Why Move to Land?
1. More light
2. Minerals - plants were the first "miners"
3.Symbiotic association with fungi - mychorrhizae - mineral and water uptake
Major adaptations to life on land
1. prevention of water loss - cuticle/stomata
2. structural support to resist gravity - vascular system to move water and photosynthate
3. new reproductive strategies - symbiotic assoication with emerging animals
4. prevention of becoming a food source for emerging animals
5. symbiotic assoication with fungi - mycorrhizae. Absolutely important for extracting minerals from the soil.
All green algae and land plants shared a common ancestor a little over 1 BYA. Kingdom Virdiplantae. A single species of freshwater green algae gave rise to the entire terrestrial plant lineage.
a monophyletic group of organisms that share homologous features dderived from a commmon ancestor.
Have multicellular haploid and diploid stages. Trend toward more diploid embryo protection. Trend toward smalled haploid stage. Diploid stage dominates life cycle and larger.
Why diploid dominance?
1. About 550 million years ago, UV radiation was still high - water protected.
2. Coming to land menay DNA damage. being diploid - could mask DNA damage in recessice gene on alternate chromosome.
3. Larger number of alleles exist in gene pool - greater diversity
Multicellular diploid stage
Sporophyte - 2N.
Produces haploid spores by meiosis. Diplois spore mother cells (sporocytes) undergo meiosis by sporangia. Produces 4 haploid spores and first cells of gametophyte generation.
Multicellular haploid stage
gametophyte - N
Spores divide by mitosis. Produces gametes by mitosis. Gametes fuse to form diploid zygote. First cell of next sporophyte generation.
All land plants are haplodiplontic.
Relatice size of generation vary.
Moss - large gametophyte. Small, dependent sporophyte.
Angiosperm-small dependent gametophyte, large sporophyte.
Clade of streptophytes. Also green algae. Distinguised from chlotophytes by close phylogenetic relationship to land plants. Haplontic life cycles. Evolution of diplontic embryo and haplodiplontic life cycle occured after move to land.
2 Candidate Charophyta clades
Charales and Coleochaetales. One species of Chara must have successfully inched its way onto land through adaptations to drying
Closest living descendants of the first land plants. Called nontracheophytes because they lack tracheids. Do have other conducting cells. Mychorrhizal assoiciations important enhancing water uptake.
conspicuous and photosynthetic. Sporophytes - small and dependent. Requires water for sexual reproduction.
Gametophytes consist of small, leadlike strucutres around a stemlikek axis. Not true leaves - no vascular tissue. .
Anchored to substrate by rhizoids.
Cooksonia, the first vascular land plant.
Appeared about 420 MYA. Only a few centimeters tall, no roots or leaves. Homosporous - only one type of spore.
Vascualar plants include seven extant phyla grouped in three clades.
1. Lycophytes (club mosses)
2. Pterophytes (ferns, whisk ferns, and horsetails)
3. seed plants
Worldwide distribution - sbundant in tropics. Lack seeds. Superficially resemble true mossess.
Limitations of Early Land Plants
Stems - early fossils reveal stems but no roots or leaves. Lack of roots limited early tracheophytes
Roots- provide transport and support. Lycophytes diverged before true roots appeared.
Leaves- Increased surface area for photosynthesis. Evolved twice - Euphylls (true leaves) found in ferns and seed plants and Lycophylls found in seed plants
Phylogenetic relationship among ferns and their relatives is still being sorted out. Common ancestor gave rise to 2 clades. All form antheridia and archegonia.
Found in tropic, Sporophyte consists of evenly forking green stems without true leaves or roots. Some gametophytes develop elements of vascular tissue.
All living species are homosporous. Ribbed, jointed photosynthetic stems that arise from branching rhizomes with roots at nodes. Silica deposits in cells - "scouring rush" of pioneer days.
Most abundant group of seedless vascular plants. Coal formed from forest 300 MYA. Conspicuous sporophyte and much smaller gametophyte are both photosynthetic. Fern life cycle differs from that of moss. Much greter devlopment, independence and dominance of ther fern's sporophyte. Gametophyte lacks vascular tissue.
