32.1 Reproductive Development and Structure

Plants reproduce through a cycle that alternates between two distinct generations, and understanding this cycle is the foundation for everything else in plant reproduction. This topic covers how that cycle works, how flowering plants build their reproductive structures, and how those structures set the stage for fertilization.

Plant Reproductive Development and Structure

Alternation of Generations in Plants

All plants alternate between two multicellular body forms during their life cycle: the diploid sporophyte (2n) and the haploid gametophyte (n). This is called alternation of generations.

Here's how the cycle flows:

1. The sporophyte produces spores through meiosis (so the spores are haploid).
2. Each spore develops into a gametophyte.
3. The gametophyte produces gametes (sperm and egg) through mitosis, not meiosis, because the gametophyte is already haploid.
4. Gametes fuse during fertilization to form a diploid zygote, which grows into a new sporophyte.

Which generation dominates the life cycle depends on the plant group:

• In non-vascular plants (mosses, liverworts, hornworts), the gametophyte is the larger, longer-lived generation. The sporophyte is small and depends on the gametophyte.
• In vascular plants (ferns, gymnosperms, angiosperms), the sporophyte is dominant. The gametophyte is reduced, and in seed plants it's microscopic.

Male vs. Female Angiosperm Gametophytes

In flowering plants, the male and female gametophytes are tiny structures that develop inside the flower.

Male gametophyte (pollen grain):

1. A microsporocyte (2n) inside the anther undergoes meiosis, producing four haploid microspores.
2. Each microspore divides by mitosis to form a pollen grain containing two cells: a generative cell and a tube cell.
3. The generative cell later divides to produce two sperm cells.
4. The tube cell grows the pollen tube, which delivers the sperm to the ovule after pollination.

Female gametophyte (embryo sac):

1. A megasporocyte (2n) inside the ovule undergoes meiosis, producing four haploid megaspores.
2. Three of the four megaspores degenerate. The one surviving megaspore undergoes three rounds of mitosis.
3. This produces the embryo sac: a 7-celled structure with 8 nuclei.
4. The embryo sac contains the egg cell, two synergid cells (which help guide the pollen tube), three antipodal cells, and a large central cell with two polar nuclei.

Key Plant Reproductive Structures

• The stamen is the male reproductive structure. It consists of the anther (where pollen is produced) and the filament (the stalk that supports the anther).
• The pistil (also called the carpel) is the female reproductive structure. It has three parts: the stigma (sticky surface that receives pollen), the style (a tube connecting the stigma to the ovary), and the ovary (which contains one or more ovules).
• Each ovule inside the ovary houses the embryo sac, surrounded by protective layers called integuments. A small opening in the integuments called the micropyle is where the pollen tube enters.

Components of Complete Flowers

A complete flower has all four whorls of modified leaves, arranged from outside to inside:

• Sepals (collectively the calyx): the outermost whorl, typically green, protecting the flower bud before it opens.
• Petals (collectively the corolla): the second whorl, often brightly colored to attract pollinators.
• Stamens: the third whorl, the male structures (anther + filament).
• Pistil/carpel: the innermost whorl, the female structure (stigma + style + ovary).

Two additional structures to know:

• The receptacle is the base of the flower where all the whorls attach.
• Nectaries are glands that secrete nectar to attract pollinators. They can be located on the receptacle, petals, or sepals.

Gymnosperm Sporangium Development

Gymnosperms (conifers, cycads, etc.) also produce male and female spores, but their reproductive structures differ from those of angiosperms.

• Microsporangia contain microsporocytes (2n) that undergo meiosis to produce microspores (n), which develop into pollen grains.
• Megasporangia contain a megasporocyte (2n) that undergoes meiosis to produce megaspores (n). Typically only one megaspore survives and develops into the female gametophyte, which stays inside the megasporangium.
• The megasporangium is wrapped by an integument, and together they form the ovule.

The key distinction: gymnosperm ovules sit exposed on the surface of a megasporophyll (a modified leaf, often part of a cone). They are not enclosed in an ovary. This is why gymnosperms are called "naked seed" plants, while angiosperms, with their ovary-enclosed ovules, are "enclosed seed" plants.

Reproductive Processes in Flowering Plants

Pollination is the transfer of pollen from an anther to a stigma. This can happen via wind, water, insects, birds, or other animals. Pollination is not the same as fertilization; it just gets the pollen to the right place.

Double fertilization is a process unique to angiosperms, and it involves both sperm cells from the pollen grain:

1. The pollen tube grows down through the style and enters the ovule through the micropyle.
2. One sperm cell fuses with the egg cell, forming a diploid zygote (2n), which develops into the embryo.
3. The other sperm cell fuses with the two polar nuclei in the central cell, forming the triploid endosperm (3n), which serves as a nutrient source for the developing embryo.

After fertilization, the ovule develops into a seed (containing the embryo and endosperm), and the ovary develops into a fruit that surrounds and protects the seed.