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Seed Plants 1: Seed Plants w/out Flowers ("Gymnosperms")…
Seed Plants 1: Seed Plants w/out Flowers ("Gymnosperms")
Division Progymnospermophyta: Progymnosperms
Archaeopteridales
Abundant wood & secondary phloem
Stems of Archaepteris had a siphonostele
which is a pith surrounded by a ring of primary xylem bundles
Reproduction in archaeopterids:
is heterosporous
Evolution of Seeds
Investigating the life cycle of extinct plants
Based on fossil material can be difficult
In free-sporing species,
spores can be identified w/ sporophytes if,
some spores were trapped in a sporangium attached:
to leaves or wood during fossilization
Micropyle:
hole in the integument that permitted
sperm cells to swim to the egg after the megaspore had developed
into a megagametophyte & had produced eggs
Aneurophytales
Contains the more relictual progymnosperms
Such as:
Protopteridium
Proteokalon
Aneurophyton
Tetraxylopteris
Triloboxylon
Eospermatopteris
All had a vascular cambium & secondary growth
But the primary xylem of the stems was a protostele
Like that of rhyniophytes & trimerophytes
3rd group to evolve from trimerophytes;
was the now extinct progymnosperms
Named this because some:
gave rise later to conifers, cycads, & other gymnosperms
Division Pteridospermophyta: Seed Ferns
Earliest seed ferns appeared in the Upper Devonian Period
They form a grade;
a level of evolution
Thought to have evolved from the Aneurophytales
because they had a 3-ribbed protostele
Progymnosperms gave rise to another line of gymnospermous plants
which is cycadophytes
these are classified into three divisions:
Cycadophyta
Cycads, extant
Cycadeoidophyta
Cycadeoids, all extinct
Pteridospermophyta
Seed ferns, all extinct
Division Coniferophyta: Conifers
Diverse, all are trees of moderate to gigantic size
Conifers are never vines, herbs, or annuals
They never have bulbs or rhizomes
Leaves are always simple needles or scales
Leaves of conifers are perennial,
persisting for many years
Venation of conifer leaves is often simple
W/ just one or two long veins running down the center;
of a needle-shaped leaf or several parallel veins;
in a scale-shaped leaves
Division Cycadeoidophyta: Cycadeoids
Cycadeoids would never be considered
distinct from cycads, BUT
microsporophylls & megasporophylls
each ovule had a stock
& the megasporangium ws surrounded by
an integument that extended out into a long micropyle
individual cones of them contained both;
All are exinct
Had vegetative features
which were almost identical to cycads
differ in stomatal complexities
Division Cycadophyta: Cycads
Most cycads are short plants
less than 1 or 2 tall
Trunk is covered w/ bark & persistent leaf bases
which remain on the plant even after;
the lamina & petiole have abscised
internally cycad stems are similar to seeds ferns
w/ a thick cortex containing secretory ducts;
that surround a small amount of manoxylic wood
tracheids are long & wide & rays are massive
Division Gnetophyta
Contains 3 groups of enigmatic plants:
Ephedra
w/ about 40 species
mostly tough shrubs & bushes that are common in desert regions
Gnetum
w/ 30 species
mostly vines or small shrubs w/ broad leaves
Welwitschia mirabilis
only species in the genus
few living plants
exist only in deserts
leaves are reduced and scale like
pollen cones of gnetophytes are
compound & contain small bracts
seeds cones are also compound & contain
extra layers of tissue around the ovules
the tissue is variously interpreted as an extra:
integument
bract
sporophyll
Division Gingkophyta: Maidenhair Tree
Contains a single living species
Ginkgo biloba
"maidenhair tree"
is an unusual tree
looks like a large dicot tree
BUT, its wood is like a conifer
it lacks vessels & axial parenchyma
has "broad leaves"
BUT, have dichotomously branched veins like seeds ferns
Reprodution
is dioecious & gymnospermous
BUT, cones are not produced
pollen is produced in an organ that resembles a catkin
Concepts:
Life cycle of vascular cryptograms
Alternation of independent
Heteromorphic generations
Disadvantage of this life cycle
The new sporophyte, while developing from the zygote,
Is temporarily dependent on a tiny gametophyte for its start in life
Many new sporophytes perish
