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Seed Plants 1: Seed Plants Without Flowers ("Gymnosperms")…
Seed Plants 1: Seed Plants Without Flowers ("Gymnosperms")
Concepts
evolution of seeds was preceded by evolution of a vascular cambium :check:
lignophytes are a group of plants that their cambium thought to arose once to monophyletic woody plants :check:
spermatophytes evolved shortly after being, seed origin, establishing seed plants :red_flag:
two major suits of characteristics have developed
manoxylic wood are plants that produce small amounts of soft, spongy parenchymatous wood with large compound leaves and radially symmetrical seeds :star:
pycnoxylic wood are hard, strong wood with little parenchyma and small simple leaves and flattened seeds :check:
free living gametophytes develop on the soil whereas retained ones develop high on plant strobil :star:
this position is out of reach of swimming sperm
modifications to compensate this are megasporophylls with megagametophytes were arranged in upright cone-like shapes
allows microspores to be carried by wind and dropped into the megasporangiate cone
old classification from 1800 grouped seed plants into single division, Spermatophyta with two classes :star:
gymnosperms are plants with naked ovules that are located on flat sporophylls such as pine cones :check:
Angiosperms are flowering plants, those with carpels, which are believed to be sporophylls that form a tube-like closed structures :red_flag:
mutations that improve the survival of the gametophyte or embryos contributes to reproductive success of maternal sporophyte :check:
5 different divisions are used and accepted by book
Coniferophyta
Ginkgophyta
Cycadophyta
Gnetophyta
Magnoliophyta
Division Progymnospermophyta: Progymnosperms
Aneurophytales
contain the more relicual progymnosperms that all had vascular cambium, and secondary growth :check:
primary xylem of their stems was a protostele liek that of rhiniophytes and trimerophytes
resemble trimerophytes in having little webbing between their ultimate branches. Could not yet be called leaves :red_flag:
Archaeopteridales
had siphonostele, pith surrounded by ring of primary xylem bundles, much like modern conifers :check:
fronds of archaeopterids resembled fern leaves revels plantaed branch systems and ultimate leaflets really spiraled simple leaves :check:
more derived progymnospore. were up to 8.4m tall with abundant wood and secondary phloem :red_flag:
reproduction was heterosporous, sporangia were terminal on short branches mixed with sterile leaf-like branch systems :<3:
progymnosperms are an extinct group and the 3rd group to evolve from trimerophytes :check:
similar to ferns in development of megaphylous leaves but also evolved a vascular cambium with unlimited growth potential and capable of producing secondary xylem and phloem :check:
wood almost indistinguishable from modern conifers with elongated trachieds, little to no axial parenchyma
tall and uniseriate rays with procumbent ray tracheids with strong wood that is effective at conduction and supporting a large mass :black_flag:
named so because some gave rise to conifers, cycads, and other gymnosperms :red_flag:
Evolution of Seeds
micropyle is a hole in the integument that permitted the sperm cells to swim to the egg after megaspore had developed into megagametophyte and had produced eggs :check:
pollen chamber is a holding chamber on top of the megasporangium because the place where microspores settle :red_flag:
integument is a megasporangium that is surrounded by a layer of tissue :star:
In free sporing species, spores can be identified with sporophytes if spores are trapped in a sporangium attached to leaves or wood during fossilization :star:
spores cannot be identified with gametophytes except when the gametophytes is microscopic and develops within the spore wall :black_flag:
Division Cycadophyta:
unlike seed ferns, cycad foliage leaves do not bear ovules, they produce seed cones and pollen cones :star:
cycads are always dioecious
upon germination, pollen grains produce a branched pollen tube and large multiflagellated sperm cells :red_flag:
seed cones are variable, in some species they are large loose aggregation of leaf like pinnately compound megasporophylls :black_flag:
was a much larger group with many species in earlier times, contains only 9 or 10 genera now, modern cycads highly prized ornamentals in warm part of the US :black_flag:
most are short plants less than 1-2 m tall and trunk is covered with bark and persistent leaf bases that remain on plant even after lamina and petiole have abscsied :star:
almost all tropical with unusual distribution, from Cuba and Mexico to Asia and Africa :world_map:
modern cycads frequently confused with either ferns or young palm trees :check:
Division Gnetophyta
shrubs are found in the deserts of the US and Mexico and dry mountains of South America :mountain:
are mostly vines or small shrubs with broad leaves similar to those of dicots. Native to tropical regions such as the Amazon :palm_tree:
few other species are shrubs in the deserts of South Africa or cultivation