Produce distinctive sporangia in clusters called sori on the back of the fronds. Diplois spore mother cells in sporangia produce haploid spores by meiosis. Spores germinate into gametophyte. Rhizoids but not true roots - no vascular tissue. Flagellated sperm.
Evolution of Seed Plants
Appeared 305-465 MYA. Evolved from spore bearing plants known as progymnosperms.
Success attributed to evolution of seed
Protects and provides food for embryo. Allows the clock to be stopped to survive harsh periods before germinating, later development of fruits enhanced dispersal.
Seed-internal fertilization, escape from water
Embryo protected by integument (can escape desiccation and hardened seed coat), megasporangium divides meiotically inside ovule to produce haploid megaspore, megasporie produces egg that combines with sperm to form zygote and also contains food supply for embryo.
2 kinds of gametophytes
Male - pollen grains, dispersed by wind or a pollinator, no need for water.
Female - devlop within an ovule, enclosed within diploid sporophyte tissue in angiosperms.
Plants with naked seeds.
All lack glowers and fruits of angiosperms, all have ovule exposed on a scale.
Most familiar gymnosperm phylum.
Pines, sprices, firs, cedars and others. Found in colder and sometimes drier regions of the world. Conifers are sources of important products. Timber, paper, resin and taxol (anti-cancer)
Example of Conifer. More than 100 species, all in Northern hemisphere. Leaves have thick cuticle and recessed stomata to retard water loss. Leaves have canals with resin to deter insect and fungal attacks.
Male gametophytes (pollen grains) - develop from microspores in male cones by meiosis.
Femal pine cones form on the upper branches of the same tree. Female cones are larger and have woody scales. Two ovules develop on each scale. Each contain a megasporangium. (each will become a female gametophyte)
Female cones take 2 or more season to mature. During the first season the pollen grains drift down between open scales. (pollen grains drawn down into micropyle, scales close).
A year later, female gametophyte matures. Pollen tube is digesting its way thorugh and mature male gametophyte has 2 sperm.
15 months after pollinaiton, pollen tube reaches archegonium and discharges contents. One sperm united with egg=sygote. Other sperm degenerates.
Slowing growing gymnosperms of tropical and subtropical regions. Sporophytes resemble palm trees. Female cones can weigh 45 kg. Have the largest sperm cells of all organisms.
Only gynmonsperm with vessels in their xylem. Contain three genera
Only one living species remains - now a cultivar. Ginkgobiloba. Flagellated sperm, Dioecious (male and female reproductive structures form on different trees)
Flowering plants, Ovules are enclosed in diplois tissue at the time of pollination. Carpel, a modified leaf that covers seeds, develops into fruits.
Origin as early as 145-208 MYA.
Oldest known angiosperm in the fossil record is Archaefructus. Closest living relative to the orginal angiosperm is Amborella.
Modified stems bearing modified leaves. Primordium develops into a bud at the end of a stalk called the pedicel. Pedicel expands at the tip to form a receptable, to which other parts attach. Flower parts are organized in circles called whorls.
Carpel's 3 regions
Ovary - swollen base containing ovules
Stigma - tip where pollen lands
Style - neck ot stalk
Single megaspore mother cell in ovule undergoes meiosis. Produces 4 megaspores, daughter nuclei divide to produce 8 haploid nuclei and integuments become seed coat.
occurs in the anthers. It is similar but less complex than female gametophyte formation. Diploid microspore mother cells undergo meiosis to produce four haploid microspores Binucleate microspores become pollen grains.
Mechanical transfer of pollen from anther to stigma. May or may not be followed by fertilization. Pollen grains devlop a pollen tube that is guided to the embryo sac. One of the two pollen grain cells lag behind, this generative cell divided to produce two sperm cells and no flagella on sperm.
One sperm united with egg to form the diploid zygote. Other sperm united with the two polar nuclei to form the triploid endosperm. Seed may remain dormant for many years.
Ecological Propertoes of C. americana
1. lacking chlorophyll, but possessing all other traits of Angiosperms - flowers, seeds.
2. parasitic on roots of plants, but causes the host no discernable damage.
3. DNA has been sequenced and the genes for photosynthesis proteins are lacking
4. achlotophyllus/parasitic life style arose by mutation