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Introduction to Stamen
A stamen is the male reproductive organ of a flower that produces pollen grains. The flower stamen, also referred to as the microsporophyll, comprises three main components: a filament, anther, and connective.
The number and length of stamens are variable in flowers of different species. A sterile stamen is called staminode. The flower stamens are typically arranged in a whorl and collectively form the androecium.
1.0Parts of Stamen
A stamen is composed of two parts– the long and slender stalk called the filament, and the terminal generally bilobed structure called the anther. The filament serves as a stalk-like structure, while the anther is responsible for producing and releasing pollen.
The filament:
- A flower male part, the stamen consists of a long stalk-like structure called filament and an anther. The filament carries nutrients and water to the anther and positions it to help spread pollen. The proximal end of the filament is attached to the thalamus or the petal of the flower, while at the distal end, the filament terminates or the anther is located.
Anther:
- The anther is the part of a flower that produces and stores pollen grains. It is usually located at the tip of a long, slender stalk called the filament. The anther consists of two lobes connected by tissue called the connective. The bilobed nature of an anther is very distinct in the transverse section of the anther. Each anther lobe contains two pollen sacs or microsporangia, situated at the corners of the four-lobed anther.
- In its youthful state, the anther is roughly oblong in shape in section, comprising a homogeneous mass of meristematic cells. As it matures, the anther develops four pollen sacs (microsporangia), each containing numerous pollen grains.
Structure of microsporangium:
- The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe. The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains.
- In a transverse section, a typical microsporangium appears near circular in outline. It is generally surrounded by four wall layers– the epidermis, endothecium, middle layers and the tapetum.
- The outer three wall layers perform the function of protection and help in the dehiscence of anther to release the pollen. The innermost wall layer is the tapetum. It nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
Microsporogenesis:
- When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the centre of each microsporangium.
- As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads. As each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is a potential pollen or microspore mother cell.
- The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad . As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains. Inside each microsporangium, several thousand microspores or pollen grains are formed that are released with the dehiscence of the anther.
Also Read: Cell Cycle and Cell Division
Inside each microsporangium, several thousand microspores or pollen grains are formed that are released with the dehiscence of the anther.
Structure of Pollen Grain
- The pollen grains represent the male gametophytes. Pollen grains are generally spherical, measuring about 25-50 micrometers in diameter.
- It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.
- Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent.
- Pollen grains are well preserved as fossils because of the presence of sporopollenin. The exine exhibits a fascinating array of patterns and designs.
- The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grain is surrounded by a plasma membrane.
- When the pollen grain is mature it contains two cells, the vegetative cell and generative cell. The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus.
- In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
2.0Types of Stamen
- Polyandrous stamens: when the stamens are free from each other, then the conditions are allied polyandrous conditions. For example, mustard ( belonging to the Brassicaceae or Cruciferae family).
- Cohesion of stamens: Cohesion of stamens refers to the fusion or attachment of the individual parts of the stamen, such as the filament and the anther, to each other. It is of the following types-
- Adelphy of stamens: Adelphy of stamens refers to the cohesion of stamens primarily through their filaments, with various types of cohesion observed in different plant species:
- Monadelphous: In this type, all the filaments are merged into a single bundle or cluster, while the anthers remain free. This cohesion results in the formation of a tube around the gynoecium, known as the staminal tube. For example, the China rose (belonging to the Malvaceae family) exhibits this type of adelphy.
- Diadelphous: Here, the filaments are grouped into two bundles, but the anthers remain unattached. A classic example is the pea plant (belonging to the Papilionatae family) such plants, typically out of 10 stamens, 9 are fused into a bundle, while one stamen remains independent.
- Polyadelphous: In this scenario, filaments are united into more than two bundles. A well-known example is found in citrus plants.
- Syngenesious: This term describes a condition where only the anthers are fused together in a bundle, while the filaments remain separate and free. A typical example can be seen in the Compositae family.
- Synandrous: In this type of cohesion, both the anthers and the filaments of the stamens are united throughout their entire length. Examples include plants like Colocasia, Alocasia, and species within the Cucurbitaceae family.
3.0Adhesion of Stamens
Adhesion of stamens refers to the condition where the stamens are fused or attached to other parts of the flower. It is of following types:
- Epipetalous: In this arrangement, the stamens are attached to the petals of the flower. An example is observed in Brinjal, a member of the Solanaceae family.
- Epiphyllous or Epitepalous: Here, the stamens are affixed to the tepals or perianth segments of the flower. This configuration is seen in flowers like the Lily, belonging to the Liliaceae family.
- Gynandrous: In this scenario, either the complete stamens or only the anthers are attached to the gynoecium (female reproductive structure) of the flower. Examples include Calotropis and Aristolochia.
Stamens on the basis of attachment of anther to the filaments:
- Adnate Attachment: Filament continues into the connective, which is almost as broad, seen in Ranunculus.
- Basifixed Attachment: Filament ends at the base of the anther, resulting in an erect anther, as in Brassica.
- Dorsifixed Attachment: Filament attaches on the connective above the base, resulting in a somewhat inclined anther, as in Sesbania.
- Versatile Attachment: Filament attached nearly at the middle of the connective, allowing the anther to swing freely, as in Lilium and grasses.
Stamens on the basis of position and orientation:
- Centripetal Positioning: Stamens develop from the outside to the inside, with the oldest stamens towards the periphery.
- Centrifugal Positioning: Stamens develop from the center towards the periphery, resulting in the oldest flowers being towards the center.
- Introrse Stamens: Slits of the anther face towards the center.
- Extrorse Stamens: Slits of the anther face towards the outside.
4.0Length of Stamens
In certain flower species there exists a variation in filament length.
- Didynamous Condition: In flowers with four stamens, two may be long and two short, a condition termed didynamous. For instance, this arrangement is observed in the Lamiaceae/Labiatae family, such as in Salvia.
- Tetradynamous Condition: In flowers with six stamens, arranged in two whorls, the outer whorl comprises two short stamens while the inner whorl consists of four long stamens. This arrangement characterizes the tetradynamous condition, found notably in the Cruciferae family, which includes plants like mustard, radish, and turnip.
5.0Functions of Stamens
- The primary function of stamens is to produce pollen, which contains the male gametes (sperm cells) necessary for fertilization. Pollen is produced within the anthers, which are the enlarged sacs located at the tip of the stamen.
- Stamens undergo dehiscence, which is the process of opening of the anthers to release pollen. Proper timing and mechanism of anther dehiscence ensure that pollen is released when flowers are receptive and pollinators are available.
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