Stamen, Microsporangium and Pollen Grain

Stamen, microsporangium and pollen grains are associated with the male reproductive unit. In unisexual flowers, only either of the reproductive parts is present. Almost all flowering plants show sexual reproduction. Flowers are the most attractive parts of the plant and the site of sexual reproduction in flowering plants. Bisexual flowers bear both male (stamens) and female (carpels or pistils). 

This article will learn what stamen is, its parts, and its function. We will also learn the development of anther and the formation of pollen grains. Last but not least, the development of male gametophytes from pollen grain will also be discussed in brief.

Androecium

The stamens in flowers are collectively called the androecium. It is the male reproductive part of a flower. The androecium is the third whorl in a flower that arises inside the corolla and is made up of stamens or microsporophylls. Based on several anther lobes, androecium can be of two types:

1. Monothecous: Anthers with a single anther lobe bearing a single locule are called monoecious. For example, members of Malvaceae.
2. Dithecous: Anther with two anther lobes is said to be dithecous. For example, Vinca, Pea.

Stamen and Microsporangium

Stamens are the male reproductive units of angiosperms. Each stamen is composed of a slender shaped stalk known as a filament, and anther is present at the tip of the filament. 

Each anther consists of two lobes connected by a tissue called connective. Anther lobe has four pollen sacs placed longitudinally. These pollen sacs represent a microsporangium that contains numerous pollen grains or microspores. A stamen is, therefore, a microsporophyll bearing four microsporangia (tetrasporangiate).

Fig: Stamen and Structure of Anther

Descriptive Terms Related to Stamen

1. Adelpy: When only stamens unite and not anthers, it is called adelpy or adelphous. It is of various types:
a. Monadelphous: In this type, the filaments of all the stamens are fused to form a single bundle. For example, Hibiscus.
b. Diadelphous: In this type, the filaments unite to form two bundles. In members of Papilionaceae, nine stamens form one bundle, and the tenth remains as the second bundle. For example, Pea.
c. Polyadelphous: In this type, the filaments are fused to form two or more than two (many) bundles. For example, Bombax malabarica (silk-cotton tree).

2. Epipetaly: It is a condition where the stamens are partially or completely attached to the petals. For example, Brinjal.

3. Syngeny: It is a condition where all anthers of a flower are united, but filaments are. For example, Sunflower.

4. Synandry: It is a condition where all anthers and stamens are united along their length. For example, Cucurbita.

Fig: Types of Androecium

Anther 

The anther has distinct bilobed nature when seen under in the transverse. It is a four-sided (usually termed as tetragonal) structure that bears four microsporangia located at the corners, two in each lobe. Each microsporangium develops into a pollen sac. These extend longitudinally throughout the length of an anther and are packed with pollen grains. Let us learn more about parts of the anther by studying its transverse section.

Microsporangium 

The transverse section of a mature anther reveals the presence of anther cavity surrounded by a four-layer anther wall. The anther is bilobed, where each lobe has two theca (called dithecous). Under the transverse section, a typical microsporangium appears nearly circular in outline.

1. Anther wall: The mature anther wall comprises four layers, namely the epidermis, endothecium, middle layers, and the tapetum.
2. Epidermis: It has only one layered and is protective in function. The cells undergo repeated anticlinal divisions to face the rapidly growing internal tissues.
3. Endothecium is generally a single layer of radially elongated cells present below the epidermis. The inner tangential wall forms bands (sometimes radial walls also) of α cellulose (sometimes slightly lignified too). The cells are hygroscopic. The cells present along the junction of the two sporangia of an anther lobe deficient of these thickenings. This region is called stomium. 
4. Middle layers: Two to three layers of cells followed by endothecium constitute middle layers. They disintegrate or get pressed during maturity.
5. Tapetum: The fourth and the innermost layer is called the tapetum. It attains its maximum development at the tetrad stage of microsporogenesis. It is obtained partially from the peripheral wall layer and partially from the connective tissue of the anther lining the anther locule. Thus, the tapetum is dual in origin. It nourishes the developing sporogenous tissue, microspore mother cells and microspores. It comprises cells with dense cytoplasm and has more than one nuclei. It also regulates the fertility or sterility of the microspores or pollen grains.
6. Sporogenous tissue: A group of compactly organized homogenous cells occupies each microsporangium’s centre in a young anther. 
7. Microspore mother cell: Each cell of sporogenous tissue undergoes meiosis to form microspore tetrads. Each cell of the tetrad is called a microspore mother cell.
8. Anther Cavity: The anther cavity is packed with microspores in the early stages or with pollen grains at maturity. The meiotic division of microspore mother cells produces haploid microspores in nature.

Fig: T.S of Mature Anther

Microsporogenesis

The cells of the sporogenous tissue undergo multiple meiotic divisions to form microspore tetrads. Each cell of the sporogenous tissue is known as pollen mother cell (PCM) or microspore mother cell. The process of formation of microspore from a pollen mother cell through meiosis is called microsporogenesis. The microspores are developed in the form of microspore tetrad. As the anther mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. Let us understand this process in further detail.

