Gymnosperms

Gymnosperms

the first seed plants, came to be during the Mesozoic period

sporophyte dominant- male gametophyte dispersed into wind and germinate within female cone

Characteristics of Gymnosperms

produce seeds in strobili-like structures; seeds are not protected

gametophyte does not grow independent of the sporophyte

most are monoecious- male and female reproductive structure on one organism

all modern gymnosperms are woody: trees, shrubs, or lianas (woody vines)

Reproductive Characteristics

cycads and gingkoes- some reproductive characteristics (flagellated sperm) more like seedless plants

conifers and gnetophytes- some reproductive characteristics (non-flagellated sperm, double fertilization) more like Angiosperms

Cycads

tropical and subtropical; often grown as houseplants

all species are endangered in their natural habitats

resemble palms but stems are covered with tough, persistent leaf bases

can be large (up to 40 ft); leaves up to 29 ft long, seed cones up to 30 in long and weighing 40 kg that emit an odor when mature that attracts beetle pollinators

Cycads as Food

food of native peoples in Australia and some Pacific Islands; Florida has one used for arrowroot powder

however, all cycads produce toxins and are difficult to remove

seed flour contains BMAA amino acid that can increase chance of dementia

Cycad Roots

many produce coralloid roots- grow above ground and are symbionic with cyanobacteria which fix nitrogen and produce toxins 

Ginkgo

only remaining species is ginkgo biloba (evolved 251-286 MYA)

probably extinct in the wild, especially due to global warming event

dioecious- individual trees produce either pollen (male parts) or ovules and seeds (female parts), which have a bad smelling seed coat

Conifers

first known from fossils and believed to be extinct until rediscovered in 1944

pinewood contains primarily tracheids and no fibers, canals contain resin

evergreen; shed needles after 2-4 years; low photosynthesis during cold seasons

some are deciduous and shed needles all at one time (ex: tamarack, bald cypress, dawn redwood)

Conifer Adaptations

conical shape and flexible branches to shed snow; leaves adapted to dry conditions, surface area is reduced with a thick, waxy cuticle to prevent water loss; stomata is sunken; hypodermis protects underlying cells

resin may trap insects and protects leaves against pathogens and predators

Gnetophytes

gnetum- grow in vines, shrubs or trees in Asia and Africa 

have broad leaves not seen in other gymnosperms; considered gymnosperm due to its reproductive cycle similarity

Gnetophytes Historyish

absence of archegonia- lost during evolution as female gametophytes decreased in size

DNA evidence shows that gnetophytes had conifer ancestors; fossils indicate that angiosperms evolved from gymnosperms; not known whether gnetophytes were ancestral link to angiosperms

Ephedra (Mormon Tea)

a well-known gnetophyte found in southwestern US

leaves are tiny brown scales

contains the alkaloid ephedrine often used for weight loss (can have adverse cardiac effects); purpose of alkaloid is actually to prevent predation

Welwitschia

gnetophyte that grows in the coastal deserts of southwestern Africa

has long taproot; two very long leaves; obtains much of its moisture from coastal fog

climate change is threatening them since fog is reduced

Production & Structure of Ovules

heterosporous- two spores: megaspore and microspore 

diploid megasporangium goes through meiosis and produces haploid

seed plants produce microspores in microsporangium and release microspores containing male gametophyte pollen grains

megaspores are in megasporangia

Lifecycle of Pine

Ecological Advantage of Seeds

can remain dormant in the soil for years

seed walls are thicker than spores- offer more protection against pathogens/mechanical damage; store more food than spores

seed coats can be modified (through adaptation) to facilitate dispersal by wind or animals

most seed tissues are diploid, so 2 alleles for each gene