There are five Divisions of seed plants with living representatives. We will only cover four.
 

Division: Magnoliophyta (angiosperms)

Refer to the lesson on "Flowers" of angiosperms. Examine the life cycle of flowering plants.

Refer to diagram handed out in class.

Division:  Coniferophyta (Gymnosperms)  Contain seeds but no fruit nor double fertilization.

Living Gymnosperms

The female gametophyte of gymnosperms produces several archegonia.
Polyembryony- more than one egg may be fertilized and several embryos begin to develop within a single ovule.
Pollen grain- partly developed male gametophyte is transferred mainly by wind to the area of the female gametophyte (called pollination), water is not required for the sperm to reach and fertilize the egg.
Pollen tube- after pollination the endosporic male gametophyte produces a tubular outgrowth.
 

Most numerous and widespread of the gymnosperms.
(includes: pine, cedar, fir, sequoia, cypress, juniper)

Pines are conifers with a unique leaf arrangement, in pine seedlings the leaves are spirally arranged, after a year or two the pine produces its leaves in bundles which contains specific numbers of leaves wrapped at the base by scale like leaves.

Most pines retain their needles for 2 to 4 years, secondary growth begins early and forms large amounts of secondary xylem (wood).

Micro and megasporangia are borne on the same tree but in different cones (monoecious).  Micro (pollen producing) are on the lower branches and the mega are on the upper branches. A young microsporangiate contains microsporocytes (produce microspores that develop into a winged pollen grain consisting of  2 prothallial cells, generative cell, and a tube cell), this four celled pollen grain is the immature male gametophyte.
 
 

Refer to diagram handed out in class.
 
 

Ovuliferous scales- bear the ovules, entire modified determinate branch systems known as seed scale complexes, the scales are arranged spirally around the axis of the cone. Each megasporangium contains a single megasporocyte that undergoes meiosis which gives rise to a series of 4 megaspores (only 1 is functional).
A month after pollination the 4 megaspores are produced, one develops into a megagametophyte.
 

Refer to diagram handed out in class.
 

A year after pollination the generative cell of the 4-celled male gametophyte undergoes division giving rise to a sterile stalk (stalk cell) and a spermatogenous cell (body cell).

The pollen tube reaches the egg of an archegonium where it discharges its sperm into the egg cytoplasm, the eggs are fertilized and develop into embryos.

The conifer seed consists of 2 diploid sporophytic generations (seed coat and embryo) and 1 haploid gametophytic generation.

 The gametophyte serves as a food reserve, the embryo consists of a hypocotyl root axis (a rootcap and apical meristem at one end and an apical meristem and several cotyledons at the other)
 

Refer to diagram handed out in class.

  Note:  In Conifers the Integument develops into the seed coat and the zygote develops into an embryo.
             However; since there is no ovary no fruit will develop and no double fertilization no endosperm

Division:  Cycadophyta
 

Sporophyte - Palm like plants found in tropical regions, large plants, often toxic and can harbor cyanobacteria. Short trunk with fern or palm like leaves.  Plants are dioescious

Reproductive units are reduced leaves with attached sporangia that are loosely clustered into cone like structures near the apex of the plant (microsporophylls – male; megasporophylls – female), the pollen and seed cones are borne on different plants (dioecious) , each male gametophyte produces 2 sperm, beetles aid in pollination

Refer to diagram handed out in class.
 

       Pollen Cone  - Androstrobilus (microsporophylls with sporangia)
                 Pollen Grain (Androgametophyte) contains prothallial cell, tube cell,
                         androgenous cell (sperm) with flagella(male gametophyte). Compare to
                         angiosperms  and gymnosperms.
 

                     Inside the sporangia a microspore mother cells (2n) will form which undergoes meiosis forming micropsores(1n)
                     that develope into pollen grains.
 
 
 
 
 
 
 
 
 

       Seed Cone Gynostrobilus (megasporophylls with female gametophyte)
                             Integument may be fleshy, archgonium, micropyle
                             Ovule present. No ovary!
 

Refer to diagram handed out in class.

                  Inside the sporangia a megaspore mother cells (2n) will form which undergoes meiosis forming 4 megasores(1n),
                  3 become nonfunctional and surviving spores develops into female gametophyte with archegonium. 
 
 
 
 
 

 
      Pollination by beetles, fertilization (no double fertilization).
      Integument develops into seed coat and zygote develops into
      embryo with cotyledons – epicotyl, hypocotyl.
                           Draw Life Cycle showing Alternation of Generation
 

Division: Ginkgophyta
 

Sporophyte – Tree like, deciduous, leaves dichotomous venation. Ginkgo biloba is the only surviving genus which is resistant to air pollution.  Reproductive structures consist of  small conelike  borne on short stalks.  Dioecious.

Refer to diagram handed out in class.
 

     Male gametophyte-  Catkin-like microsporophylls (androstrobilus ) with
                            microsporangia.  Microspore mother cells undergo meiosis to form
                            spores develop into pollen grains (male gametophyte). prothallial
                            cell, tube cell,  androgenous cell (sperm) with flagella.

                  Inside the sporangia a microspore mother cells (2n) will form which undergoes meiosis forming micropsores(1n)
                  that develope into pollen grains.
 
 
 
 
 
 
 
 
 
 

             Female gametophyte- Two young ovules borne on the ends of short stalks and
                                ripen to produce fleshy seeds, fertilization of the ovules occur after
                                they have been shed from the parent tree.
                                Integument fleshy, archgonium, micropyle

Refer to diagram handed out in class.

Inside the sporangia a megaspore mother cells (2n) will form which undergoes meiosis forming 4 megasores(1n),
                  3 become nonfunctional and surviving spores develops into female gametophyte with archegonium. 
 
 
 
 
 

Draw Life Cycle showing Alternation of Generation

Division:   Gnetophyta
Contains members with angiosperm like features:  similarity of their strobili to angiosperm flowers, presence of similar vessels in their xylem, lack of archegonia
 
 
 
 
 
 

Evolution of the Seed

All seed plants are heterosporous, producing megaspores and microspores that give rise to mega and microgametophytes. (andro/gyno)

Seed- mature ovule containing an embryo, the immature ovule consists of a megasporangium surrounded by 1 or 2 layers of tissue called integuments

Evolution of the seed includes:

Retention of the megaspores within the megasporangium which is fleshy (called the nucellus- no longer releases spores).

Reduction in the number of megaspore mother cells in each megasporangium to 1
survival of only 1 of the 4 megaspores produced by the spore mother cell.

Formation of a highly reduced megagametophyte inside the single functional megaspore.

Development of the embryo within the megagametophyte retained within the megasporangium

Formation of an integument that completely surrounds the megasporangium except for an opening at the apex called the micropyle
 

Modification of the apex of the megasporangium to receive pollen grains

Seed consists of an embryo, stored food, and a seed coat
 
 
 
 

Compare the four plants bearing seeds:
 
                                                   Angiosperms   Gymnosperms     Cycads    Ginkgo
 

Double Fertilization

Fruit formed

Flowers

Cones

Strobili

Alternation of Generation
(gametophyte/sporophyte)

Heterosporous

Results of meiosis

Pollen

Archegonium

Embryo with cotyledons (epicotyl, hypocotyl)

Monoecious/Dioecious

Ovary

Ovule

Fleshy Seeds