Supergroup Archaeplastida II – Seed Plants – Background Reading

In the previous lab (Plants, part 1) you examined five plant phyla that released their spores to the environment.  In each of those phyla the spores germinated to form free-living gametophyte generations.  In this lab, you will be examining the five phyla of seed plants, collectively known as the spermatophytes.  

Spermatophytes have a specialized form of the sporic cycle.  All seed plants are heterosporous, forming microspores in microsporangia and megaspores in megasporangia.  After their formation, the spores are retained within the sporangia where they develop into gametophytes, surrounded and nourished by sporophyte tissues.  Seed plant gametophytes are highly reduced, in some cases to just a few cells.  The microspores develop into sperm-forming male gametophytes (commonly called pollen grains or microgametophytes) and the megaspores develop into egg-forming female gametophytes (commonly called embryo sacs or megagametophytes).  Megasporangia and female gametophytes are formed within a protective sporophyte structure called the ovule.  

Male gametophytes (pollen) are released from the sporophyte.  Pollination occurs when pollen is transferred to the vicinity of a receptive ovule.  Sperm are formed and released by the male gametophyte and ultimately fertilize the egg cell (within the female gametophyte inside of its ovule).  Following syngamy, the zygote develops into a sporophyte embryo, and the ovule develops into a seed.  A generalized spermatophyte life cycle is represented here:

Key Points in a Seed Plant Life Cycle:

  • sporophyte is the dominant (long-lived and free-living) generation
  • sporophytes are heterosporous
  • gametophytes (male and female) are short-lived and highly reduced 
  • gametophytes are retained and nourished within the sporophytes
  • female gametophytes develop within a protective sporophyte structure called the ovule
  • zygotes are formed and develop into embryos within ovules
  • each fertilized ovule (containing a new embryonic sporophyte and a food supply) develops into a seed

A key to understanding seed plant reproduction is understanding ovules.  Ovules are ovoid structures formed by the sporophyte generation.  They consist of the megasporangium surrounded by one or two layers of sterile tissue (called integuments).  An opening through the integuments called the micropyle allows the passage of sperm or pollen into the interior of the ovule.  Each megasporangium forms one functional megaspore, resulting in the formation of one female gametophyte per ovule.  Following fertilization and development of an embryonic sporophyte, the integuments of the ovule develop into a protective seed coat and the entire structure develops into a seed.

The five phyla of seed plants are:

  • Pinophyta (conifers)
  • Cycadophyta (cycads)
  • Ginkgophyta (ginkgoes)
  • Gnetophyta (gnetophytes)
  • Magnoliophyta (flowering plants)

The first four of these phyla are relatively small clades with a combined number of species of around 900. The members of these four phyla are commonly called gymnosperms (naked seeds) because seed are formed on the surface of scales or other structures.  The flowering plants, on the other hand, are the largest clade in the plant kingdom with about 250,000 species.  These organisms are commonly called angiosperms (vessel seeds), because the seeds develop within an enclosing structure (the carpel) which ultimately develops into a fruit.  Gymnosperms share commonalities in their life cycles, and will be described first. Angiosperms have a fairly specialized life cycle and will be described second. 

I. Non-flowering Seed Plants 

The non-flowering seed plants (commonly called gymnosperms) include four clades that have similar life cycles. Microsporangia(and ultimately pollen) is formed on the surface of modified leaves called microsporophylls.  The microsporophylls are grouped into microsporangiate cones.  Microsporangiate cones are often small and ephemeral.  Megasporangia (in ovules) are formed on the surface of modified leaves called megasporophylls or (in Ginkgo) on sporangiophores.  Megasporophylls are grouped into megasporangiate cones (also called ovulate cones).  Seeds are therefore formed on the surface of cone scales or on sporangiophores.  Megasporangiate cones are often larger and woody.

