In a flower, stamen is considered as the male reproductive organ. Each stamen consists of filament, connective and anther. Anther may be monothecous or dithecous. A monothecous anther consists of two locules or two sporangia. So it is said to be bilocular or bisporangiate. A dithecous anther consists of four locules or four sporangia. So it is said to be tetralocular or tetrasporangiate.
Development of microsporangium is eusporangiate. A very young anther in transverse section shows epidermis and archesporium. The archesporial cells divide periclinally giving rise to primary parietal cells. On the outer side and sporogenous cells towards innerside. The cells of the parietal layer divide periclinally and anticlinally forms endothecium, middle layers and tapetum. The cells of the primary sporogenous tissue differentiated into pollen mother cells or microspore mother cells.
The anther wall consists of following walls layers.
1.Epidermis: Epidermis is the outermost single layer. It is compactly arranged and usually protective in function. Epidermal stomata is reported in Alangium.
2.Endothecium: The cells of the endothecium are radially elongated and shows fibrous bands. The fibrous bands are made up of callose an arise from the inner tangential walls. Usually fibrous bands are “U” shaped. Fibrous bands are absent in Hydrocharitaceae, Saprophytes and Cleistogamous flowers. Endothecial thickenings are also absent in Musa, Sesamum, Annona, Ipomea etc but in these members the anther epidermis has deposition of cutin and lignin. The fibrous bands are hygroscopic in nature. Endothecial cells help in the dehiscence of anther at maturity. Because of the presence of fibrous bands, this layer is otherwise called fibrous layer. It is single layered but in Coccinia double layered.
3.Middle layers: Below the endothecium 2-3 layers of cells are present which constitute middle layers. These middle layers are ephemeral and become crushed by early meiosis in pollen mother cells. These cells act as storage centres for starch.
4.Tapetum: Tapetum is the innermost layer of anther walls, and it completely surrounds the sporogenous tissue. The cells contain dense cytoplasm with prominent nuclei. Usually tapetum consists of single layer of cells. As the tapetum completely surrounds the sporogenous tissue major part of it is derived from parietal cells and a small part developed from the sporogenous tissue. Tapetum transports the nutrients to the developing sporocytes. Tapetal cells are pigmented and it is red brown in apple or violet in Anemone
Base on the behaviour, two kinds of tapetum were recognised.
a) Amoeboid tapetum: The inner and radial walls of the tapetum break down due to the action of hydrolytic enzymes and their protoplasts penetrates between the pollen mother cells and developing pollen grains. After intrusion, they fuse with each other and forms a mass of tapetal periplasmodium. This tapetal plasmodium remains associated with the pollengrains till their maturity. When the anther gets drying up the tapetal periplasmodium gets dehydrated and coated over the surface of pollengrains, thereby helping in the formation of exine. Amoeboid tapetum is considered as the primitive type. It is also called periplasmodial tapetum. Eg:- Alisma, Tradescantia, Typha, Saggitaria, Potamogeton.
b) Glandular tapetum: The cells of glandular tapetum remains intact throughout microspore development. They secrete their substances from their innerfaces. Secretary tapetal cells are thin and possess almost all cell organells like mitochondria, plastids, dictyosomes etc. some spherical structures called proubisch bodies are also present. Just before the pollen mother cells undergo meiosis, the walls of the tapetal cells become thick and there is considerable increase in the no. of ribosomes and pro-ubisch bodies with the completion of pollen development proubish bodies pass into the anther locule from the tapetal cells and they are now called ubisch bodies and they coated over the pollengrains Eg:- Higher monocots and many dicots.
Functions of tapetum:
1. The nutrients are transported through tapetum to the sporogenous tissue.
2. Tapetum is involved in the synthesis of callose which release microspores in a tetrad by degrading callose wall.
3. Tapetum plays an important role in the formation of exine.
4. Pollen kit (Lipids and carotenoids) is formed by tapetal layer. It is a insect attractant & protect pollen from ultra violet.
Sporogenous tissue: The primary sporogenous tissue directly functions as microspore mother cell. The microspore mother cell undergoes reduction division forms four microspores or pollengrains. Now they are called pollen tetrads. Depending upon the manner of wall formation, pollen grains are formed in two ways.
1. Successive type
2. Simultaneous type
Successive type: The cell wall is laid down between the two daughter nuclei immediately after first meiotic division. The second meiotic division results in two haploid nuclei in each of the two daughter cells. This results in the formation of isobilateral tetrads. Eg:- Monocots.
Simultaneous type: The nucleus of the microspore mother cell divides twice forming four haploid nuclei. Later wall formation taking place between the nuclei. This results in the formation of tetrahedral tetrad Eg:-Dicots.
Pollen tetrads: The microspore mother cell divides meiotically and produces four haploid microspores. The microspores are grouped into microspore tetrad. On the basis of microspores arranged in a tetrad, 5 types of microspore tetrad are recognised.
a) Tetrahedral: In this type microspores are arranged like the quadrants of a sphere. Eg:- Angiosperms
b) Isobilateral: In this type microspores are arranged in one place or at the corners of a square. Eg:- Zea mays
c) Deccusate: Microspores are arranged in two pairs standing at right angles to each other. Eg:- Magnolia, Cornus.
d) T-shaped: Here two microspores are arranged just opposed and the other two microspores are arranged one above the other. Eg:- Aristalochia, Botomopsis.
e) Linear: here the microspores are arranged one above the other in a line Eg:- Hydrocharitaceae, Asclepiadaceae.
Microspore: The microspore is oval or round contains dense cytoplasm, centrally placed nucleus. The size of the pollen grain varies from nearly less than 10 µ (Myosotis) to 200 µ (Uirabilis jalapa). The microspore is surrounded by a stratified wall called sporoderm. The sporoderm is made up of two layers
1. Outer exine
2. Inner intine
Outer exine: Exine is made up of sporopollenin. Which is derived from carotenoids by oxidative polymerisation. Sporopollenin is resistant to physical and biological decomposition. Because of this only pollen grains are protected during their movement and are preserved for longer periods. Exine is further divided into outer thick extexine (sexine) and inner thin enderxine ( nexine). Extexine exhibits three major zones.
3. Foot layer
The exine shows un-thickened areas called germpores
Inner intine: Intine is made up of outer pectic layer and inner cellulosic layer. Near germ pore intine is made up of callose. Intine contains enzymatic proteins near the germ pore. Intine helps in the formation of pollen tube. The microspores of a tetrad usually separate from each other as the anther matures. But sometimes they do not separate even at maturity and even the tetrads are stuck together in groups which may contains as many as 64 pollen grains. Such groups are called compound pollen grains. In Asclepiadaceae all pollens in a pollen sac are united into a single compact mass known as pollinium. Pollinia are also formed in Orchidaceae but in certain members of this family the pollinium is less compact as it comprises smaller groups of pollen grains. Such loose groups are formed as Massulae. In Ericaceae and Empetraceae the pollen grains are permanently united in tetrahedrons.