what are lycophytes?
First vascular plants
Look like ferns, not ferns
Large, long-lived autotrophic plants
Some are aquatic
Evolved leaves for the first time!
Only about 1500 species but lots of morphological diversity
how did vascular plants evolve?
Hornwort vertical axial sporophyte gave rise to vascular plants (unlike upward growth in moss)
Photosynthetic sporophytes in hornworts caused evolutionary radiation
Gametophytes in moss were short-lived, temporarily photosynthetic and growth does not give access to light so did not evolve further
fossil evidence of sporophyte evolution
rhyniophyta fossils around 420 million years old
10-40cm in height (sporophytes)
Had rhizomes (not true roots)
First sporophytes to have branches - more photosynthetic area and more sporangia
Leaves had not yet evolved, just stems
Vascular strand (stele) that provided conduction and mechanical support
Thought that rhyniophytes appeared between hornworts and lycopodium
early lycopod
Zostereophyllophyta
About 410 million years ago
Similar to rhyniophytes with stele, rhizomes and branching stems
No leaves, but enations (flaps of stem) that produce more photosynthetic area
Lateral sporangia on branches - further increasing of sporangium capacity
what are the features of lycophytes?
- Autotrophic phase is sporophyte, gametophyte is less important
- Microphyll: small leaves accessed by one vascular strand
- Dichotomous branching (growing tip divides in two) - seed plants do not do this
- Lateral sporangia: one associated with the base of leaves
- Sporophylls: can be dispersed among vegetative leaves (look similar) or can be clustered in a separate morphological organ called a strobilus
describe photosynthetic gametophytes
Sporophytes produce spores by meiosis
Spores grow mitotically to produce gametophytes (haploid)
Gametophytes are small and photosynthetic - sperm splash in water to archegonium to produce diploid zygote
Diploid zygotę forms a sporophyte still attached to the gametophyte
describe underground gametophytes
Saprophytic underground gametophytes
Supported by fungi
Still produces sperm and eggs to produce sporophytes
what are strobili and what is heterospory?
- Strobili: sporangia produce either 4 large spores (megaspore) or many tiny spores (microspores)
- Heterospory - important in land plant evolution as it leads to seed heads on plants
- Megaspores produce female gametophytes (only produce eggs) - get resources from nutrients in megaspore
- Microspores produce microgametophytes that swim through water to fertilise eggs
what is an example of a past lycopod?
- Lepidodendron
- 80 million years ago
- Only woody lycopods (gone now)
- Heterosporous
- Typical lycopod branching with microphylls
- So abundant that they drew down carbon dioxide concentration
- Lived in swamp forests where organic material accumulated when they died (fossil fuels)
