SEED PLANTS I: SEED PLANTS WITHOUT FLOWERS (GYMNOSPERMS) (DIVISION…
SEED PLANTS I: SEED PLANTS WITHOUT FLOWERS (GYMNOSPERMS)
the gymnosperms are those plants with naked ovule that is located on flat sporophylls. angiosperms are the flowering plants with carpels which are believed to be sporophylls that form a tube like closed structure.
DIVISION PROGYMNOSPERMOPHYTA: PROGYMNOSPERMS
have abundant wood and secondary phloem. reproduction was heterosporous. sporangia were terminal on short branches mixed with sterile, leaf-like branched syatems. megaspores were released from the sporangia, not retained. seeds werenot produced.
EVOLUTION OF SEEDS
in earliest known progymnosperm species the megasporangium produced only one megaspore mother cell, and this produced only one large viable megaspore and three small aborted cells. integument surrounds the megasporangium. micropyle permitted the sperm cells to swim to the egg after the megaspore had developed into a magagametophyte and had produced eggs.
contains the more relictual progymnosperms. they all had vascular cambium and secondary growth. resembled trimerophytes in having little webbing between their ultimate branches.
DIVISION PTERIDOSPERMOPHYTA: SEED FERNS
the earliest seed ferns appeared in the upper devonian period. not all are closely related to each other. they form a grade rather than clade. seed ferns were any woody plant with fern-like foliage that bore seeds instead of sori on its leaves. leaves of seeds fern were similar to those of true ferns in overall organization. however the foliage leaves of seed ferns bore seeds.
DIVISION CONIFEROPHYTA: CONIFERS
they 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. conifers leaves are always simple needles or scales. leaf veins have an endodermis and a tissue consisting of parenchyma cells and transfusion tracheids. pines like other conifers have two types of shoots that is long shoots and short shoots. pollen cones are simple cones that bears microsporophylls.
DIVISION CYCADOPHYTA: CYCADS
Modern cycads are frequently confused with either ferns or young palm trees because they have stout trunks with pinnately compound leaves. the trunk is covered with bark and persistent leaf bases that remain on the plant even after the lamina and petiole have abscised. cycads foliage leaves donot bear ovules. cycads produce seed cones and pollen cones. tracheids are long and wide and rays are massive.
DIVISION CYCADEOIDOPHYTA: CYCADEOIDS
had vegetative features almost identical to those of cycads. differ in differentiation of stomatal complexes and in leaf trace organization. individual cones contained both microsporophylls and megasporophylls. each microsporophyll is cup shaped and contained numerous microsporangia.
DIVISION GINKGOPHYTA: MAIDENHAIR TREE
Contains a single living species. look like large dicot tree with a stout trunk and many branches. it lacks vessel and axial parenchyma. it has broad leaves but they have dichotomously branched veins like seed ferns, not reticulate venation like dicots. cones are not produced. ovules occur in pairs at the ends of a short stalk and are completely unprotected at maturity.
Gnetums are mostly vines or small shrubs with broad leaves similar to those of dicots. plants of ephedra are tough shrubs and bushes and their leaves are reduced and scale like. all three genera are unusual in being gymnosperms aith vessels in their woods.