Sexual Reproduction in Plants

Sexual and Asexual Reproduction in Plants

Let's start by looking at the meaning of sexual and asexual reproduction. There are two types of reproduction: sexual and asexual. Plants can reproduce sexually, asexually, or both!

Asexual reproduction in plants occurs with the generation of a new individual from a part of a parent plant. It can take place in various ways.

For example, lilies can be propagated simply by replanting their bulbs, while cacti can be propagated by stem cuttings. Strawberries can also reproduce asexually using modified stems called stolons (or runners) that extend from the crown at the base of the parent plant, take root in the soil, and produce new daughter plants.

Compared to sexual reproduction, asexual reproduction demands less energy because it does not involve forming flowers, attracting pollinators, or dispersing seeds. The new individual also matures faster than a sexually produced offspring.

While offspring produced via asexual reproduction would survive under stable conditions, their lack of genetic variation can be a disadvantage when faced with changing environmental conditions (such as climate change or the emergence of a new disease).

Figure 1 below shows how sexual and asexual reproduction occurs in strawberries.

Examples of Sexual Reproduction in Plants

While you may immediately think of bees pollinating flowers, sexual reproduction in plants does not always involve pollination! Neither does it always involve flowers, fruits, or seeds. Sexual reproduction in plants largely varies across plant groups.

• For example, angiosperms (or flowering plants) produce flowers which contain the male and female reproductive organs and gametes (sex cells). Pollination brings the gametes together. The fusion of the gametes through fertilization results in the formation of a seed enclosed in a fruit.

• On the other hand, gymnosperms (or nonflowering seed plants) like pines and cypresses produce male and female cones where the fusion of gametes to the formation of naked seeds. Unlike angiosperms, their seeds are not enclosed in a fruit.

• Seedless vascular plants like ferns and nonvascular plants like mosses and liverworts do not produce flowers, seeds, or fruits at all; instead, they reproduce through spores.

Now, let's look at some examples involving asexual reproduction (more on sexual reproduction later!)

Steps of Sexual Reproduction in Plants

Now, let's focus on the steps involving sexual reproduction in plants. Sexual reproduction in plants is characterized by an alternation of generations.

In other words, during alternation of generations, plants alternate between two different phases: haploid gametophytes and diploid sporophytes.

The basic steps involved in the alternation of generations is as follows.

1. Haploid gametophytes produce male and female gametes (sperm and egg, respectively) through mitosis.

2. When the male and female gametes combine, a diploid zygote is formed.

3. The diploid zygote undergoes mitosis, forming a diploid sporophyte.

4. When it reaches maturity, the diploid sporophyte produces haploid spores through meiosis.

5. These spores undergo mitosis, producing haploid gametophytes.

This life cycle varies across plant groups.

Types of Sexual Reproduction Mechanisms in Plants

There are two main sexual reproduction mechanisms in plants.

• Gymnosperms and angiosperms reproduce via seeds.

• Nonvascular and seedless vascular plants reproduce via spores.

Sexual Reproduction via Spores

As mentioned earlier, nonvascular plants are haploid gametophytes for most of their life cycle.

During their gametophyte phase, gametes are developed for sexual reproduction. Gametophytes can have multiple multicellular sex organs called gametangia. Egg–producing gametangia are called archegonia, while sperm-producing gametangia are called antheridia. An archegonium contains one egg, while an antheridium can produce numerous sperm cells.

The sperm of a nonvascular plant needs to travel on a layer of moisture to reach the egg. The fusion of the gametes leads to the formation of the diploid zygote and later, the sporophyte.

The egg is typically fertilized within the archegonium, so young sporophytes rely on gametophytic tissue. In fact, sporophytes are attached to their parent gametophyte during most if not all of their life, depending on their parent for water and nutrients.

Like nonvascular plants, seedless vascular plants also reproduce by spores and require moisture for fertilization. Figure 3 below shows the life cycle of a fern, a vascular plant.

Sexual Reproduction via Seeds

With the emergence of seeds in evolutionary history, the requirement for water to reproduce disappears. The seed also provides an advantage to seed plants, as it contains the nutrients needed for the embryo’s survival.

