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Seed Plants I: Seed Plants Without Flowers ("Gymnosperms")…
Seed Plants I: Seed Plants Without Flowers ("Gymnosperms")
Concepts
life cycle of vascular cryptogams
alternation of independent
heteromorphic generations
disadvantage:
new sporophyte - temporarily dependent on tiny gametophyte
for its start in life
many new sporophytes parish
Advantagous
if the embryo could use photosynthetic + absorptive capacity
of the leaves and roots of the previous sporophyte
Division Cycadophyta: Cycads
frequently confused with ferns or young palm trees
they have stout trunks
with pinnately compound leaves
most are less than 1 or 2 m tall
Macrozamia
can reach up to 18 m
trunk
covered w/ bark
persistent leaf bases
internally
similar to those of seed ferns
thick cortex
secretory ducts surrounds manoxylicwood
tracheids
long and wide
rays are massive
Division Coniferophyta: Conifers
very diverse
about 50 genera and 550 species
all the trees are moderate to gigantic size
Sequoiadendron giganteum
venation
often siimple
never veins, herbs, or annuals
never have bulbs or rhizomes
Division Ginkgophyta: Maidenhair Tree
single living species
Ginkgo biloba
very unusual
looks like a large dicot tree
w/ stout trunk and many branches
wood is like that of conifers
lacks vessels and axial parenchyma
Division Gnetophyta
3 groups of enigmatic plants
Ephedra
40 species
Welwitschia mirabilis
only species
Gnetum
30 species
double fertilization
do not produce multicellular archaegonia
have vessels in the xylem
Division Cycadoidophyta: Cycadeoids
all extinct
had vegetative features
almost identical to those of cycads
#
differ only in subtle details
of differentiation of stomatal complexes
and in leaf ttrace organization
cycadeoids would never be considered
distinct from cycads
can contain both microsporophylls and megasporophylls
microsporophylles - below the cluster of megasporangium
curved upward
envelopes the megasporophylls
each ovule
had a stalk
megasporangium - surrounded by an integument
extended out into a long micropyle
in between - thick, fleshy scales
Division Progymnospermophyta: Progymnosperms
trimerophytes :red_flag:
important group of plants
evolved from rhyniophytes
Progymnosperms :red_flag:
third group to evolve
gave rise later to conifers, cycads, other gymnosperms
also developed megaphyllous leaves
evolution of a vascular cambium w/ unlimited growth potential
capable of producing both secondary xylem/phloem
Aneurophytales
more relictual progymnosperms
:
Aneurophyton, Protopteridium, Proteokalon, Tetraxylopteris, Eospermatopteris
Archaeopteridales
more derived progymnosperns
order
Archaeopteris
#
stems had siphonostele
pith surrounded by primary xylem bundles
much like modern conifers and dicots
up to 8.4 m tll
abundant wood + secondary phloem
reproduction
was heterosporous
megaspores
up to 300 micrommeter in diameter
microspores
only 30 micrometers in diameter
Evolution of Seeds
free-sporing species
spores identified with sporophytes
if spores were trapped in a sporangium
attached to leaves or wood
during fossilization
spores - no identification with gametophytes
Division Pteridospermophyta: Seed Ferns
cycadophytes
3 divisions
Cycadophyta
Cycadeoidophyta
pteridospermophyta
earliest seed ferns
appeared in the upper devonian period
others appeared later
not all are closely related
form a grade
level of evolution
any woody plants
with fern-like foliage
bore seeds instead of sori
on its leaves
many resembled modern tree ferns
thought to have evolved from Aneurophytales
earliest seed ferns had three - ribbed protosteles
