We will define androecium. A typical flower is divided into 4 whorls. They are calyx, corolla, androecium and gynoecium. The androecium in 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 androecium meaning, stamen, microsporangium and pollen grains.
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 anther wall. They form the microspore mother cells.
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Figure: Androecium of a flower
Anther Wall Layers
This will help us to clearly understand and define androecium more. The anther is made up of 4 walls. They are:
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.
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 of hygroscopic nature. In hydrophytes, these bands are absent.
These cells are ephemeral. This means that they are short-lived. They are 1-3 layered and degenerates at maturity.
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, 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:
This is also known as glandular tapetum. Cells of these tissues secrete sporopollenin. They also secrete pollenkitt 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.
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.
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. The 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 is the areas 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:
In this type of androecium, the stamens of the flower are free. Androecium of hibiscus is of this type.
In this type of condition, the filaments of the anther are fused to form one group.
In this type of condition, the filaments of the anther are fused into two groups.
In this type of condition, the filaments of the anther are fused to form more than one group.