all three genera are unusual in being gymnosperms with vessels in their wood :check:
contains three groups of enigmatic plants :star:
Ephedra
contains 40 species
:
Welwitschia mirabilis
contains only one species
Gnetum
contains 30 species
unlike pollen cones of all other gymnosperms, those of gnetophytes are compound and contain small bracts :<3:
seed cones are also compound and contain extra layers of tissue around ovules, tissue is variously interpreted as an extra integument, bract, or sporophyll :red_flag:
few fossil of organ or tissue exist but are too recent to help with evolution and ancestry of the group
pollen is spindle-shaped and has narrow ridges, easy to recognize :star:
certain aspects of anatomy and reproduction have been interpreted to indicate gnetophytes and flowering plants constitute two sister clades with a common ancestor :black_flag:
group that would form would be called Division Anthophyta or anthophytes but too little data acquired to be confident :checkered_flag:
Division Ginkgophyta: Maidenhair Tree
Ginkos have short shoots which bear leaves and long shoots cones are not produced during reproduction :warning:
ovules occur in pairs at the ends of a short stalk and are completely unprotected at maturity,
wood is like conifers, lacks vessels adn axial parenchyma, has broad leaves with dichotomous veined leaves
do not have reticulate venation like dicots :red_flag:
pollen is produced in an organ that resembles a catkin, having a stalk and several sporangiospores that each have 2 microsporangia :black_flag:
contains a single living species, looks like a large dicot tree with stout trunk and many branches :check:
exact ancestors are not known, suspected to be one of the seed ferns or closely related to that group :red_flag:
Division Pteridospermophyta: Seed Ferns
form a grade rather than a clade are any woody plant with fern-like foilage that bore seeds instead of sori
Pteridosperms are thought to have evolved from the Aneurophytales because the earliest seed ferns had 3-ribbed prostele :check:
earliest seed ferns appeared in the Upper Devonian Period and not all are closely related. :red_flag:
most central cells of stem have parenchyma and ring of vascular bundles surrounding a pith
within seed fern ovule, megasporangium was large and bundles of vascular tissue ran through it
seed fern wood was manoxylic, much softer and less dense than wood of conifers and progymnosperms :check:
contained thick cortex with distinctive radial plates of sclerenchyma below epidermis
leaves were similar to true ferns, large, compound, planer, foilage leaves of seed ferns bore seeds
Division Cycadeoidophyta:
2 groups differ only in subtle details of the differentiation of stomatal complexes and in leaf trace organization :check:
each ovule had a stalk and the megasporangium was surrounded by an integument that extended out into a long microspyle :green_cross:
cycadeoids are all extinct and had vegetative features almost identical to those of cycads :star:
between ovules was thick, fleshy scales and microsporophytes were located below cluster of megasporophylls and curved upward enveloping megasporophylls :red_flag:
each microsporophyll was cup shaped and contained numerous microsporangia
Division Coniferophyta: Conifers
trees are monopodial, contain trachieds with annual rings, spring wood, summer wood, thin and tall rays
tiny papery leaves occur on long shoots and in their axils are short shoots that produce familiar long needle leaves :star:
venation of leaves simple, with 1-2long veins running down center of needle shaped leaves, all have pollen and seed cones :check:
pines have both pollen cones and seed cones :check:
pollen cones are simple cones with a single short unbranched axis that bears microsporophylls :star:
seed cones are more complex than pollen cones, they are compound cones consisting of a shoot with axiillary buds :checkered_flag:
the short axis bears leaves called cone bracts rather than sporophylls and each bract has a axillary bud containing the megasporophylls :black_flag:
the axillary bud is microsopic and its megasporophy are fused laterally forming a ovuliferous scale :red_flag:
are very diverse and all are trees of moderate to gigantic size, never vine, herbs, or annual and never have bulbs :star:
development can take as long a a year but walls form, converting the coenocyte into a cellular megagametophyte :check:
2-3 archegonia form as sets of cells each surrounding a large egg
conifer eggs are gigantic cells loaded with carbohydrate and protein :green_cross:
egg nucleus is haploid but swollen to a volume much larger than typical for entire cells
conifer pollen arrives before egg is mature and more than a year can pass between pollination and fertilization :!:
zygote does not immediately form an embryo in conifers, instead some of the first cells elongate as a suspensor that pushes cells deep into megagametophyte :star:
proembryo develop into the embryo and no double fertilization occurs :check:
massive pollen tube slowly digests its way toward megagametophyte as the egg forms, passageway necessary
