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.

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.

Flower Structure

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 flower has both stamens and pistils.  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.