All gymnosperms are woody plants, either shrubs or trees.  All have roots, stems, and leaves.  Leaves are of the megaphyll type.  Most gymnosperms have substantial fossil histories.

Pinophyta (Conifers)

Conifer sporophytes are typically large, long-lived and form copious amounts of wood (the stems and roots of older sporophytes are mostly wood.  Anatomists describe them as pycnoxylic.   Conifer leaves are typically reduced and needle-like or scale-like and often evergreen.  Sporophytes are heterosporous.  Microsporangia are formed in microsporangiate cones where microspores develop into pollen grains (male gametophytes/microgametophytes).  Megasporangia are formed in megasporangiate cones.  Megagametophytes develop within ovules.  Following pollination and fertilization, ovules develop into seeds on the upper surface of cone scales.  Most conifers are monoecious (both pollen and ovules are formed on each individual).  This familiar group includes pines, spruces, firs, redwoods, junipers, and hemlocks (as well as others).  About 625 species.

The following images show common conifer features:

Western bristlecone pine showing: microsporangiate cones in early development (left), and developing and mature megasporangiate cones (right).  Note the distinctive apical bristle on the mature pine cone.  Also note the needles is clusters (fascicles) of five.  Western bristlecones include some of the oldest trees on Earth, with one known individual over 5,000 years in age.
Image credit: Bill Tanner
Douglas fir showing: microsporangiate cones (left), young megasporangiate cone (middle), and mature megasporangiate cones (right).  Note the distinctive three-lobed bracts that are at the base of each cone scale of the megasporangiate cones.  These are unique to this genus (Pseudotsuga).
Image credit: Bill Tanner
Other cones: Utah juniper “berries” (left) are actually small flesh cones.  Blue spruce cones (center). Note detached twigs with pegs where leaves were attached.  Cones of a subalpine fir (right). Fir cones stand upright and lose their scales leaving the erect cone axis (visible on the branches just below the cones.
Image credit: Bill Tanner

Cycadophyta (Cycads)

Cycad sporophytes produce only small amounts of wood (they are manoxylic).  Stems are unbranched or sparingly branched and armored with persistent leaf bases.  Leaves are large, (usually) pinnately compound, and borne on the crown of the plant.  Plants often form coralloid roots at the soil surface (roots which contain an endosymbiotic nitrogen-fixing cyanobacterium).  Pollen is formed in microsporangiate cones, seeds are formed in megasporangiate cones.  Plants are dioecious (individuals form microsporangiate or megasporangiate cones, not both).  About 185 species.

The following images show common cycad features:

Two cycads showing typical growth form: unbranched stems bearing a crown of large, pinnately compound leaves (left and center). Plant on right has a large microsporangiate cone located in the center of its crown of leaves. 
Image credit: Bill Tanner
Cycad megasporangiate cones.
Image credit: Bill Tanner

Ginkgophyta (Ginkgoes)

Ginkgo sporophytes are deciduous woody (pycnoxylic) trees which bear broadly triangular leaves which have open dichotomous venation.  Pollen is formed in microsporangiate catkin-like structures.  Seeds are formed at the tips of a forked structure (sporangiophore).  Plants are dioecious.  This division is monotypic, with a single extant species (Ginkgo biloba).  Ginkgo is widely planted, but may be extinct in the wild. 

Gnetophyta (Gnetophytes)

Gnetophyte sporophytes are pycnoxylic shrubs or vines whose wood contains vessels.  Other sporophyte features are highly variable.  Pollen is formed in microsporangiate cones and seeds are formed in megasporangiate cones.  Plants are dioecious.  Pollination results in a double fertilization (but endosperm is not formed).  This division contains three very dissimilar genera (Gnetum, Ephedra, and Welwitschia) and about 80 species.

