What is Angiosperm Reproduction?

The process of sexual reproduction in flowering plants is known as angiosperm reproduction. In flowering plants, reproduction begins with pollination, or the transfer of male gametes (pollen) from the male reproductive organ (anther) to the female reproductive organ (pistil), and ends with fertilization of the female gametes (egg), which produces a zygote, which then grows to become a mature plant.

Plant structures involved in Angiosperm Reproduction

Angiosperms are mostly terrestrial plants. The seeds in angiosperms are present in the ovary/fruit. Based on the number of cotyledons present in a seed, these plants are further divided into monocotyledons or monocots and dicotyledons or dicots. Seedlings of monocots have one cotyledon (seed leaf), whereas seedlings of dicots have two cotyledons.

The flower, which contains the male (stamen/androecium) and female (pistil/gynoecium) parts, is the principal structure involved in the flowering plant's reproduction. Flowering plants that have both male and female parts in the same flower are considered complete and are also known as androgynous or hermaphroditic plants. Flowering plants that have either male or female structures on distinct individual flowers are called staminate (only male structures) or carpellate (only female structures).

The calyx (sepals), corolla (petals), stamen (male portion), and pistil (female part) are the four primary parts of a flower. Only the stamen and pistil are directly involved in the reproductive system of flowering plants. A stamen has a filament (slender stalk) and an anther (sac-like structure). The filament supports the anther, where meiosis produces microspores, which grow into pollen grains. The female organ, the gynoecium or pistil, consists of a style, stigma, and ovule. A pistil consists of several joined carpels. The thick tissues of the carpel develop and protect the megaspores and female gametophytes. The sticky stigma, where pollen is deposited, is connected to the ovary, which is enclosed in the carpel by a long, thin structure called a style. One or more ovules reside in the ovary, each of which develops into a seed after fertilization.

The process involved in Angiosperm reproduction

In angiosperms, the sporophytic stage is the principal stage of the reproduction/life cycle. The adult sporophyte goes through hormonal changes to generate flowers with male (stamen) and female (pistil) reproductive organs. Both the male and female portions have sporangia structures that contain spore-producing cells known as sporocytes, which produce spores as either eggs or sperm depending on their origin. Microspores are male haploid spores formed by microsporangia (made up of four fused pollen sacs) via meiosis within anthers at the apex of the stamen. These microspores, in turn, undergo mitosis and give rise to pollen grains (male gametophyte). Each pollen grain contains two cells, the sperm-producing cell called generative cell that divides into two sperms and a second cell that becomes the pollen tube cell. The tapetum, specialized nutritive cells found in anther are the source of nourishment of pollen grains and precursor to pollen coat, exine.

In the female reproductive organs, the ovary that holds the ovule comprises a megasporangium, which is covered by two layers of integuments and an ovary wall inside the pistil. The megasporocyte inside the megasporangium goes through meiosis, producing four female megaspores; three small and one big. Only the bigger spore survives to give birth to the embryo sac or megagametophyte containing the egg cell, which then splits three times in the second phase of meiosis to generate an eight-cell stage. The four cells from these move to each pole of the embryo sac; two of the cells reach the equator and fuse to produce a 2n polar nucleus. In the next step, antipodals are formed by the three cells which are furthest away from the egg, and synergids or helper cells are formed by the two cells closest to the egg. The total number of cells in the embryo sac adds up to seven cells; three antipodal cells, two synergid cells, one egg cell and a central cell. Even though these are seven cells, the number of nuclei in the cells is equal to eight; hence they are known as eight-nucleate cells.

Pollen (microgametophyte) produced from the anthers must reach the sticky stigma at the tip of a carpel for sperm to fertilize the egg. The pollen grain's tube cell then creates a pollen tube, which extends down the carpel to the ovule through a small opening known as a micropyle. Pollination permits pollen grains that produce sperm to reach the egg-containing embryo sac. Pollen grains can be moved by wind, water, or animals while the embryo sac is motionless.

Angiosperms undergo double fertilization that results in the formation of an embryo (2n, diploid zygote) and a nutrient storage endosperm (the 2n polar nucleus fuses with another sperm). The embryo and endosperm are packed into a seed coat that forms a seed. Ovules mature into seeds, and the ovary develops into a fruit. Fruits help in the protection and distribution of the seeds. This completes the life cycle of a flowering plant.

Effects of Angiosperm reproduction

Flowering plants (Anthophyta) have evolved to dominate terrestrial ecosystems, becoming the second most common species after insects. The reproductive structures, flowers, fruits, and pollen grains, are responsible for their reproductive success. Flowers attract pollinators such as insects, thanks to their vibrant colors and aroma. They also protect the ovule and growing embryo. Due to the fruity fragrance and taste, herbivores readily accept the fruits that contain the seeds as food, and the undigested seed is then distributed through the herbivores' feces. Some of the seeds are also wind-pollinated. The outer covering or exine of the pollen grain is made up of an inert polymer known as sporopollenin, which protects it from chemical and mechanical damages enabling it to survive for longer periods of time. In addition to sexual reproduction, angiosperms also reproduce via asexual reproduction.

Self-incompatibility, a genetic program that prevents the plant from self-pollination and self-fertilization to favor cross-pollination in angiosperm, is also feasible in angiosperms. Pollen tube production, fertilization, and embryo development are all blocked when a pollen grain produced by a plant reaches the stigma of the same plant by preventing self-pollination and self-fertilization. This self-incompatibility process is based on protein-protein interactions and is controlled by a single locus known as 'S,' which has various alleles for different species. Angiosperms' existence on earth is mostly due to cross-pollination, which is facilitated by self-incompatibility.

Context and Applications

This subject is important in professional exams for undergraduate, graduate, and post-graduate studies, particularly for

• Bachelors of Science in Botany
• Maters of Science in Agriculture Practice
• Maters of Science in Botany

Practice Problems

1. How many cotyledons does a seedling of a monocotyledon have?
a. One
b. Two
c. Three
d. Four
Answer- a

Explanation- Monocotyledons such as rice, corn, and grass contain only one cotyledon or a seed leaf.

2. The reproduction in angiosperms is characterized by ______?

a. Single-fertilization
b. Quadruple-fertilization
c. Double-fertilization
d. Triple-fertilization

Answer- c

Explanation- The double fertilization in angiosperms results in the formation of an embryo (2n, diploid zygote) and a nutrient storage endosperm.

3. What are the specialized nutritive cells found in anther known as?

a. Tapetum
b. Tapium
c. Exine
d. Pollen tube

Answer- a

Explanation- Tapetum is a nutritive cell group found in the outer layer of the anther that is involved in pollen grain nutrition and the production of the pollen coat, exine.

4. What is the outer coating of a pollen grain known as?

a. Xine
b. Tapetum
c. Axine
d. Exine

Answer- d

Explanation- Exine is a weather-resistant polymeric covering that encapsulates a pollen grain.  

5. Embryo sac is also known as

a. Pollen grain
b. Megagametophyte
c. Micro gametophyte
d. Gametophyte

Answer- b

Explanation- Another term for embryo sac which contains the egg cell is known as the megagametophyte.

Common Mistakes

Angiosperm reproduction and gymnosperm reproduction are sometimes confused by students. To put it another way, angiosperm reproduction occurs in flowering plants, whereas gymnosperm reproduction occurs in non-flowering plants.

Related Concepts

• Asexual reproduction in plants
• Gymnosperm reproduction
• Fertilization in plants
• Vegetative propagation in plants
• Plant biology
• Plant systematics
• Self-incompatibility