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Seed Plants Without Flowers (division progymnospermophyta: progymnosperms,…
Seed Plants Without Flowers
division progymnospermophyta: progymnosperms
trimerophytes evolved from rhyniophytes
evolutionary trend: pseudomonopodial branching & megaphylls
trend continued in horsetails & ferns
progymnosperms
3rd group evolved from trimerophytes
features: megaphyllous leaves, vascular cambium, able to produce large amounts of secondary xylem & phloem
aneurophytales
relictual progymnosperms; varied from shrubs to large trees
vascular cambium & secondary growth present
primary xylem was a protostele
webbing btwn ultimate branches; could not be called leaves yet
archaeopteridales
trees up to 8.4 m tall
abundant wood & secondary phloem
stems w/ siphonostele, pith surrounded by a ring of primary xylem
"fronds" really are spirally arranged simple leaves
reproduction: heterosporous; no seeds produced
gave rise to conifers, cycads, & other gymnosperms
evolution of seeds
integument
layer of tissue surrounding megasporangium that projected upward
micropyle
a hole in the integument that permitted the sperm cells to swim to the egg
pollen chamber
space at the top of the megasporangium became the place where microspores settled
megasporangia evolved into ovules w/ integuments
other telomes on nearby branches became modified into capsules
microspores evolved into pollen grains
division pteridospermophyta: seed ferns
progymnosperms gave rise to cycadophytes
classified as 3 divisions
pteridospermophyta (seed ferns, all extinct)
cycadophyta (cycads, extant)
cycadeoidophyta (cycadeoids, all extinct)
seed ferns
the earliest appeared in the Upper Devonian Period
they form a grade rather than a clade bc that are not all closely related
seed ferns: any woody plant w/ fern like foliage that bore seeds on its leaves
long-lived vascular cambium that produce both xylem & phloem
wood was manoxylic, much softer & less dense than wood of conifers & progymnosperms
division coniferophyta: conifers
all trees of moderate to gigantic size
never vines, herbs, or annuals, & never bulbs or rhizomes
leaves of most are perennial
1-2 long veins run down the center of the leaves
veins have endodermis & transfusion tissue (transfusion parenchyma & tracheids)
lack vessels & sieve tubes
2 shoot types w/ characteristic leaf types
long shoots
tiny papery leaves
short shoot
axils, produce long needle leaves
cone types
simple cones
pollen cones; single short unbranched axis bears microsporophylls
compound cones
seed cones; shoots w/ axillary buds
cone bracts
leaves beared on short axis rather than sporophylls
each bract has an axillary bud that bears megasporophylls
ovuliferous scale
megasporophylls fused laterally in axillary bud
inside a megasporangium, a single large megaspore mother cell undergoes meiosis
only 1 of the 4 cells survive as the megaspore
a zygote does not immediately form an embryo in conifers
suspensor
instead, some of the first cells elongate & push other cells deep into the megamaetophyte
proembryo
other cells being pushed by a suspensor; develop into the embryo
division cycadophyta: cycads
most are short plants less than 1-2 m tall
trunk is covered w/ bark & persistent leaf bases that remain on the plant even after lamina & petiole have abscised
thick cortex w/ secretory ducts surrounding a small amount of manoxylic wood
long & wide tracheids, rays are massive
most support is provided by the tough leaf bases
prominent pith contains secretory canals
foliage leaves do no bear ovules
produce seed & pollen cones on separate plants
always dioecious
division cycadeoidophyta: cycadeoids
(all extinct) vegetative features almost identical to cycads
differ from cycads in subtle details; stomatal complexes & leaf trace organization
individual cones contain both microsporophylls & megasporophylls
each ovule had a stalk,& megasporangium was urrounded by integument that extended out into long micropyle
division ginkgophyta: maidenhair tree
single living species
Ginkgo biloba
looks very muchlike a large dicot tree w/ a stout trunk & many branches, wood like conifers (lacks vessels & axial parenchyma)
broad leaves w/ dichotomously branched veins like seeds ferns
short shoots bear most of the leaves & long shoots
reproduction: dioecious
no cone production
ovules occur in pairs at the end of a short stalk & several sporangiophores that each have 2 microsporangia
division gnetophyta
contains 3 groups:
Gnetum
30 species,
Ephedra
~40 species,
Welwitschia mirabilis
only species is the genus
Gnetums
mostly vines or small shrubs w/ broad leaves similar to dicots
Ephedra
tough shrubs & bushes, leaves are reduced & scale like
Welwitschia
short, wide stem & only 2 leaves, but leaves grow from perennially from a basal meristem, becoming increasingly longer
all 3 have vessel wood
pollen cones are compound & contain small bracts
seed cones are compound & contain extra layers of tissue around the ovules (interpreted as an extra integument, bract, or sporophyll)