The following images show common gnetophyte features:

Ephedra from southern Utah. They are shrubby plants with bright green photosynthetic stems.
Image credit: Bill Tanner
Ephedra from southern Utah. Microsporangiate cones (left) and megasporangiate cones (right).  The small black or brown scale-like leaves are visible as well.
Image credit: Bill Tanner

II. Flowering Plants

Flowering plants form reproductive structures called flowers that consist of several types of floral appendages that are interpreted as modified leaves.  These are discussed below (see Floral Structure and Variation).  Microspores are formed within the anthers of the stamens, and megaspores are formed within ovules in pistils of the flower (see Flowering Plant Life Cycle for a discussion).  Sexual reproduction results in seeds that are enclosed within fruits (which develop from modified ovaries and supporting tissues).

Magnoliophyta (Flowering Plants)

Sporophytes may be herbaceous or woody (pycnoxylic) and are highly variable in morphology.  The xylem contains specialized conducting cells called vessels.  Pollen and ovules are formed within a flower.  Pollination results in a double fertilization and the formation of a nutritive tissue called endosperm.  Ovules are formed within an enclosing ovary which develops into a seed-containing fruit following pollination and fertilization. This large and diverse group includes most familiar plants.  Systematics of this phylum has undergone major revision in the last decade.

The following images show common flowering plant features:

Floral variation showing: numerous sepals and carpels (left), fusion of petals into a corolla tube (center), and bilateral symmetry (right).
Image credit: Bill Tanner
Buttercup showing flowers and subsequent fruit development. Upper left: Flowers in early spring. Yellow pistils (carpels) are present but difficult to see in the center of the flower. Upper right: Floral appendages other than carpels have been shed.  Carpels in early fruit development. Lower left: Fruits just starting to mature.  Fruit at “4 o’clock” is starting to split open. Lower right: Fruits have matured and split open to release the black seeds.  These are follicles (dry, dehiscent fruits that split open along one line).
Image credit: Bill Tanner

A. Flowering Plant Life Cycle

Sexual reproduction in flowering plants is dependent on the stamens and pistils of a flower.  

1. The formation of microspores (microsporogenesis) occurs in the anthers of the stamen. Microspore and microgametophyte development is summarized below:

  • each microspore mother cell (2n) divides by meiosis to form a tetrad of microspores (n).  Microspores separate.
  • each microspore (n) divides by mitosis to form a 2-celled pollen grain (= microgametophyte/male gametophyte).  Cytokinesis results in a smaller cell (the generative cell) within a larger cell (the tube cell).  Pollen is shed at this stage.
  • pollination triggers the tube cell to form a pollen tube to conduct the generative cell to the micropyle of an ovule.  As the generative cell moves through the pollen tube it divides by mitosis to form two sperm (n).

2. Megaspore and megagametophyte development is summarized below:

  • the megaspore mother cell (2n) within the ovule divides by meiosis to form four megaspores(n). 
  • in the most common type of development (Polyganum-type development), only one megaspore survives.  It takes three mitotic divisions (without cytokinesis) resulting in an eight-nucleate cell (megagametophyte).
  • cytokinesis then takes place resulting in three small cells at either end of the megagametophyte, and two nuclei in the residual cytoplasm in the central region.
  • the haploid cell closest to the micropyle is the egg cell (n).  The two nuclei in the central region are polar nuclei (n+n).  The cells on either side of the egg cell are the synergids, and the three cells at opposite end from the egg cell are antipodals. 

Several alternative forms of megagametophyte development are found in different groups of flowering plants.  Although the Polyganum-type development described above is most common, the type available for viewing in lab today is the Fritillaria-type.  Key features of Fritillaria-type development include:

  • following meiosis three of the four megaspore nuclei fuse, resulting in a triploid nucleus and the remaining haploid nucleus.  Each nucleus takes two mitotic divisions resulting in eight nuclei (four haploid and four triploid).
  • cytokinesis takes place (as described above) resulting in antipodal cells (3n), polar nuclei (3n and n), synergids (n), and an egg cell (n).

