Seed Plants I: Seed Plants Without Flowers (Gymnosperms) (Division…
Seed Plants I: Seed Plants Without Flowers (Gymnosperms)
Division Progymnospermophyta: Progymnosperms
The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the trimerophytes
contains relictual progymnosperms
they varied in stature from from shrubs to large trees.
they all had vascular cambium an secondary growth
they had little webbing between their ultimate branches
more derived form of progymnosperm
these were trees up to 8.4m tall
stems had siphonostele
reproduction was heterospoous.
contains three groups of enigmati plants: Gnetum(30 species), Ephedra(40 species), Welwitschia mirabilis
Gnetums are small shrubs with broad leaves
plants of ephedra are tough shrubs and bushes.
the free iving plants of Welwitshia exist only in deserts of South Africa. they have short, wide stem and only two leaves, but the leaves grow perenialy.
Division Ginkgophyta: Maidenhair tree
this division contains single living species
it looks like a large dicot tree with a stout trunk and many branches but its wood is like that of conifers
it has vessels an axial parrenchyma
they have oh short shoots and long shoots
reproduction is dioecious and gymnospemous but cones are not produced.
efers to several distinct polyphyletic groups of extinct seed-bearing plants
The earliest fossil evidence for plants of this type is the genus Elkinsia of the late Devonian age
they are thought to havve evolved from Aneurophylates
Division Coniferophyta: Conifers
Conifers are generally large trees and include pines, spruce, firs, cedars, redwoods, junipers, larches, cypresses, and yews
The leaves are needle-shaped and have a thick cuticle for protection and to decrease water loss
Conifers are so named because of the reproductive cones they produce
Division Coniferophyta comprises the conifers, the most abundant gymnosperms today.
conifers have a life cycle following an alternation of generations pattern.
Division Cycadeiodophyta: Cycadeoids
they have vegetative features and identical to those of cycads.
they differ in stomatal complexes and leaf trace organization.
cones of cycadeoids consists of microsporophylls and megasporophylls
Division Cyadophyta: Cycads
Cycads are an ancient group of seed plants which are characterized by a large crown of compound leaves and a stout trunk.
They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves
Cycads are found across much of the subtropical and tropical parts of the world.
All living cycads are slow-growing, long-lived, dioecious, pachycaul, woody perennials