23,24- Gymnosperms & Angiosperms

Background - Gymnosperms

  • A disadvantage of alternation of independent heteromorphic generation is new sporophyte, while developing from the zygote, is temp. dependent on a tiny gametophyte

    • It would be advantageous if the embryo could use the photosynthetic capacity of the leaves and roots of the previous sporophyte

  • Evolution of seeds

    • evolution of a vascular cambium gave rise to a monophyletic group of woody plants

    • shortly after, seeds originated establishing the seed plants, spermatophytes

    • the gymnosperms of those plants with “naked ovules,” ovules located on flat sporophylls

  • Division of living seed plants

    • Division Cycadophyta

    • Division Coniferophyta

    • Division Ginkgophyta

    • Division Gnetophyta

    • Division Magnoliaphyta (flowering plants)

Progymnosperms

  • 390 million years ago, Progymnosperms evolved (now extinct)

  • the vascular cambium that evolved could function indefinitely, producing large amounts of both secondary xylem and phloem

  • although they were similar to conifers, the two groups must be kept separate because Progymnosperms do not have seeds

  • simple reproduction

Evolution of Seeds

  • the megasporangium was surrounded by a layer tissue, integument

  • there were also a micropyle, a hole in the integument that permitted the sperm cells to swim to the egg

  • as megasporangia evolved into ovules with integuments, other telumes on nearby branches became modified in cupules

    • microspores evolved into pollen grains

  • Progymnosperms gave rise to another line of gymnospermous plants, cycadophytes

    • classified as three divisions:

      • Pteridospermphyta (seed ferns, all extinct)

      • Cycadophyta (cycads, extant)

      • Cycadeoidophyta (cycadeoids, all extinct)

    • not all seed ferns are closely related ; they form a grade instead of a clade

  • Seed Ferns

    • any woody plant with fernlike foliage that bore seeds instead of sori

    • leaves were similar to those of true ferns in overall organization- large, compound, and planar

  • Conifers

    • most diverse group and all are trees of moderate to gigantic size

    • leaves are always simple needles or scales, and most are perennial

    • venation is simple, one or two long veins running down the center of a needle-shaped leaf or several parallel veins in scale-shaped leaves

    • wood of modern conifers lacks vessels, and their phloem lacks sieve tubes

    • composed completely of tracheids

    • pollen cones are simple cones, with a single short unbranched axis that bares microsporophylls

    • cone bracts: short axis of seed cones leaves

    • each bract has an axillary bud that bears megasporophylls

    • the microspore has two large air bladders that increase buoyancy in air

  • Cycads

    • stems are similar to those of seed ferns

    • cycad foliage leaves do not bear ovules

    • produce seed cones and pollen cones, each on separate plants (dioecious)

    • almost all are tropical

  • Cycadeoids

    • had vegetative features almost identical to those of cycads

    • individual cones of cycadeoids contained both microsporophylls and megasporophylls

  • Maidenhair Tree

    • contains a single living species

    • its wood is like that of conifers, lacks vessels

    • reproduction is dioecious and gymnospermous but cones are not produced

    • the megasporangiate (female) produce seeds; the outer fleshy layer of the seed emits butyric acid which has a putrid odor

Background - Angiosperms

  • all are classified together in a single division known as magnoliophyta

  • mutually beneficial interactions with animals

    • pollination and seed distribution

    • evolutionary changes involved in the conversion of gymnosperms to angiosperms

      • double fertilization

      • ability to produce bisexual flowers

      • appearance of vessel elements and sieve tubes

      • ancestral flowering plants were woody perennials

      • annual growth habit

  • wind pollinated trees (elms, oaks, planes) were grouped together in a subclass Hamamelidae and were considered the most relictual living flowering plants

    • small and simple flowers without sepals and petals

  • monocots usually have parallel veins because the leaves are elongate and strap shaped

    • vascular bundles are distributed throughout the stem

    • no ordinary secondary growth

    • flowers with parts arranged in groups of three

  • eudicots are more diverse and include greater number of families, genera, and species

    • have two cotyledons

      • reticulate venation in the leaves

      • vascular bundles form one ring in the stem

      • can be woody, herbaceous, or succulent`

      • flower parts occur in sets of four or five

  • basal angiosperms

    • contain the living descendants of several groups that originated while angiosperms were still a young clade

  • monocots

    • lack ordinary secondary growth

    • early monocots diverged into a series of clades whose extant members are classified into about 10 orders

    • Alismatales

      • flowers are large and showy, with three sepals and three petals but in others they are highly modified

    • Liliales

      • important features are the presence of spots or lines on the petals and o rather ordinary nectaries formed at the bases of tepals or stamens

    • Asparagales

      • extremely diverse in morphology, ranging from small, delicate bulbs like chives and onion to vining epiphytes such as many orchids

  • Eudicots

    • constitute a much larger group than the monocots and are divided into numerous clades

    • mosses vs. liverworts vs. hornworts

      • hornworts do not have the capsule like moss

      • moss has both gametophyte and sporophyte

      • thorns on hornworts are thicker

      • the hapaphyta (third division of bryophytes) has a broader and flatter leaves compared to hornworts and mosses (liverworts)

    • basal eudicots

      • ranunculales: are believed to be clades that diverged at early stages in eudicot evolution

        • flowers have little fusion of parts

      • caryophyllales

        • most flowering plants have anthocyanin pigments in their flowers

      • santalales

        • a small order of highly modifies plants most of which are parasitic

    • rosid clade

      • consists of many families that are so diverse with respect to vegetative body, flowers, chemistry, and ecology that its difficult to see they are all related

      • five orders make up 75% of the species in the clade

      • important character is the presence of pinnately compound leaves

    • asterid clade

      • majority can be easily distinguished from other angiosperms on the basis of:

        • their petals are fused together into a tube

        • have just a few stamens, not more than the numbers of petal lobes

        • stamens alternate with petals

  • monocots vs. eudicots

    • monocots

      • seed: one cotyledon

      • roots: fibrous

      • vascular: scattered

      • leaf: parallel

      • flower: multiples of 3

    • dicot

      • seed: cotyledon

      • roots: tap

      • vascular: ringed

      • leaf: net-like

      • flowers: 4 or 5

DIVISION PTERIDOSPERMOPHYTA:
SEED FERNS

DIVISION PTERIDOSPERMOPHYTA:
SEED FERNS