The Diversification of Seed Plants Flashcards
How did seed plants revolutionise reproduction
Seed plants (spermatophytes) no longer required water for fertilisation. They protected and nourished the spore within the sporophyte, increasing seed survival
Why did seed plants become dominant by the Permian
By the Permian (~260 mya), drier climates from Pangaea’s formation favoured seed plants over spore-producing plants like ferns and lycophytes
What is heterospory in plants
The production of two types of spores:
Microspores (male) → pollen grains
Megaspores (female) → megagametophyte within the sporophyte
How many megaspores survive in seed plants
Only one megaspore per megasporangium survives and develops into a megagametophyte; others degenerate (fossil evidence shows 3 degenerate spores)
What is the function of integuments in seed plants
Integuments are layers of sterile sporophyte tissue that enclose the megasporangium, forming the ovule and leaving a micropyle for pollen entry
What makes a plant a seed plant
A seed plant retains and nourishes a single megaspore within the nucellus (modified megasporangium), which becomes an ovule and eventually a seed
What is the structure of an ovule
Nucellus: Megasporangium
Integuments: Protective tissue layers (form seed coat)
Micropyle: Opening for pollen
A fertilised ovule develops into a seed containing the embryo and stored nutrients.
How does a gymnosperm ovule develop
- One of four megaspores survives and forms the megagametophyte.
- A pollen grain enters via a droplet and fertilises the egg.
- The zygote develops into an embryo; the integuments form the seed coat.
What is endospory in seed plants
means the male gametophyte develops inside the spore so pollen grains develop within microspores on the sporophyte
How is pollen adapted for fertilisation without water
Pollen grains are wind or animal dispersed and form pollen tubes to reach the egg removing the need for water
What cells are in a mature pollen grain
Two vestigial prothalial cells
One generative cell (divides into two sperm)
One tube cell (forms the pollen tube)
What are the evolutionary advantages of seeds
Embryo protection
Nutrient storage
Independence from water
Dormancy during harsh conditions
Enhanced dispersal
What are euphyllophytes
include all seeds plants and ferns and are plants with true leaves - megaphylls
What were progymnosperms
They were extinct woody, spore-bearing plants believed to be the ancestors of gymnosperms
What was Archaeopteris and why is significant
A woody spore-producing plant that showed features of both ferns and seed plants - an important transitional form towards seed plant evolution
What is Wattieza known for in plant evolution
The first tree-sized plant marking an evolutionary step in the rise of tall vascular plants
What type of plant was Medullosa
An early seed fern which showed clear signs of seed-bearing structures in a fern-like plant
What was a Cordaites
A gymnosperm ancestor with thick, strap-like leaves - considered an important precursor to modern conifers
When did gymnosperms evolve and dominate
Gymnosperms first appeared in the Carboniferous (~350 mya) and expanded during the Permian (~300 mya), replacing spore-based forest
What does ‘gymnosperm’ mean
Gymnosperm means ‘naked seed’ - ovules are exposed, not enclosed in fruit
What were early gymnosperm groups
Pteridosperms: Seed ferns
Cordaitales: Conifer-like trees
How did climate change affect plant dominance
The formation of Pangaea led to arid conditions and seasonal extremes.
This meant that seed plants thrived whilst spore plants declined
What is an eustele and why is it important
It is a vascular system with discrete bundles, proving better transport and support, important because its development allowed larger seed plants to exist
What is the vascular cambium
A meristem that allows lateral (secondary) growth by producing secondary xylem (wood) and phloem - common in woody plants like gymnosperms
