The anther is a part of the flower’s male reproductive system or stamen. It carries the reproductive structures or the male gametes (pollen grains) which germinate and cause fertilization of an ovule. A male reproductive part of a flower typically consists of two elements:
Filament: A filament appears as a long, slender and stalk-like structure found in the stamen.
Anther: It is generally a bilobed structure that carry pollen grains in the pollen sacs.
A filament of a stamen emerges out from the thalamus or peduncle and gives rise to a knob-like anther. The number, length and position of a stamen can vary within the different range of flowers.
The anther is a structure found in the male reproductory part of the flower refers to as “Stamen” or “Androecium”. It has a knob-like structure, which usually consists of two lobes joined together by the connective tissue. It carry the pollen grains within the microsporangia, which further develops into a pollen sac. The pollen grains released out during the process of dehiscence occurs after the process of Anthesis. The pollen grains are moved from the anther to the stigma of either same or different flower during pollination. Thus, pollen transfer can occur via self or cross-pollination methods.
To understand the detailed structure of an anther, we must focus on its external and internal structure as well. Morphologically, an anther is a knob-like, bilobed, where the connective tissue connects both the lobe.
But, if we look at the transverse section of an anther, we will come to know about many of its exciting features. As we can see in the diagram, anther consists of two lobes and four chambers. The parenchymatous connective tissue separates the two lobes, and each lobe contains two thecae, which means anther is dithecus. The external longitudinal groove separates the four chambers refers to as “Stromium”.
The thecae consist microspores and thus refers as “Microsporangium”. Due to the presence of four microsporangium, an anther refers as “Tetrasporangiate”. An epidermis forms the outer lining and protects the anther. On maturation, the microsporangium develops “Archaesporial cells”. The Archaesporial cells are the large, individual cells with dense cytoplasm and remain hypodermal in condition.
Then archaesporial cells undergo “Periclinal division”. A periclinal division is a kind of transverse division that divides the cells into:
- Peripheral primary parietal cells
- Inner primary sporogenous cells
Parietal cells provide protection to the inner sporogenous cells and further undergoes some periclinal and anticlinal divisions to form three differentiated layers of cell:
Outer endothecium: Parietal cell first undergoes mitosis and differentiates into outer endothecium which functions at the time of dehiscence.
Middle layer: Parietal cell further divides and differentiates to form a middle layer. An intermediate layer usually comprises of 1-3 layers of the cell.
Anther can classify in many kinds, based on its structure and attachment with the filament.
Based on Structure
Based on the structure, it can classify into the following types:
Bilobed: It is the most common type, which consists of a bilobed structure occurs in all mature stamens of the Angiosperms.
Unilocular: It consists of a single lobe like in the members of the Malvaceae family. The unilocular condition arises due to:
- Destruction of one lobe.
- Destruction in the connective tissue that holds the two lobes.
- And, by the destruction of longitudinal grooves that separates the microsporangia.
Linear: This occurs in the genus Acalypha.
Rounded: It appears in the genus Merculiaris.
Sagittate: It occurs in the genus Vinca.
Sinuous: It has a peculiar filamentous type appearance and occurs in the members of Cucurbitaceae.
Reniform: China rose consists of a Reniform anther.
Appendiculate: It appears in the Erica cinerea of Ericaceae.
Based on Attachment to the Filament
An anther can be of the following types, based on the attachment with the filament:
Adnate: In this type, an anther is present in continuation with the filament-like in Magnolia and water lily.
Innate: Here, a filament is attached to the basal region of an anther like in mustard plants. It also refers as “Baxifixed” anther.
Dorsifixed: In this type, a filament attaches to the dorsal side or back of an anther like in Sesbania etc.
The functional role of an anther can be understood by knowing the distinct character of the individual components:
Epidermis: It forms the lining of an anther that acts as a “Protective sheath” by providing structural integrity and strength. Epidermis also plays an essential role in the process of gaseous exchange between an anther and surrounding. It protects against dehydration and extreme conditions.
Endothecium: It is the second layer after the epidermis, which functions in the anther dehiscence and structural support to the internal structures.
Middle layer: It is the third layer, present after a layer of large endothecium cells that serves to store starch. The starch mobilizes to the pollen grain during the maturation.
Tapetum: It is the fourth and the last layer that radially surrounds the microsporangium. Tapetum is a nutrient-rich layer that provides nourishment to the pollen grains during its development. It consists of auxin hormone and callose enzyme, promotes the growth of pollen grain. Tapetum forms a compatible protein which means a pollen grain those which are compatible with the female gametophyte will only germinate.
Anthesis: It can define as the process when the anthers dangle out from the spikelet of the growing flower. The process of dehiscence occurs during the stage of anthesis when a flower attains maturity.
Dehiscence: On maturation, the anther becomes ripe and burst out by discharging pollen grains. Thus, the process of pollen release from an anther is a process refers as “Dehiscence”. Based on the direction of dehiscence, an anther releases pollen grain in the following ways like:
- Transverse dehiscence: In this, pollen releases away from the centre of the unilocular anther.
Example: Members of the Malvaceae family.
- Longitudinal dehiscence: Here, the pollen releases laterally or longitudinally, neither towards nor away from the axis.
- Poricidal dehiscence: In this type, pollen releases from the apical or distal region.
Example: Potato, Brinjal etc.
- Valvular dehiscence: In this type, the anther wall opens like trap doors from where the pollen releases.
Example: Berberries, Laurus etc.
Pollination: It is also involved in the process of pollination, where the pollen grains released due to dehiscence are captured by the stigma via pollinators.
Thus, an anther plays a crucial role in flower fertilization and development.