Development of Anther and Formation of Pollen Grains

1. The young anther consists of a homogenous mass of parenchymatous cells surrounded by the epidermis. 
2. It shortly becomes four-lobed, and each of the four lobes, along with some of the hypodermal cells, begin to act as archesporial initials. 
3. Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell.
4. The primary parietal cell splits to form 3-5 wall layers, i.e., endothecium, middle layers and tapetum. 
5. The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes.
6. Each microspore mother cell divides meiotically to form four haploid pollen grains and remains arranged in tetrads. 
7. The arrangement in the tetrads can be tetrahedral, isobilateral, linear, T-shaped and decussate.
8. Now the microspores are separated from tetrad. 
9. In Drosera, Typha, Elodea, Hydrilla, etc., all the four pollen grains do not separate and thus form compound pollen grains. 
10. In the family Cyperaceae (Cyprus), out of four pollen in a tetrad, three degenerate and one remains alive. So one meiosis produces one pollen. 
11. Sometimes more than four pollens are produced from one microspore mother cell. It is called polyspory, e.g., Cuscuta. 
12. In Calotropis (Asclepiadaceae) and some orchids, all the pollen grains of an anther lobe form a typical structure called pollinium.

Pollen Grains

The pollen grains are the male gametophytes. Microspore or pollen grain is haploid, uninucleate, and minute spores. These are produced in large numbers due to meiosis in microspore mother cells inside the microsporangia. 

Fig: Pollen Grains

Following are some characteristics of pollen grains:

1. Pollen grains are generally spherical in shape, measuring about 25-50 micrometres in diameter.
2. It has double-layered cell walls, exine and intine. 
3. Intine is the thin, delicate inner layer composed of pectin and cellulose. 
4. The exine is the outer layer composed of sporopollenin, one of the most resistant organic materials. It can resist high temperatures, strong acids or alkali and is resistant to physical and biological decomposition. Thus, pollen walls are preserved for long periods in fossil deposits.
5. The exine is very thin or absent at one or more places. These regions are called germ pores.
6. A plasma membrane surrounds the cytoplasm of the pollen grain. 
7. In the majority of the angiosperms, pollen grains are released at a 2-celled stage, i.e. the vegetative cell and generative cell. 
8. The vegetative cell is bigger with sufficient reserved food and a large irregularly shaped nucleus. 
9. The generative cell is relatively small and floats in the cytoplasm of the vegetative cell. The generative cell is spindle-shaped along with dense cytoplasm and a nucleus.

Microgametogenesis

The development of male gametophytes from pollen grain is called microgametogenesis. This process can be better understood in two developmental stages of microspores mentioned below:

Pre-pollination Development

1. Microspore begins development In situ, i.e., while enclosed inside the microsporangium or pollen sac and is called precocious.
2. Microspores may be best termed as partially developed male gametophytes. 
3. Microspore nucleus divides mitotically to form a smaller generative cell present next to spore-wall and a much larger vegetative cell (or tube cell). 
4. A callose layer is formed around the generative cell. The generative cell separates from the wall of the microspore and becomes accessible in the cytoplasm.
5. The callose layer then dissolves, and pollen grains are shed from the anther at this bicelled stage (rarely three celled).

Post-pollination Development 

1. The released pollen grains are carried to the receptive surface of the carpel (i.e., stigma) by the pollination.
2. Here, the pollen grain absorbs water and swells within a few minutes. It releases the wall-held recognition factors that determine whether the pollen grain will germinate on the stigma or not. 
3. After pollen-pistil interaction, the vegetative cell or tube cell expands and comes out through one of the apertures in the form of a pollen tube. 
4. The wall of the pollen tube is the extension of the intine. 
5. The tube secretes special enzymes to form a passage for its entry. 
6. It receives nourishment from the transmitting tissue of the style. 
7. Progressively, the tube and generative nuclei are carried by the pollen tube. 
8. The generative cell divides to form two non-motile male gametes. The tube nucleus has no vital function and may disintegrate.

Fig: Microgametogenesis

Summary

The androecium is the total number of stamens, with each stamen consisting of an anther at the end of a stalk called a filament. The anther usually is composed of four elongated lobes called pollen sacs. These pollen sacs represent a microsporangium that bears numerous pollen grains or microspores. Pollen grains of some species may cause various allergies and bronchial disorders in some people. Pollen grains are rich sources of nutrients. Hence, pollen tablets are used as food supplements.

In the early development stage of the anther, each pollen sac carries a group of dividing cells called microsporocytes. Each microsporocyte undergoes meiosis to form four haploid (n) microspores. The nucleus of each of these microspores undergoes mitosis to form a two-celled pollen grain that bears a tube cell and a comparatively small generative cell. The function of this two-celled, haploid male gametophyte is to produce sperm cells for fertilization.

FAQs

Q.1. What is stamen and microsporangium?
Ans: Stamens are the male reproductive units of angiosperms. Each stamen is composed of a slender stalk called filament and at the tip of it is the anther. Anther lobe has four pollen sacs placed longitudinally. These pollen sacs represent a microsporangium that contains numerous pollen grains or microspores.

Q.2. What is the difference between microsporangia and pollen sac?
Ans: Microsporangia is also called pollen sac. These microsporangia produce microspores, also known as pollen grains. Pollen sacs are borne on the lower surfaces of the microsporophylls.

Q.3. What is pollen grain?
Ans: Pollen grains are the male gametophytes. Microspore or pollen grain is haploid, uninucleate, and minute spores. These are produced in large numbers due to meiosis in microspore mother cells inside the microsporangia. 

Q.4. What is the relationship between stigma and stamen?
Ans: The male reproductive parts of the flower are called the stamens. These are made up of the anther at the top and the filament that supports the anther. The female reproductive parts are collectively called the pistil. The top of the pistil is called the stigma, which is a sticky receptive surface for pollen.

Q.5. Why is anther called Tetrasporangiate?
Ans: The anther is four-sided. It has four locules that consist of four microsporangia located in each locule. These microsporangia develop further and become the pollen sacs. Thus, the anther is called tetrasporangiate due to the presence of four microsporangia.

Learn The Concept Of Pollination Here

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