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Seed Plants I: Seed Plants Without Flowers "Gymnosperms"…
Seed Plants I: Seed Plants Without Flowers
"Gymnosperms"
Division Progymnospermophyta
did not have seeds or ovule precursors
produced true woody trees
developed megaphyllous leaves
360 mya
had little or no axial parenchyma
the wood contained elongate tracheids with circular bordered pits
the vascular cambium produced both primary and secondary xylem and phloem
Progymnosperms
Aneurophytales
all had a vascular cambium and secondary growth
resembles trimerophytes in having webbing between their ultimate branches
contains more relictual progymnosperms
Aneurophyton, Protopteridiujm, Tetraxylopteris
Archaeopteridales
stems consist siphonostele, pith surrounded by a ring of primary xylem bundles
reproduction was heterosporous
Archaeopteris, more derived progymnosperm
sporangia were terminal on short branches
seeds were not produced
Evolution of Seeds
micropyle permitted sperm cells to swim to the egg
microspores settled in the pollen chamber
began with retention of megaspore and then with formation of an integument in progymnosperms
Concepts
Gymnosperm seeds are unprotected by an ovary or fruit
includes conifers, cycads, Ginkgo, and gnetophytes
naked ovules located on flat sporophylls
Division Pteridospermophyta: Seed Ferns
a ring of vascular bundles surrounding a pith
long-lived vascular cambium that produced both xylem and phloem
thought to have evolved from the Aneurophytales
Three divisions
Pteridospermophyta (seed ferns, all extinct)
Cycadophyta ( cycads, extant )
Cycadeoidophyta ( cycadeoids, all extinct )
Division Coniferophyta: Conifers
both cones
pollen cones
simple cones with a single short unbranched axis
bears microsporophylls
seed cones
compound cones consisting of a shoot with axillary buds
short axis bears leaves with cone bracts
Pines
2 types of shoot
tiny papery leaves on long shoots
short shoots in axils
axillary bud is microscopic
megasporophylls fuse laterally forming ovuliferous scale
perennial, leaves are simple needles or scales
Division Cycadophyta: Cycads
parenchymatous wood
simple seed cone consists of an axis and leaflike megasporophyll
modern seed plants with compound leaves
tracheids are long and wide, rays are massive
cycad foliage leaves do not bear ovules
Division: Cycadeoidophyta: Cycadeoids
individual cones contained both microsporophylls and megasporophylls
each ovule had a stalk
vegetative features almost similar to those of cycads
megasporangium extended into a long micropyle
Division Ginkgophyta: Maidenhair Tree
resembles dicot tree except leaves have dichotomous veins
bears naked ovules surrounded by neither a cone nor a carpel
Ginkgo biloba is the sole living species
Division Gnetophyta
leaves are reduced and scale like
pollen cones and seed cones are compound
mostly vines or small shrubs with broad leaves
reproduction is gymnospermous
three groups
Ephedra
40 species, resemble dicots
Welwitschia mirabilis
only two leaves
grow indeterminately from basal meristems
Gnetum
30 species, resemble dicots
