The bilobed anther is a part of the stamen and serves as the site of pollen production and dispersal. Typically, an anther is bilobed, dithecous, and tetrasporangiate in most angiosperms with a few exceptions. The structure of a bilobed anther consists of two lobes, each containing a pair of microsporangia or pollen sacs called thecae. These thecae are where pollen grains develop and mature.
The two sporangia in each lobe become confluent at maturity and cause lysis of the septum separating them. At maturity, the anther dehisces along a slit termed the stomium to release the pollen. Mature pollen carries sperm cells to the stigma of the female reproductive part for fertilization. In this article, we will read about the bilobed anther diagram and the structure, and function of the bilobed anther.
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What is a Bilobed Anther?
A bilobed anther is a type of anther with two lobes separated by a septum. This type of anther is commonly found in the androecium of most angiosperms. The androecium consists of the stamens, the male reproductive part of a flower. A stamen has two parts: a long, slender stalk called the filament and a two-lobed pollen-bearing unit called an anther.
Some angiosperms, such as Hibiscus, Cotton, etc., exhibit single-lobed anthers instead of the typical bi-lobed structure. Inside each lobe is a cavity which houses the pollen sacs, producing hundreds of pollen grains for pollination.
Bilobed Anther Diagram
The diagram of a bilobed anther is given below:
Structure of a Bilobed Anther
The anther is a knob-like structure that sits on a slender stalk called a filament. The filament delivers water and nutrients to this pollen-producing part of the stamen. The structure of bilobed anther consists of two lobes (bilobed) and contains two microsporangia in each lobe. The lobes each have a cavity, known as the theca, to house the microsporangia. An anther is tetrasporangiate, meaning it has a total of four microsporangia or pollen sacs.
The non-sporangial tissue that connects the two lobes is called the connective and contains a single vascular strand. The two microsporangia in each lobe are separated by sterile tissue called the intersporangial septum. As the anther matures, enzymatic lysis of the septum causes the two sporangia to merge into a single locule or theca.
In plants like Hibiscus rosa-sinensis, the anther has only one lobe with two microsporangia, which fuse at maturity to form a single theca, hence it is monothecal. Anthers may be linear (Acalypha), rounded (Mercurialis), sagittate (Vinca), sinuous (Cucurbita), or reniform (China rose) in shapes.
T.S of Bilobed Anther
A transverse cross-section reveals the internal layers of the bilobed anther. A well-differentiated anther wall has four layers-
- Epidermis: It is the outermost layer of the anther and has a protective function. It prevents water loss and provides structural support. The epidermis also helps in anther dehiscence. The point of dehiscence, known as the stomium, can be longitudinal, transverse, apical, or valvular. In the stomium region, epidermal cells differentiate into specialized cells that split open at anther maturity, allowing the anther to burst open and release pollen grains.
- Endothecium: The endothecium is a layer beneath the epidermis, called the hypodermal layer. It is typically single-layered and found in the protruding part of the anther in most angiosperms. As the anther matures, the endothecium cells elongate radially to aid in dehiscence. It also provides structural support to the anther.
- Middle layers or intermediate layer: The intermediate layer generally consists of 1 to 3 layers. The thin-walled cells of the intermediate layer contain food material such as starch, which nourishes the microspores. During pollen development, the reserved food gets mobilized. Middle layers are usually short-lived and are crushed during meiosis in the microspore mother cells.
- Tapetum: The tapetum is the innermost single layer of the anther wall and surrounds the sporogenous tissue. It secretes necessary hormones and enzymes for pollen development and pollen germination. In many angiosperms, the tapetum has a dual origin: the outer part is derived from the parietal layer, and the inner part is formed by the connective tissue. During micro-gametogenesis, the tapetum begins to degenerate and is fully degraded by the time of dehiscence.
Functions of a Bilobed Anther
The main function of a typical anther is to help in plant reproduction. The following are its functions:
- Pollen production- A typical bilobed anther of angiosperms acts as a site for the production of microgametophytes or pollen grains. The pollen grains are the carriers of male gametes which play a central role in plant reproduction.
- Pollen maturation- Anther supports the maturation of the microsporocytes or pollen mother cells. These microsporocytes undergo meiosis to produce haploid microspores. The microspores then develop into the microgametophytes or pollen grains following two mitotic divisions.
- Pollen storage- Inside the thecae of a bilobed anther remain pollen sacs or microsporangia to store large amounts of pollen grains.
- Pollen release- When an anther matures, it dehisces at the stomium, to release the pollen grain content inside.
- Pollen dispersal- The bilobed anther is strategically positioned on a stalk-like filament to assist in pollen dispersal. This allows bees and other passing pollinators to come in contact with the released pollen and transport it to the stigma.
Conclusion: Bilobed Anther - Structure of an Anther
A bilobed anther is a crucial part of the stamen required for the reproductive process of angiosperms. Its distinctive two-lobed structure houses innumerable pollen grains inside the pollen sacs in thecae. The anther plays a vital role in pollen production, maturation, storage, and release. It also enables pollen dispersal by aiding pollinators to carry pollen to the stigma for fertilization. The bilobed anther is critical for ensuring the continuity of plant life.
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