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Gymnosperms
Chapter 18
Seeds
 All seed plants are heterosporous, with megspores and microspores
 Seed is simply a mature ovule containing an embryo
 embryo, stored food and seed coat
 Immature ovule consists of megasporangium surrounded by tissues called the
integuments
Evolution of ovule
 Retention of megaspores within the megasporangium
 Reduction in the number of megaspore mother cells to one
 Survival of only one megaspore
 Formation of megagametophyte inside single megaspore, retained within
megsporangium
Evolution of ovule
 Development of embryo within the megagametopyte
 Formation of integument with micropyle
 Modification of apex of megasporangium to receive microspores
 Result is dispersal of seeds instead of spores
Seed plants – five extant phyla
 Cycadophyta
 Ginkgophyta
 Coniferophyta
 Gnetophyta
 Anthophyta
Gymnosperms
 Seeds not contained in fruits
 Seeds are exposed on sporophylls
 Four phyla of extant gymnosperms
 Cycadophyta
 Ginkgophyta
 Coniferophyta
 Gnetophyta
Pollen grains
 Water is not required for fertilization
 Cycads and Ginkgo have flagellated sperm
 Microgametophyte develops into pollen grains
 Pollen is transferred during process of pollination
 Pollen produces a tubular outgrowth called a pollen tube
Phylum Coniferophyta
 Most widespread and ecologically important of the gymnosperm phyla
 Diversified during drying of Permian period
Genus Pinus
 Needles (leaves) in bundles called fasicles
Pine leaves
 Adapted for low water conditions
 Cuticle covers epidermis
 Hypodermis below is tightly packed
 Sunken stomata
 Maintain needles for 2-4 years
Conifers
 Remember that:
 Xylem are trachieds
 Phloem are sieve cells
Pine life cycle
 Microsporangia and megasporangia are borne on cones or stroboli
 Megasporangia normally on top of tree
Microsporangate cones
 Young microsporangium contains many microsporophytes (microspore mother
cells) which undergo meiosis to for four haploid microspores
 Each microspore develops into a winged pollen grain or microgametophyte
 Two prothallial cells
 Generative cell
 Tube cell
Megasporangate cones
 Larger and more complex
 Ovule is megasporangium surrounded by integument with micropyle opening
 Each megasporangium contains megaspore, which undergoes meiosis to form
four megaspores
 Only one is functional
Pollination
 Occurs in spring
 Pollen grain germinates and pollen tube digests tissue to get to developing
megagametophyte
 One year after pollination, generative cell undergoes division to form sterile
cell (stalk cell) and spermatogenous cell (body cell), which then divides to
become two sperm
 Megaspores and then archegonia produced one year after pollination
 Egg meets sperm 15 months are pollination
Cypress
Juniper
Yew
Norfolk Island Pine
Sequoiadendron
Cycadaceae
 Tropical and subtropical areas
 Resemble palms
 Often toxic
 Contain cyanobacteria
 Separate males and females
 Insect pollinated
Cycad sperm
Gingkophyta
 Gingko biloba
 Deciduous
 Preserved on temple grounds
 Separate males and females
 Fruits contain butanoic and hexanoic acid
 Rancid butter and Romano cheese
Gnetophyta
 Members have angiosperm-like features
 Flower-like stroboli
 Similar vessels in xylem
 Lack of archegonia
 Double fertilization
Welwitschia