Plant Reproductive Cycle

Modes of Reproduction in Plants

Let's start by looking at the definition of the two modes of Reproduction in plants: asexual and sexual.

For example, the plant Kalanchoe pinnata can reproduce asexually by forming miniature plantlets along the edges of its leaves, with each one able to fall off, grow roots, and become a new plant identical to its parent.

For example, when a tomato plant produces seeds, each seed grows into a plant that is slightly different from the parent: maybe it is taller, or perhaps it is more resistant to diseases.

Plants reproduce sexually, asexually, or both!

Figures 1-2. Kalanchoe producing plantlets asexually (left) and tomato seedlings (right).

Plant Reproductive Cycle in Biology

Two distinct multicellular stages characterize a plant's reproductive cycle: haploid gametophyte and diploid sporophyte, one giving rise to the other, a phenomenon known as alternation of generations.

The alternation of generations sets plants apart from Animals, which only have one multicellular stage. Whereas an animal's diploid Cells divide by Mitosis, starting from the zygote, to produce its diploid body, its haploid cells do not undergo mitosis--they simply become single-celled haploid gametes.

On the other hand, in plants both haploid and diploid Cells undergo Mitosis, which means that plants produce two kinds of multicellular bodies. This is what we mean by two multicellular stages.

Before we go into detail about this reproductive cycle, let's go over the definition of some terms you'll be encountering throughout the article:

Plant Reproductive Cycle Diagram

The plant life cycle (also called reproductive cycle) is generalized in the diagram below. First, we'll go over the different stages in this diagram, and then later we will get into detail as to how this cycle varies among different plant groups.

Plant Reproductive Cycle Steps

The steps of the generalized plant reproductive cycle are summarized as follows:

• A mature diploid sporophyte produces four haploid spores through Meiosis.

• These haploid spores undergo mitosis and develop into a mature multicellular haploid individual called a gametophyte.

• The gametophyte produces male and/or female gametes (sperm and egg, respectively) through mitosis.

• Fertilization, or the fusion of male and female gametes, forms a diploid zygote.

• The zygote undergoes mitosis, developing into a multicellular diploid sporophyte.

• …and the cycle begins again!

Plant Reproduction Cycle Characteristics and Functions

One of the two generations can be dominant, meaning that the individual in the dominant generation lives longer and grows larger than the less dominant generation. On the contrary, individuals in the non-dominant generation are typically tiny and live in or on the dominant individual.

Generally:

• Nonvascular Plants have a dominant gametophyte generation, with the sporophyte dependent on the gametophyte.

• Seedless Vascular Plants have a dominant sporophyte generation. The gametophyte is not dependent on the diploid generation.

• Flowering plants (Angiosperms) and nonflowering plants (Gymnosperms) have a dominant sporophyte generation, with the gametophyte dependent on the sporophyte.

Reproductive Cycle of a Flowering Plant

Now that we have a general idea of how plants alternate between two stages, let's zoom into variations of this reproductive cycle across plant groups, beginning with flowering plants.

Flowering plant reproductive structures

But first, here is a quick recap of the reproductive parts of a flowering plant:

Figure 4. This diagram shows the basic parts of the flower.

The pistil represents the female reproductive parts of the flower. It is made up of the ovary, style, and stigma.

1. The ovary contains the ovules.

   • Each ovule contains a megasporangium, which holds the megasporocytes.

     • When a megasporocyte undergoes meiosis, it produces four megaspores, but only one survives. It then undergoes mitosis to become the embryo sac. The embryo sac is the female gametophyte.

2. The style brings up the stigma over the ovary and other flower parts.

3. The stigma is a structure with a sticky surface that traps pollen.

The stamen represents the male reproductive parts of the flower. It is made up of anther and filament.

1. The anther is made up of microsporangia, which contain microsporocytes.

   • Microsporocytes divide by meiosis to produce microspores that develop into pollen.

   • Each pollen grain is a sperm-producing male gametophyte. It contains a generative cell and a tube cell.

2. The filament links the anther to the flower.

Flowering Plant Reproduction

So, how does a flowering plant (angiosperm) reproduce? Let's start from Pollination.

When pollination occurs under the right conditions, the pollen grain germinates, causing the tube cell to grow within the style. The generative cell then enters the tube and undergoes mitosis, forming two sperm cells. The pollen tube passes through the micropyle, which is an opening in the ovule.

One sperm cell fertilizes the egg cell, forming a diploid zygote. The other sperm cell fertilizes two polar nuclei in the embryo sac, forming a triploid cell, which later develops into the endosperm that will nourish the embryo.

The fertilized ovule now forms the seed, while the ovary forms the fruit that surrounds the seed. When the seed germinates, the embryo develops into a new sporophyte individual.

The angiosperm sporophyte generation is more dominant than its gametophyte generation: the gametophyte is attached to and is dependent on the sporophyte.

The angiosperm reproductive cycle is illustrated in the diagram below.

Reproductive Cycle of Nonvascular Plants

Let's move on to another plant group: nonvascular plants.

Unlike angiosperms and gymnosperms, nonvascular plants have a dominant haploid gametophyte stage in their reproductive cycle.

During its gametophyte phase, a nonvascular plant develops several gametangia (multicellular sex organs). Female gametangia are called archegonia, while male gametangia are called antheridia. A mature archegonium contains one egg cell, while a mature antheridium produces many sperm cells.

The fusion of these gametes leads to the diploid sporophyte phase. For fertilization to take place, most nonvascular plants require moisture as the sperm typically travels on a layer of water (like rainwater or dew) to reach the egg.

The egg is usually fertilized within the archegonium so that the young developing sporophyte is protected and fed by gametophytic tissue. For most of their life cycle, the sporophyte remains attached to the parent gametophyte, depending on the parent for water, sugars, and other nutrients.