The steps for both types of development are summarized below.  Note that resulting female gametophytes look similar, but the resulting antipodal cells and (more importantly) polar nuclei differ genetically.  Under the microscope the triploid nuclei usually look larger than the haploid nuclei:

Polyganum-type development (below)

Fritillaria-type development (above)

3. Following gametophyte development, reproduction involves the following steps:

  • pollination: transfer of pollen to a portion of the pistil called the stigma 
  • germination of a pollen grain to form a pollen tube that grows through tissue of the pistil to deliver two sperm to an ovule (through the micropyle)
  • double fertilization: one sperm fusing with the egg cell to from the diploid zygote and the second sperm fusing with the two polar nuclei to form a polyploid (either 3n or 5n) primary endosperm nucleus.
  • seed formation:
    • the zygote develops into an embryonic sporophyte (2n)
    • the primary endosperm nucleus develops into endosperm (a nutritive tissues that will be absorbed by the embryonic sporophyte during development (either 3n or 5n)
    • each fertilized ovule develops into a seed
    • each ovary develops into a fruit

B. Floral Structure and Variation

Flowers are highly specialized structures for sexual reproduction that show a tremendous variation in structure.  Flowers may be visualized as compressed shoot systems which bear up to four types of floral appendages:

  • Sepals – Sepals make up the lowermost series of floral appendages.  Sepals are often green and leaf-like, and usually protect the developing flower while in bud.  The sepals of a flower are collectively called the calyx.
  • Petals – The petals of a flower are often the largest and showiest portion of the flower, although in many cases they are highly reduced (or absent).  Petals are located just above the calyx of the flower.  Petals usually function in the attraction of pollinators to the flower.  The petals of a flower are collectively called the corolla, and the sepals and petals together are collectively called the perianth of the flower.
  • Stamens – Stamens are located in the central part of the flower, above the perianth.  Each stamen usually consists of a pollen-producing anther borne on a stalk-like filament.  The stamen is one of the two places within a flower where meiosis takes place.  Meiosis occurs within the anther to produce haploid microspores.  Each microspore will ultimately develop into a pollen grain (= microgametophyte ormale gametophyte).  Each pollen grain forms two sperm.  The stamens of a flower are collectively called the androecium
  • Carpels/Pistils – Carpels make up the series of floral appendages found in the center of the flower, above the androecium.  One or more carpels fuse together to form a pistil.  A simple pistil is formed from a single carpel.  A compound pistil is formed by the fusion of two or more carpels.  Each pistil (whether simple or compound) usually consists of a stigma (= surface which is receptive to pollen), style (= stalk which holds the stigma to an elevated position within the flower), and ovary (= which contains ovules).  The pistil is the second place within the flower where meiosis takes place.  Meiosis occurs within the ovules of the ovary to produce haploid megaspores.  Each megaspore will develop into an embryo sac (= megagametophyte or female gametophyte).  The embryo sac produces the egg cell.  The pistils of a flower are collectively called the gynoecium.
  • The floral appendages are attached to a short length of stem called the receptacle.  Flowers may be sessile, or borne on a stalk called a peduncle (or pedicel if the flower is borne in an inflorescence).  Flowers are quite variable, and may differ from each other in size, color, symmetry, number of floral appendages, degree of fusion between floral parts, specialization of floral parts, and absence of one or more series of floral appendages.

Common Variations in Floral Morphology

1. Fusion – Fusion is common in flowers, and is usually considered to be derived from an ancestral free condition in which each floral appendage has a separate point of attachment to the receptacle.  Fusion may occur between similar floral appendages (called coalescence) or between dissimilar floral appendages (called adnation).  Some common examples of fusion include:

  • Fusion of sepals into a calyx tube
  • Fusion of petals into a corolla tube
  • Fusion of sepals and petals into a perianth tube
  • Fusion of stamens with petals
  • Fusion of carpels into a compound pistil

2. Symmetry – Flowers may have radial symmetry (in which case they are described as being regular or actinomorphic) or 

bilateral symmetry (in which case they are described as being irregular or zygomorphic).  Irregular flowers are generally 

considered to be derived.

