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Seed Plants I: Seed Plants Without Flow (Division Progymnospermophyta:…
Seed Plants I: Seed Plants Without Flow
Division Coniferophyta: Conifers
Never vines, herbs, or annuals
Leaves are always simple needles or scales
Leaves of most conifers are perennial
Venation of conifer leaves is often simple
Leaf veins have an endodermis and a tissue called transfusion tissue
Never have a bulb or rhizomes
Pines
Have two types of shoot, each with a characteristic type of leaf
Tiny papery leaves occur on long shoots
In their axils are short shoots that produce the familiar long needle leaves
The leaves have many xeromorphic characters
Thick cuticle
Sunken stomata
Cylindrical shape
Pines have both pollen cones and seed cones
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Pollen cones are simple cones with a single short unbranched axis that bears microsporophylls
Ovuliferous scale
The axillary bud is microscopic, and its megasporophylls are fused laterally
Diverse (approximately 50 genera and 550 species)
And all are trees of moderate to gigantic size
Division Cycadophyta: Cycads
Modern cycads are confused w/ either ferns or young palm trees
Cycad foliage leaves do not bear ovules
Instead they produce seed cones & pollen cones each on separate plants
Cycadophyta contains 9-10 genera & approximately 100 species
Division Pteridospermophyta: Seed Ferns
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Woody plant w/ fern-like foliage that bore seeds
Seed ferns had a long-lived vascular cambium
Produced xylem and phloem
Earliest seed ferns appeared in the Upper Devonian Period
Others appeared later
Pteridosperms are thought to have evolved from the Aneurophytales
Division Progymnospermophyta: Progymnosperms
Archaeopteridales
More derived progymnosperm was Archaeopteris
Trees up to 8.4 m tall w/ abundant wood & secondary phloem
Reproduction in archaeopterids was heterosporous
3rd group to evolve from trimerophytes
Progymnosperms
So named because some gave rise to conifers, cycads, and the other gymnosperms
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360 million years ago, the vascular cambium that evolved was capable of undergoing radial longitudinal divisions
Could function indefinitely, producing large amounts of xylem and phloem
Also developed megaphyllous leaves
Evolution of Seeds
The space at the top of the megasporangium became the place where microspores settled, acting as a pollen chamber
Megasporangium was surrounded by a layer of tissue called integument
There was a large microphyle
A hole in the integument that permitted the sperm cells to swim to the egg
After the megaspor had developed into a megagametophyte
Had produced eggs
Projected upwards
Aneurophytales
Contains the more relictual progymnosperms
All had a vascular cambium and secondary growth
Primary xylem of their stems was a protostele
Varied in stature
Division Gnetophyta
Contains three groups of enigmatic plants
Ephedra with about 40 speceis
Welwitschia mirabilis, the only species in the genus
Gnetum with 30 species
Division Anthophyta
Fern like
Pollen
Cycads
