21: Seed Plants (Gymnosperms)

Wollemi Pine

"living fossil" found in Sydney, Australia -prehistoric-looking Aracucaia -southern conifer

Coniferophyta (leaders)

Largest group of gymnosperms: species RICH -Record holders -Pinus aristata (bristlecone pine): longest-lived individuals in world -Sequoiadendron gigantea (Giant Sequoias): widest girth trees -Sequoia sempervirens (coatal redwoods): tallest trees in world

Integuments

(2n) parental tissue flaps that enclose megaspore -creates ovule by enveloping sporangium

Summary---*

(female) megagametophyte never leaves the megasporangium which is part of the sporophyte parent plant -Can take up to year for sperm to fertilize egg--- one reason why we don't see rapid generation turnover among gymnosperms

Advantages of Seed Production

-Seed coat allows protection and extended dormancy -multicellular embryo + food storage give new sporophyte head start to germination -Larger + more complex than simple spores -More resistant to harsh conditions-- increased dispersal adaptions **More costly to produce Quality> quantity

Coniferophyta characteristics

-bract-scale morphology in female cones -pollen non-flagellated; delivered by wind to ovules -wood bears only tracheids in xylem, no vessel elements -very diverse repoductively, juniper+ yew "berries" + classic pine cones -needle leaves + scale leaves -Mostly monoecious- seperate strobili -dimorphic

Cycadophyta: the cycads

-palm-like plants, but produce cones, not flowers -quite ancient 300 species today -seperate female (seed) and male (pollen plants) -mostly insect pollinated: cones produce heat+ odor to attract and repel thrips and bettles -NOT enclosed megasporangia -Largest sperm cells- flagelluated (vestigial) -sperm not released until pollen already germinated inside the ovule-- discharges sperm in enclosed pollen chamber

Ginkgophyta: Ginkgo biloba

-single living species, not known in wild -another ancient lineage, lots of fossil species known -unusual leaf venatioin -pollen from male cones, ovules produced in stalked pairs, not in cones -fleshy seeds (NOT fruites) -pollination in spring, fertilization in fall (very slow growing pollen tube) --embryo continues to develop up until the following spring

Heterosporous

-some progymnosperms -precondition for seed production

gymnosperms

900+ species -phylogency not completely settled yet -- sister to angiosperm or grade leading up? -seeds exposed to air, not protected by sporophyll

Monoecious

Both sexes on same plant

Pine Pollen

Dispersed by smaller male cones: produce clouds to be dispersed by wind -have lateral air bladders to help it float

Wood Evolution

Homoplasy convergent evolution: evolved three different times in distinct linages -secondary xylem (wood) develops in multiple linages

Big trees?

Lycophytes: unable to divide in RADIAL direction within cambium, no girth increase -addition of layers towards the inside outer cells shed Lignophytes (modern woody plants): capable of radial division -allow increase in girth of camium -outer cells can accomodate inner expansion

Advantages of Pollen> Spores

Male gametophyte is protected (sporopollenin) -Pollen disperse over long distances to find suitable mate -Fertilization does not require water-- find way to egg by pollinator or wind

Pollen

Male gametophytes -can travel through air, without water -reduced microgametophyte

Seed

Mature ovule with three seperate generations of plant tissue 1. Seed coat (derived from integument)- 2n parental tissue 2. Food supply (female gametophyte tissue)- n 3. Embryo (new sporophyte)- 2n

Innovation 2 of Seed Plants

Megaspores do not disperse or develop into independent gametophytes -(female) megasporangium produces single megaspore mother cell -divides meiotically -produces single haploid (n) spore (other 3 cells abort) -divides mitotically to produce archegonia-- remains enclosed by parental tissues (protected, not dispersed) -when pollen reaches ovule, fertilizes it, and embryo develops, all encased in parental tissue **3 tissue types 1. Integrument (2N) outer 2. Megasporangium (2N) 3. Female Gametophyte (N) 4. Eggs (N)>>>Embryo (2N)

Development of Microgametophyte

Microspore has three mitotic divisions to produce microgametophyte with 3 nuclei -once reaches ovule tube -Sterile cell -Spermatogenous cell (sperm) -Tube cell **male gametophytes in gymnosperms have more nuclei than angiosperms

Bifacial Cambium

Produce secondary phloem towards the outside of cambium -Lignophytes

Microsporangia

Produce tetrads of haploid microspores by meisois -each microspore surrounded by resistant sporopollenin coating-- goes through mitotic divisions -Four seperate pollen grains

Innovation 1 of seed plants

Protection of the sperm -encapsulate sperm so can spread through air NOT water -pollen: reduced male microgametophyte-- only able to be produced in heterosporous plant

Gnetophyta

Three different genera: 1. Gnetum: tropical, have vessels- 30 species 2. Ephedra: temperate (look like horsetails), male/feamle cones- adaptive to desert climates 30 species 3. Welwitschia Mirabilis: single species- "living fossil"

Unifacial Cambium

Trees that do not produce phloem towards outside -Lycophytes

Ovule

Unfertilized seed -sporangium enveloped by integuments