Seed Plants I: Seed Plants Without Flowers ("Gymnosperms")…
Seed Plants I: Seed Plants Without Flowers
:- Modern cycads are frequently confused with either ferns or young palm trees because they have stout trunks with pinnately compound leaves. Most cycads are short plants less than 1-2 m tall. Trunk is covered with bark and persistent leaf bases that remain on the plant even after the lamina and petiole hae abscised. Internally, cycad stems are similar to those of seed ferns.
Division Ginkgophyta: Maidenhair tree:- Contains a
single living species, Ginkgo biloba. Seems unusual looks very much like large dicot tree with a stout trunk and many branches. but its wood is like that of conifers: lacks vessels and axial parenchyma. It has "broad leaves" but they have dichotomously branched veins like seed ferns, not reticulate venation like dicots. Have both short shoots, which bear most of the leaves, and long shoots. Reproduction is dioecious and gymnospermous but cones are not produced.
Division Cycadeoidophyta: Cycadeoids:The sycadeoids had vegetative features almost identical to those of sycads.Two groups differ only in subtle details of the differentiation of stomatal complexes
and in leaf trace organization. Individual cones of cycadeoids contained both microsporophylls and megasporophylls. Microsporophylls located below the cluster of megasporophylls and curved upward, enveloping the megasporophylls.
Division Pteridospermophyta: Seed Ferns
Pteridospermophyta: Thought to be evolved from the
Aneurophytaled because the earliest seed ferns, such as Stenomyelon, had a three-ribbed protostele. Most central
stem cells of the stem differentiated as parenchyma, nottrachieds
Division coniferophyta: Conifers:- Are familiar
plants, are diverse, and all are trees of moderate to gigantic size
Conifers are never vines, herbs or annuals, and they never
have bulbs or rhizomes. They are always simple needles or scales. Leaves are perennial, persisting for many years
Have two cones
Compound Cones: Are more complex than pollen cones each
consisting of a shoot with axilary buds
Axillary bud is microscopic, and its megasporophylls are fused
laterally, forming an ovuliferous scale
Simple cones: Pollen cones are simple cones with a single short
unbranched axis that beard microsporophylls.
Have two shoots
Long Shoots: Tiny papery leaves occur
Short shoots: in the Axils which produce long needle leaves
Division Gnetophyta: Contains three groups of
Gnetum with 30 species: Are mostly vines or small shurbs
with broad leaves similar to those of dicots
Ephedra with about 40 species : They are tough shurbs and
bushes that are very common in desert regions in Northern Mexico and southwestern United States and dry mountains of South America
Welwitschia mirabilis: The few living plants of Welwitschia
exist only in deserts of South Africa or in cultivation. They have a short, wide stem and only two leaves, but the leaves
grow perennially from a basal meristem, becoming increasingly longer.
Division Progymnospermophyta: Progymnosperms:- Third group evolve from
Trimerophytes, so named because some gave rise later to conifers, cycads, and the other gymnosperms. Progymnosperms also developed megaphyllous leaves, another significant feature- the evolution of a vascular cambium with unlimited growth potential and capable of producing xylem and secondary phloem
Archaepteridales: These were trees up to 8.4 m tall with
abundant wood and secondary phloem. Stem had a siphonostele, pith surrounded by a ring of primary xylem bundles. Reproduction was heterosporous.
: Aneurophytales Contains the more relictual progymnosperms. Varied in
stature from shurbs to large trees. All have vascular cambium and secondary growth, but the primary xylem of their stems was a prostele
Evolution of Seeds: The megasporangium produced onlyone
megaspore mother cell, and this produced only one large, viable megaspore and three small, aborted cells.
Megasporangium is surrounded by a layer of tissue, an integument, that projected upward. There is large micropyle,
a hole in the integument that permitted the sperm cells to swim to the egg after the megaspore had developed into a
megagametophyte and produced eggs.