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Seed Plants I: Seed Plants without flowers (Division Progymnospermophyta:…
Seed Plants I: Seed Plants without flowers
Concepts
The life cycle of vascular cryptogams is an alteration of independent, heteromorphic generations
Disadvantage is that the new sporophyte is temporarily dependent
Evolution of seeds was preceded by evolution of a vascular cambium
Believe to have arose just once in a group of woody plants known as, the lignophytes
Seeds began to originate, establishing the seed plants called spermatophytes
Two major suites of characters
Some plants produce a small amount of very soft, spongy, parenchmatous wood (manoxylic wood), have large, compound leaves, and radially symmetrical seeds
Consists of hard, strong wood with little parenchyma (pycnoxylic wood), small, simple leaves and flattened seeds
Old classification from the 1800s grouped all the seed plants together in a single division
Spermatophyta
Two classes
Gymnospermae
Gymnosperms
Plants with "naked ovules"
Class Angiospermae
Angiosperms
Flowering plants, those with carpels, which are believed to be sporophylls that form a tube-like, closed structure
Fruits are mature carpels
Division Progymnospermophyta: Progymnosperms
3rd group to evolve from trimerophytes
Progymnosperms
So named because some gave rise to conifers, cycads, and the other gymnosperms
Also developed megaphyllous leaves
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
Aneurophytales
Contains the more relictual progymnosperms
Varied in stature
All had a vascular cambium and secondary growth
Primary xylem of their stems was a protostele
Archaeopteridales
More derived progymnosperm was Archaeopteris
Trees up to 8.4 m tall with abundant wood and secondary phloem
Reproduction in archaeopterids was heterosporous
Evolution of Seeds
Megasporangium was surrounded by a layer of tissue called integument
Projected upwards
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 and had produced eggs
The space at the top of the megasporangium became the place where microspores settled, acting as a pollen chamber
Division Pteridospermophyta: Seed Ferns
Earliest seed ferns appeared in the Upper Devonian Period
Others appeared later
Seed ferns were any woody plant with fern-like foliage that bore seeds instead of sori on its leaves
Seed ferns had a long-lived vascular cambium
Produced xylem and phloem
Pteridosperms are thought to have evolved from the Aneurophytales
Division Coniferophyta: Conifers
Diverse (approximately 50 genera and 550 species)
And all are trees of moderate to gigantic size
Never vines, herbs, or annuals
Never have a bulb or rhizomes
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
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
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
Division Cycadophyta: Cycads
Modern cycads are frequently confused with either ferns or young palm trees
Cycad foliage leaves do not bear ovules
Instead they produce seed cones and pollen cones each on separate plants
Cycadophyta contains 9-10 genera and approximately 100 species
Division Gnetophyta
Contains three groups of enigmatic plants
Gnetum with 30 species
Ephedra with about 40 speceis
Welwitschia mirabilis, the only species in the genus
Division Anthophyta