Both angiosperms and gymnosperms produce seeds. However, there is a major difference in how their seeds are developed: the seed of the angiosperm is enclosed in an ovary, while that of a gymnosperm is not.

Let’s go through the mechanisms of reproduction for these two plant groups.

Sexual reproduction in angiosperms

Sexual reproduction in angiosperms can be characterized by the production of flowers to attract pollinators, double fertilization, and the development of fruits for seed dispersal.

Flowers function primarily in sexual reproduction. The reproductive parts of the flower can be found in the carpel and the stamen.

• The carpel holds the female reproductive parts of the flower. It consists of the ovary, style, and stigma.

  • The ovary contains the ovules, which become seeds after fertilization, and the embryo sac, which is the female gametophyte that contains the egg cell.

  • The style elevates the stigma over the ovary and other parts of the flower.

  • The stigma is a sticky structure that traps pollen.

• The stamen holds the male reproductive parts of the flower. Its main parts are:

  • The anther is made up of sac-like microsporangia that produce pollen. The pollen grain is the sperm-producing male gametophyte.

  • The filament connects the anther to the flower.

Note that the stigma contains a generative cell and a tube cell. When a pollen grain germinates, the tube cell grows within the style. The generative cell then enters the tube, where it undergoes mitosis to form two sperm cells. The pollen tube then goes through an opening in the ovule called the micropyle.

One sperm cell fertilizes the egg to form a diploid zygote, while the other fertilizes two polar nuclei to form a triploid cell in the embryo sac. The triploid cell will later become the endosperm, which will provide nourishment to the embryo. Together, these two fertilization processes are called double fertilization.

The fertilized ovule develops into a seed, while the ovary forms the fruit that encloses it and aids in its dispersal.

Figure 4 below shows the life cycle of an angiosperm.

Sexual reproduction in gymnosperms

Sexual reproduction in gymnosperms are quite different. While angiosperms produce flowers and fruits, gymnosperms do not. Instead, their pollen and seeds are found in cones.

A gymnosperm cone has bracts called sporophylls, which are arranged around a central stalk. These sporophylls produce sporangia, which are sac-like structures that hold spores. Some gymnosperms have sporophyte tissue called integument partially covering the seeds.

• Male pollen cones have small bracts. Pollen sacs called microsporangia are found under these bracts.

• Female ovulate cones also have numerous bracts, each containing megasporangia (which are female carriers of spores).

• Microsporangia and megasporangia can be found in the same plant or on separate plants.

Diploid cells within the microsporangia and megasporangia undergo meiosis to produce haploid microspores and megaspores. These undergo mitosis to form the male gametophyte (pollen grain) and the female gametophyte.

At this point, the ovule is ready for fertilization!

Similar to the germination of pollen grains in angiosperms, a pollen tube grows and extends toward the structure that contains the egg cell. The sperm undergoes further division to produce two sperm cells.

When the nuclei of the two sperm cells make contact with the egg cell, one nucleus dies while the remaining one fuses with the cell to form a diploid zygote. When the zygote divides via mitosis, a diploid sporophyte is formed in the form of a seed.

Figure 5 below illustrates sexual reproduction in gymnosperms.

Compare and Contrast Sexual Reproduction in Plants and Animals

Both animals and plants undergo sexual reproduction and, in both, sexual reproduction involves the fusion of gametes, resulting in offspring that are genetically different from their parents.

Animal sexual reproduction can be summarized as follows:

1. Inside an adult animal, diploid cells produce haploid gametes through meiosis.

2. Male and female gametes fuse, forming a diploid zygote.

3. The zygote undergoes mitosis and matures into a new diploid individual.

4. The process repeats.

What makes plants very different from animals is that all plants are characterized by an alternation of generations, which is basically a change of ploidy (or number of chromosomes) from parent to offspring. Plants alternate between haploid gametophyte and diploid sporophyte generations, while animals do not.

In other words, there are extra steps between #1 and #2: diploid cells undergo meiosis forming haploid cells, which then undergo mitosis to form haploid individuals (male and female gametophytes). Mature gametophytes then produce haploid male and female gametes.

Another important difference is that plants produce spores (whether they reproduce by spores or by seeds), while animals generally do not.