Angiosperm origins

oldest known in the fossil record is archaefructus
125 million years old but unlikely to have been the first
lack sepals and petals

Flower morphology

modified stems bearing modified leaves

Pedicel

primordium develops into a bud at the end of a stalk

Receptacle

pedicel expands at the tip creating the place where other parts attach

Whorls

flower parts are arranged into circles

Flower whorls

sepal --> outermost
petals
stamens (androecium)
gynoecium --> innermost

Stamens

pollen bearing anther and a filament (stalk)

Gynoecium

consists of one or more carpels, houses the female gametophyte

Major regions of the carpal

ovary, stigma, and style

Ovary

swollen base containing ovules that later develops into the fruit

Stigma

tip of carpel where pollen lands

Style

neck or stalk of the carpal

Double fertilization

1. A single diploid megaspore mother cell in ovule undergoes meiosis to produce four haploid megaspores of which one survive
2. The daughter nuclei divide to produce eight haploid nuclei in two groups of four
3. Two nuclei (one from each group) migrate toward the center
4. Cell closest to the micropyle becomes the egg
5. Two other cells are synergids
6. Antipodals are the three cells at other end with no function
7. Integuments become the seed coat

Pollen production

occurs in the anthers
similar but less complex than female gametophyte formation
diploid microspore mother cells produce four haploid microspores
binucleate microspores become pollen grains

Pollenation

mechanical transfer of pollen from anther to stigma
may or may not be followed by fertilization
pollen grains develop a pollen tube that is guided to the embryo sac
the grain cell that lags behind produces the two sperm cells

Double fertilization and seed formation

one sperm unites with egg to form the diploid zygote --> new sporophyte
the other sperm unites with the two polar nuclei to form the triploid endosperm that provides the embryo with nutrients
seed may remain dormant for many years

How Seeds Protect Embryos

1. They maintain dormancy under unfavorable conditions
2. They protect the young plant when it is most vulnerable
3. They provide food for the embryo until it can provide its own food
4. They facilitate dispersal of the embryo

Metabolic activities

cease once the seed coat forms
germination cannot take place until water and oxygen reach the embryo

Germination

specific adaptations ensure that seeds will germinate only under appropriate conditions
ex: some seeds lie within tough cones that do not open until exposed to fire

Fruits

mature ovaries
often formed from the flower ovary but sometimes occurs without seed development

Pericarp

the ovary wall containing three layers

Layers of the pericarp

exocarp, mesocarp, and endocarp determine the fruit type

Fruit genotypes

three in one package
fruits and seed coat from prior sporophyte generation
remnants of gametophyte generation produced egg
embryo represents next sporophyte generation

Fruit dispersal

occurs through a wide variety of methods
ingestion and transportation by birds/vertebrates
hooked spines on birds or mammals
blowing in the wind
floating and drifting on water

Fungal species

1.5 million species
single celled/multicellular
sexual/asexual
unusual mitosis

6 major monophyletic phyla of fungi

blastocladiomycota, neocallistigomycota, chytridiomycota, glomeromycota, basidiomycota, and ascomycota

1 paraphyletic phyla of fungi

zygomycota, sometimes microsporidia

Hyphae

long slender filaments in multicellular fungi, some continuous and some divided by septa
cytoplasm flows throughout for rapid growth

Mycelium

mass of connected hyphae
grows through and digests substrate