3. Number of Floral Appendages per Series – The number of floral appendages per series may be large and somewhat variable (in which case it is described as being numerous) or reduced to a fixed number or a series of floral appendages may be absent.  Common variations include:

  • Numerous floral appendages in a series
  • Floral appendages in a series reduced to a fixed number.  Monocots commonly have floral appendages in multiplies of three, while eudicots often have their floral appendages in multiples of four or five.
  • Absence of a series of floral appendages.  The following terms are often applied to flowers:  A complete flower has all four types of floral appendages.  An incomplete flower lacks one or more type of floral appendage.  A perfect flowerhas both stamens and pistils (but may be incomplete).  An imperfect flower lacks either stamens or pistils. Imperfect flowers are either staminate (with stamens) or pistillate (with pistils).  If a plant has both staminate and pistillate flowers (i.e. on the same individual) it is referred to as being monoecious.  A dioecious plant has either staminate or pistillate flowers (and therefore requires two individuals for sexual reproduction).

4. Ovary Position – If the ovary of a pistil is situated above the receptacle it is described          as being superior.  An ovary that is “embedded” in the receptacle is described as being inferior.  Flowers are often categorized according to the ovary position, as follows:

  • Hypogynous Flower – ovary superior, other floral appendages attached to the receptacle below the ovary.
  • Epigynous Flower – ovary inferior, other floral appendages attached to the receptacle above the ovary.
  • Perigynous Flower – ovary superior, other floral appendages attached to the margin of a “floral cup” that surrounds the ovary.

C. Fruits

Following fertilization, the ovary of a flower develops into a fruit, while the ovules within become seeds.  During this development the ovary wall forms the fruit wall, which is called the pericarp.  The pericarp may consist of up to three distinct layers (the exocarp, mesocarp, and endocarp), and may be either fleshy or dry at maturity.  Fruits are generally classified according to:

  • The number of ovaries that combine to form the fruit.
  • Whether the pericarp is fleshy or dry.
  • Whether the pericarp splits open at maturity.
  • Whether non-ovarian tissues contribute to the fruit.

Some of the major fruit types include:

  1. Accessory Fruits – Derived, in part, from non-ovarian tissues.
  2. Multiple Fruits – Derived from the fusion of two or more ovaries which come from different flowers.
  3. Aggregate Fruits – Derived from the fusion of two or more ovaries, all of which come from the same flower.
  4. Simple Fruits – Derived from a single ovary.  Common types of simple fruits are listed below:

                  Fleshy Fruits:

  • Drupe – One-seeded fruit with a thin exocarp (skin), fleshy mesocarp, and hard/stony endocarp.
  • Berry – Many-seeded fruit lacking an endocarp.  Seeds are embedded in the fleshy mesocarp.  Several types of berries are often given special recognition:
  • Hesperidium – Outer layers forming a leathery skin containing aromatic oils.
  • Pepo – Outer layers forming a thick rind (not containing aromatic oils).
  • Pome – Fleshy layer of fruit is derived primarily from receptacle tissue.

                  Dry, Dehiscent Fruits:

  • Follicle – Fruit splits open along one line of dehiscence.
  • Pod/Legume – Fruit splits open along two lines of dehiscence.
  • Capsule – Fruit splits open along three or more lines of dehiscence.
  • Silique – Fruit splits open along two lines of dehiscence, leaving a false partition.

                  Dry, Indehiscent Fruits:

  • Achene – One-seeded fruit.  The seed coat is fused with the pericarp at one point.
  • Grain/Caryopsis – One-seeded fruit.  The seed coat is fused in entirety with the pericarp.
  • Samara – Pericarp forms a wing.
  • Nut – One-seeded fruit with a thick pericarp and a basal cup.