Chapter 22: Seed Plants 1
Division Progymnospermophyta: Progymnosperms
Aneurophytales
Archaeopteridales
Evolution of Seeds
Division Pteridospermophyta: Seed Ferns
Division Coniferophyta: Conifers
Division Cycadophyta: Cycads
Division Cycadeoidoohyta: Cycadeoids #almost identical vegetative to cycads
Division Ginkgophyta: Maidenhair Tree #similar wood/shoots to conifer
Division Gnetophyta
third group to evolve from trimerophytes, now extinct
produces true woody trees
contains the more widespread progymnopserms
varied from size of shrub to large tree
no actual leaves
Archeopteris: tall trees w/ abundant wood & secondary phloem
stem had siphonostele pith surrounded by primary xylem bundles
some organisms had complete webbing, considered megaphylls
reproduction heterosporous, seeds not produced
mega/microspores produced in own distinctive sporangium
megaspores released, not retained, from sporangia
earliest known heterospory progymnosperm: Chauleria
from Middle Devonian Period (approx. 390 million yrs ago)
in Archaeosperma arnoldii fossil only one megaspore mother cell produced
megasporangium surrounded by integument
integument has large hole (micropyle)
permits sperm cells to swim to egg after megaspore develops into mature megagametophyte
earliest seed ferns appeared in Upper Devonian Period
considered any woody plant w/ fern-like foliage & bore seeds
thought to have evolved from Aneurophytales
due to earliest seed ferns having 3-ribbed protostele
had long-lives vasc. cambium that produced xylem&phloem
(similar to gymnosperms/angiosperms)
tracheids longer/wider
rays many cells wide&tall
wood was manoxylic
wood was softer/less dense than conifers/progymnosperms
(also occurs in cycads & cycadeoids
leaves similar to true fern, however foliage leaves bore seeds
approx. 550 species
all trees of moderate to very large size
never vines, herbs, or annuals
never have bulbs/rhizomes
leaves always simple needles or scales
leaves of most are perennial
venation simple
1/2 veins running down center of needle-shaped leaves
several parallel veins in scale-shaped leaves
veins have endodermis & tissue (transfusion tissue)
consists of transfusion parenchyma&tracheids
parenchyma cuboidal w/ prominent circular bordered pits
mixed with the tracheids to form complex 3D pattern
facilitates transfer b/t vasc. tissue & mesophyll tissue outside endodermis
wood lacks vessels, phloem lacks sieve tubes
all have pollen cones & seed cones
2 types of shoots common
long shoots w/ tiny papery leaves
short shoots in axial, produce familiar needle leaf
pollen cones can be simple cones
single short unbranched axis w/ microsporophylls
microspore mother cell undergoes meiosis & forms microspores
each develop endosporially into small gametophyte w/ 4 cells
then shed from tree as pollen & carried by wind
seed cones more complex
compound cones consisting of shoot w/ axillary buds
short axis bears leaves called cone bracts, not sporophylls
each bract has axillary bud w/ megasporophylls
modern conifers have microscopic axillary buds
megasporophylls fuse laterally forming ovuliferous scale
inside megasporangium single megaspore mother cell undergoes meiosis
3 of resulting cells degenerate leaving 1 as megaspore
megasporangium doesn’t dehisce, megaspore retained & grows into large coenocytic megagametophyte
pollen arrives before egg is mature
more than a year can pass between pollination & fertilization
massive pollen tube slowly digests way to toward megagametophyte as egg forms
zygote does not immediately form embryo
first cells elongate as suspensor that pushes other cells deep into megagametophyte
cells pushed deeper develop into embryo (called proembryo)
often confused with ferns/young palm trees due to stout trunks w/ pinnately compound leaves
most short, 1-2 meters tall
Macrozamia can get as tall as 18 meters
trunk covered w/ bark & persistent leaf bases
stems similar to seed ferns internally, thick cortex w/ manoxylic wood #had manoxylic wood like seed ferns
tracheids long/wide, rays massive
foliage leaves do not bear ovules
seed cones & pollen cones produced instead
on separate plants, always dioecious
pollen cones consist of spirally arranged shield-shaped microsporophylls w/ clusters of microsporangia
about 100 species
thought to be fairly extensive due to range of current species
all species extinct
had vegetative features almost identical to cycads
differ only in subtle details in stomatal complexes & leaf trace organization
however, individual cones of cycadeoids contain both microsporophylls & megasporophylls
each ovule had stalk, megasporangium surrounded by integument
b/t ovules were thick fleshy scales
microsporophylls located below cluster of megasporophylls
contains a single living species
Ginkgo biloba
looks like large dicot tree w/ stout trunk & many branches
wood similar to conifers
lacks vessels and axial parenchyma
have both short and long shoots
reproduction dioecious & gymnospermous
contains 3 groups of enigmatic plants:
Gnetum
Ephedra
Welwitschia mirabilis
30 species
40 species
only species
mostly vines/small shrubs w/ broad leaves (similar to dicots)
tough shrubs/bushes common in desert regions
leaves reduced/scale like
exist only in South Africa or cultivation
have short, wide stem & only 2 leaves
leaves grow perennially from basal meristem, becoming increasingly longer
unusual due to being gymnosperms w/ vessels in wood
pollen cones compound & contain small bracts
seed cones also compact w/ extra layers of tissue around ovules
fossils of organs/tissues too recent to help with evolution of group
possibly shares ancestor with flowering plants
would form “Division Anthophyta”