How Does the Androecium Influence Plant Reproduction?
We will define androecium. A typical flower is divided into 4 whorls. They are calyx, corolla, androecium and gynoecium. The androecium in the flower is the third whorl. It arises from the inner side of the corolla. The androecium is the male reproductive part of the flower. It is composed of stamens. A stamen is further composed of anther and filament. Anthers that are present inside the flower are generally bilobed. Thus, anther and stamens are the parts of androecium. Each lobe of the anther contains two microsporangia. They are also known as pollen sacs. Inside the pollen sacs, the pollen grains are produced. The staminode is the name given to a sterile staminode. The length of the filaments can vary in different flowers. Now we will learn more about the and roecium meaning, stamen, microsporangium and pollen grains.
Stamen
The above paragraph helped us to define androecium. Stamens form the androecium. A stamen consists of anther and filament. The anther is a terminal bilobed structure. The filament is a long and slender stalk. The proximal end of the filament remains attached to the thalamus or the petal of the flower. An anther is a bilobed structure. A deep groove is present in front that separates the two anther lobes. The two anther lobes are attached by a band of tissues that are known as connective. An anther has a four-sided tetragonal structure. This structure consists of four microsporangia that are located at the corners. Two are present in each lobe. Therefore, a mature anther is tetrasporangiate. Pollen sacs are formed by microsporangia and on maturity, these microsporangia are filled with pollen grains. A microsporangium appears nearly circular in outline. Primary sporogenous cells are present in it. They are a homogenous mass of meristematic cells and are surrounded by another wall. They form the microspore mother cells.
Four Walls of the Anther
This will help us to clearly understand and define androecium more. The anther is made up of 4 walls. They are:
Epidermis
This is the outermost layer of the anther. It is a single layer and serves the purpose of protection. Some plant species such as Arceuthobium develop fibrous thickening and they are known as exothecium.
Endothecium
Alpha-cellulose fibrous bands are present in the cells of this layer. These bands arise from inner tangential walls and they help in the dehiscence of the anther. This is because they are hygroscopic. In hydrophytes, these bands are absent.
Middle Layer
These cells are ephemeral. This means that they are short-lived. They are 1-3 layered and degenerate at maturity.
Tapetum
This is the innermost layer of the anther wall. This layer surrounds the sporogenous tissue. The pollen grains receive nutrition from these cells. These cells have more than one nucleus and have dense cytoplasm. As these cells have more than one nucleus, so they are polyploid. There is also an increase in the DNA content of these cells. The increase in DNA content is achieved by Endomitosis, the Restitution nucleus and Polyteny. In endomitosis, there is DNA replication and the splitting of chromosomes. The restitution nucleus technique involves normal mitosis up to anaphase. The chromosomes at the two poles get surrounded by a nuclear membrane that is common to both. In polyteny, if the DNA replication is not supported by the splitting of chromosomes then polytechnic chromosomes are formed.
The Tapetum is of Two Types
Secretory Tapetum
This is also known as glandular tapetum. Cells of these tissues secrete sporopollenin. They also secrete pollen kitt and compatibility proteins. These cells also provide ubisch bodies. The ubisch bodies help in the orientation of exine because they have a chemical called sporopollenin that is deposited upon them.
Amoeboid Tapetum
The cells of this layer undergo breakdown and because of this, the entire protoplasm moves towards the centre to serve the purpose of providing nourishment.
The sporogenous tissue is a group of compactly arranged homogenous cells. These are formed when the anther is young. This tissue occupies the centre of each microsporangium. Microsporogenesis is the process by which haploid microspores are formed from diploid microspore mother cells. These haploid microspores are then arranged in the form of four cells and are called microspore tetrad.
Pollen Grain
As we studied above, the pollen grains represent the male gametophyte or the androecium. The pollen grains are spherical structures and are about 25-50 micrometre in diameter. Sporoderm is the name given to their cell wall. This sporoderm consists of two layers that are the exine and the intine. The exine is the hard outer layer that is made up of sporopollenin. Sporopollenin is one of the most resistant organic materials. This layer can withstand high temperatures and even strong acids and alkalis. Till now, no enzyme that can degrade sporopollenin is known. This strong layer also helps in fossilization. This means that the pollen grains can be well preserved. This layer also protects the seed from various biotic and abiotic stresses. For the taxonomic significance, this layer also exhibits various patterns and designs.
