It involves an event development of anther and Microsporogenesis, development of male gametophyte, female reproductive whorl, structure of ovule Or megasporangium, development of ovule, development of pollen grains, development of endosperm.
Introduction
Sexual reproduction
Pre fertilization structures and events
Male reproductive whorl – Androecium consists of a stamen. The number and length of stamen are variable in flowers. Stamen is equal to microsporophyll. Stamen is made up of long, thin stalks called the filament and at the end a bilobed structure known as anther. And at the end of the stamen is attached to thalamus. Anther and filament are connected with the small region that is made up of connective tissues. A typical anther in angiosperm is generally made up of bilobed; it has two theca. Anther is consists of four microsporangia located at the four corners; later these microsporangium acts as pollen sacs.
Structure of anther – microsporangium consists of four different walls
- Epidermis – outermost layer, acts as a protective layer
- Endothecium – this layer is present below the epidermis. Single celled thick layer. When the maturation of anther takes place various changes are observed in this cell. Outer walls of the endothecium are thin but the inner walls starts are thick due to deposition of alpha cellulose fibres. This layer is hygroscopic in nature and helps in the dehiscence of anther.
- Middle layer- it is made up of parenchymatous cells. It is a short lived layer as maturation takes place this layer starts vanishing the outer three layers perform the main function they act as a protective layer.
- Tapetum – innermost layer, the cells present in this layer possess a cytoplasm and one nucleus, cell division takes place inside this layer, initially cell present in that are diploid but they become polyploid due to endomitosis and multinucleate due to nuclear division.
Tapetum absorbs food from the middle layer.
Functions of the tapetum- it provides nutrition to the developing pollen. Secretion of pollenkitt substances
Development of anther- it is the mass of meristematic cells which is surrounded by single celled thick outer layers.
Development of anther and Microsporogenesis- In the young stage of anther it is the mass of the meristematic cells. First all vascular tissue is formed in the middle region as gradually the cell at the corners starts growing. These are known as archesporial cells, further these cells divide and form primary parietal cells and form a series of 3 – 5 layers. Primary parietal further divides now these cells are known as primary sporogenous cells. Later sporogenous cells differentiate into microspore mother cells. Each microspore mother cell divides to form four haploid microspore by meiotic division. This process is known as Microsporogenesis. Four Microspores are attached together with the help of a callose layer. Now they get arranged in the group of four cells. The group is called microspore tetrad. Simultaneously, spherical bodies are known as ubisch bodies; they are made up of complex substance sporopollenin. After the formation of ubisch bodies, the tapetum layer degenerates; now thick walled microspores are called pollen grains. Generally pollen grains are spherical in shape measuring about 25-50 micrometers in diameter.
Dehiscence of anther-
Middle layer degenerates due to absorption of food by tapetum. Mature anther only two layers are present epidermis and endothecium. Dehiscence of anther takes place during the dry season. Loss of water takes place through the cells of the endothecium. Due to the loss of water endothelial cells try to contract due to the loss of water. Which creates exert pulling force or tension over the surface of anther. Due to which the walls break off pollen grains present in pollen sacs released into the atmosphere.
Development of male gametophyte-
Pre-Pollination development-
Nucleus of pollen grains divides by unequal mitotic division. Nuclei surrounded by the cytoplasm further following an unequal cytokinesis results in unequal size cells being formed. Larger cells are known vegetative cells or tube cells and smaller cells are known as generative cells.
Post pollination development-
Development of pollen grains takes on the stigma. It absorbs moisture and sugar content from stigma due to which volume of the cytoplasm is increased. It exerts pressure on the both outer layers through which intines come out through any one germ pore in the form of tube-like structures called as pollen tube. First the vegetal nucleus enters into the pollen tube assuming tip position spindle shape generative cell now enters the pollen tube. Inside the pollen tube, generative cells divide mitotically to form two non motile male gametes, now the male gametophyte becomes a three celled structure, one vegetal cell and two male gametes.
Female reproductive whorl-
Development of ovule –
In the beginning of development of the ovule nucellus develops from the placenta further it forms the small rounded outgrowth like structures all the cells present in the nucellus are undifferentiated, homogenous and are meristematic cells. From these cells any one cell of the nucellus differentiate and increase in size and forms a archesporial cell divides mitotically to form a primary parietal cell and primary sporogenous cell further develops in a large cell with dense cytoplasm it divides Meiotically to form four haploid megaspores.
Megasporogenesis-
Formation of a megaspore mother cell is called megasporogenesis. Four haploid megaspores are arranged in a linear fashion. Functionally megaspores produce female gametophytes.
Development of embryo sac or female gametophyte –
Nucleus of the functional megaspore divides mitotically to form two nuclei, further mitotically they divide and form the eight nuclei. Out of which three nuclei are at the tip of the cell in two are arranged at the tip of the cell, two in the middle and three at the end of the cell. Further these are developed into the antipodal cells, polar nuclei, synergids and egg respectively.
Fertilization
Fusion of male gametes and female gametes is called fertilization. Which further includes a following steps
Germination of pollen grains
Entry of pollen tube into ovule
Entry of the pollen tube in embryo sac
Fusion of the gametes
Development of the endosperm
Endosperm development includes the maturation of ovule into the seeds and ovary into the fruits.
Conclusion
This is how the sexual reproduction takes place in plants thus, it is very complex to understand each of the part of the plant plays a very significant role. Changes occurs in the plants after the fertilization
Ovary – fruit
Ovule – seed
Ovary wall – fruit wall ( pericarp)