Germ Pore
Germ pore is the area where the sporopollenin is not present
The intine is the inner wall. It is a thin and continuous layer. It is made up of cellulose and pectin. The plasma membrane surrounds the cytoplasm of the pollen grain. The pollen grains of some species are also seen to cause pollen allergies. They are also responsible for causing bronchial afflictions. Some of the species of these pollens can also cause asthma and bronchitis. The pollen grains are rich in nutrients therefore they can be served as a food source. They are seen to cause an increase in the performance of athletes and horses. These pollen tablets are used as food supplements. Tablets and syrups of this pollen are also available in the market. Pollen viability is the period from which the pollens retain their ability to germinate on the landing of the stigma. This pollen viability also depends on temperature and humidity.
Types of Androecium
From the above paragraphs, we learned the meaning of androecium. Here we will learn about its types. There are mainly four types of androecium that are present in the flowers. They are
Polyandrous
In this type of androecium, the stamens of the flower are free. Androecium of hibiscus is of this type.
Monadelphous
In this type of condition, the filaments of the anther are fused to form one group.
Diadelphous
In this type of condition, the filaments of the anther are fused into two groups.
Polyadelphous
In this type of condition, the filaments of the anther are fused to form more than one group.
What Exactly is Androecium?
Flowers are made up of four whorls of reproductive and non-reproductive components. The calyx, corolla, gynoecium, and androecium are among these.
Calyx: The outermost whorl is made up of sepals, which are usually green, leaf-like structures.
Corolla: A whorl of petals that are often brilliantly coloured.
Androecium: Male reproductive organs called stamens are found in the third whorl.
Gynoecium: Female reproductive components termed carpels make up the innermost whorl.
The non-reproductive structures of a flower are the calyx and corolla, whereas the reproductive structures are the androecium and gynoecium. Egg cells are produced by the gynoecium, while sperm cells are produced by the androecium. The structure of the androecium will be the subject of this lesson.
Androecium's Components
Let's take a closer look at the androecium's various sections.
The androecium is divided into stamens. Stamens or stamina are the plural forms of the word. Multiple stamina make up an androecium; each one is made up of two parts: the filament and the anther.
Filament
The word filament has been derived from the Latin word ‘filum’, which means ‘thread’. In reality, the filament is a synonym for the thread. A tiny wire called a filament is heated to high temperatures in an incandescent light bulb that is devoid of oxygen to prevent the filament from catching fire while it is heated. The filament glows as it starts to light up. The original filaments were constructed of carbon, but due to their high melting point, tungsten is now commonly used.
A flower's stamen, or pollen-producing component, is made up of a slender stalk called a filament and an anther.
Anther
The stamens of the vast majority of angiosperms include separate pollen-containing components called anthers. Thecae (singular theca) are two compartments in anthers, each of which contains two microsporangia (the fusion product of which is a locule)(Anthers are frequently tetrasporangiate as a result.) The connective tissue, which connects and connects the two thecae, is where the filament (if existent) is attached. Pollen grains, the immature male gametophytes of seed plants, are produced in microsporangia.
Tapetum
This is the anther wall's deepest layer. The sporogenous tissue is surrounded by this layer. These cells provide sustenance to the pollen grains. These cells have several nuclei and a lot of cytoplasms. These cells are polyploid because they have more than one nucleus. In addition, the DNA content of these cells has increased. Endomitosis, Restitution nucleus, and Polyteny all contribute to the increase in DNA content. DNA replication and chromosome splitting occur during endomitosis. Normal mitosis up to anaphase is used in the restoration nucleus procedure. The chromosomes at both poles are encased in a nuclear membrane that is the same for both. Polytechnic chromosomes are generated when DNA replication is not supported by chromosomal splitting in polyteny.
Types of Androecium Found in Flowers Including
Polyandrous: Polyandrous refers to a condition in which a flower's stamens are unattached.
Monadelphous: Monadelphous is a condition in which the filaments of anthers in a flower are fused into one group.
Diadelphous: Diadelphous is a condition in which the filaments of anthers in a flower are fused into two groups.
Polyadelphous: When the filaments are fused in more than two groups, the condition is said to be Polyadelphous.
FAQs on Androecium: Definition, Layers, and Types
1. What is the androecium and what is its primary function in a flower?
The androecium is the third whorl of a flower, located inside the corolla. It represents the male reproductive part of the flower. Its primary function is to produce and present pollen grains, which contain the male gametes necessary for pollination and subsequent fertilisation.
2. What are the two main parts of a stamen?
A stamen, the individual unit of the androecium, consists of two main parts:
- Filament: A long, slender stalk that supports the anther. Its proximal end is attached to the thalamus or a petal.
- Anther: A terminal, typically bilobed structure that contains microsporangia (pollen sacs) where pollen grains are produced.
3. What is the difference between cohesion and adhesion of stamens?
Cohesion and adhesion describe how stamens are fused within a flower.
- Cohesion refers to the fusion of stamens among themselves. For example, in a monadelphous condition, all filaments are fused into one bundle.
- Adhesion refers to the fusion of stamens with other floral parts. For example, in an epipetalous condition, the stamens are attached to the petals.
4. Describe the different types of androecium based on the fusion of stamens (cohesion).
Based on the degree of filament fusion, the androecium can be of several types:
- Polyandrous: All stamens in the flower are free and not fused (e.g., Mustard).
- Monadelphous: Filaments are fused into a single bundle or tube, while anthers remain free (e.g., China rose).
- Diadelphous: Filaments are fused into two separate bundles (e.g., Pea).
- Polyadelphous: Filaments are fused into more than two bundles (e.g., Citrus).
5. What is the role of the tapetum in anther development?
The tapetum is the innermost nutritive layer of the anther wall. Its primary role is to provide nourishment to the developing microspore mother cells and pollen grains. It also plays a crucial part in forming the pollen grain wall by secreting enzymes like callase and substances like sporopollenin and pollenkitt.
6. Why is the anther of most angiosperms described as dithecous and tetrasporangiate?
An anther is called dithecous because it is typically composed of two lobes, which are clearly separated by a longitudinal groove. It is called tetrasporangiate because each of the two anther lobes contains two microsporangia (pollen sacs). Therefore, a mature anther has a four-sided structure with four sporangia in total, hence the term tetrasporangiate.
7. How are the androecium and stamen related? Are they the same thing?
The androecium and stamen are not the same thing, but they are directly related. The androecium is the collective term for all the male reproductive organs in a flower, representing the entire whorl. The stamen is the individual unit that makes up the androecium. Therefore, a flower's androecium is composed of one or more stamens.
8. What makes sporopollenin, a component of the exine, so resistant and what is its biological significance?
Sporopollenin is considered one of the most resistant organic materials known. Its resistance comes from its complex chemical structure, which is not degraded by strong acids, alkalis, or any known enzyme. This high resistance has significant biological importance:
- It protects the pollen grain from physical damage, UV radiation, and dehydration during its journey from the anther to the stigma.
- It allows pollen grains to be well-preserved for millions of years as fossils, which is crucial for paleontological studies.
9. What is a staminode and how does it differ from a fertile stamen?
A staminode is a sterile or rudimentary stamen that does not produce viable pollen. It differs from a fertile stamen, whose primary function is to produce and release pollen for reproduction. While a staminode is non-functional in terms of reproduction, it may sometimes be modified to serve other purposes, such as attracting pollinators by resembling a petal or producing nectar.
10. Explain the difference between secretory (glandular) and amoeboid (plasmodial) tapetum.
Both are types of tapetum that nourish developing pollen, but they do so differently.
- Secretory (Glandular) Tapetum: The tapetal cells remain intact in their original position. They actively secrete nutrients and other substances (like sporopollenin precursors) into the anther locule, which are then absorbed by the developing microspores. This is the more common type.
- Amoeboid (Plasmodial) Tapetum: The inner walls of the tapetal cells break down early, and the protoplasts move into the anther cavity. They fuse to form a multi-nucleate mass called a periplasmodium, which directly surrounds and nourishes the microspore mother cells.
