In this article we will discuss about the reproductive structures of a flower with the help of suitable diagrams.
Male Gametophyte (Stamen, Microsporangium and Pollen Grain):
Stamen:
It is the male reproductive unit of angiosperm. It consists of two parts:
(i) The long and slender stalk called the filament.
(ii) The terminal generally bi-lobed structure called the anther. The proximal end of the filament is attached to the thalamus or the petal (epipetalous) of the flower.
The number and length of the stamens are variable in flowers of different species.
Structure of Anther:
A typical anther is bilobed with each lobe having two theca, which means that they are dithecous.
In a cross-section, the anther is a four-sided tetragonal structure consisting of four microsporangia located at the corners, two in each lobe. Later, the microsporangia develops and become pollen sacs, which are packed with the pollen grains.
Structure of a Microsporangium:
A typical microsporangium is surrounded by four wall layers, i.e., the epidermis, endothecium, middle layers and the tapetum.
The outer three wall layers are protective in function and help in dehiscence of anther to release pollen grains.
Tapetum nourishes the developing microspores or pollen grain.
When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupies the center of each microsporangium.
Micro-sporogenesis (Development of Pollen Grains):
Each cell of the sporogenous tissue is a potential Pollen Mother Cell (PMC) and can give rise to microspore tetrad/pollen grains. The process of formation of microspore from a pollen mother cell through the process of meiosis is called micro-sporogenesis.
As the anther matures and dehydrates, the microspores dissociate from one another from tetrad and develop into pollen grains. Inside each microsporangium, thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.
In general, dehiscence of anther occurs through the rupture of anther lobe walls. But in family-Berberidaceae, anthers dehisce by the means of a valve or lid, through the pores. In plants like Solanum and Cassia. Irregular breaking of anther wall takes place in Najas.
Microspores and Pollen Grains:
Microspores are haploid, uninucleate and minute spores produced in large numbers by meiosis in the microspore mother cell.
Pollen grains are generally spherical measuring about 25-50 micrometers in diameter and has two layered wall; outer hard layer exine is made up of sporopollenin, which is resistant to any organic material and inner intine layer is chiefly composed of cellulose and pectin. A pollen grain is partly germinated microspore representing the male gametophyte. Each mature pollen grain in angiosperms has 2 or sometimes 3 cells; generative cell and vegetative cell.
i. The vegetative cell is larger, has abundant reserve food material and a large nucleus which is irregular in shape. Whereas, the generative cell is small, spindle-shaped and floats in the cytoplasm of the vegetative cell.
ii. Pollen grain are generally shed at this 2-celled stage in over 60% of angiosperms. While, in the remaining species, the generative cell divides mitotically and give rise to two male gametes before pollen grains are shed (3-celled stage).
iii. Pollen grains has primary distal aperture for the germination called as germ pore.
iv. Pollen grains are rich in nutrients. In Western countries, large number of pollen products in the form of tablets and syrups are used to increase the performance of athletes and derby horses.
v. Viability of pollen depends on temperature and humidity varies from 30 minutes as incase of wheat, rice and to several months as in some members of Solanaceae, Leguminosae, etc.
vi. Once pollen grain are shed, they have to land on the stigma before they loose viability if they have to bring about fertilisation.
vii. It is possible to store pollen grain of a large number of species for years in liquid nitrogen (-196°C) and such stored pollen grains can be used as pollen banks in crop breeding programmes.
viii. Pollen grains sometimes cause severe allergies and bronchial infections leading to asthma, bronchitis, etc., e.g., Parthenium.
Female Gametophyte:
(Pistil, Mega-sporangium (Ovule) and Embryo Sac):
Female gametophyte is also known as embryo sac. A typical pistil consists of a hollow basal swollen ovary, an elongated style and a terminal stigma. It may consist of one carpel (monocarpellary) or more than one carpel, two carpels (bicarpillary), three carpels (tricarpillary) or many carpels (multicarpillary).
The stigma is the receptive spot of the carpel where the pollen grains get lodged during pollination. The ovary is an important part of the carpel, which bears ovules. Gynoecium/pistil may be syncarpous (i.e., more than one pistil are fused together) when the number of pistils is more than one as in Papaver, Solarium, etc., or may be apocarpous (carpels remain free) as in Michelia, rose, etc.
Structure of Mega-sporangium (Ovule):
The ovule in a flower is an integumented mega-sporangium within which the meiosis and megaspore formation takes place, which develops into a seed after fertilisation. Each ovule usually consists of a nucleus invested by one or two integuments and a stalk called funiculus or funicle.
The junction between an ovule and a funicle is called hilum, which later becomes a scar on the seed. Each ovule has one or two protective envelopes called integuments, which encircle the ovule except at a tip where a small opening called micropyle.
The basal part of an ovule just opposite to micropyle is called chalaza. Cells of the nucellus are rich in reserve food materials. There is generally a single embryo sac or female gametophyte located in the nucellus; it has developed from a megaspore.
Mega-sporogenesis (Development of an Ovule):
The formation of megaspores from Megaspore Mother Cell (MMC) is called mega-sporogenesis.
It occurs inside the nucellus of developing ovule of angiosperms. The process begins very early when nucellus is not completely surrounded by the integuments.
MMC enlarges in size and divides by the meiosis to form a tetrad of four haploid megaspores. Out of these, soon three degenerates and only one megaspore becomes functional.
Development of Female Gametophyte:
In general, the development is monosporic, e.g., in Polygonum. In this type of development, only one megaspore situated towards chalazal end remains functional, while the remaining three megaspores gradually degenerate and finally disappear.
Following are the different stages of development of female gametophyte:
(i) The functional haploid megaspore is the first cell of female gametophyte of angiosperm.
(ii) It enlarges in size to form the female gametophyte, also called embryo sac.
(iii) Its nucleus undergoes a mitotic division and the two nuclei move to the opposite poles, forming the 2-nucleate embryo sac.
(iv) The 2-nucleate embryo sac undergo second mitotic division giving rise to the 4-nucleate stage.
(v) The third mitotic division gives rise to 8-nucleate embryo sac, which comprises of a micropylar end and a chalazal end with four nuclei at each end.
(vi) Out of four nuclei at the micropylar pole, 3 differentiates to produce an egg apparatus consisting of 2 synergids and a female gamete egg cell.
(vii) Similarly, at the chalazal end, 3 out of 4 nuclei are grouped together and are surrounded by cytoplasm and cellular wall differentiate as antipodal cells.
(viii) The remaining nuclei, one at the micropylar end and one at the chalazal end termed as polar nuclei migrate towards the center of the embryo sac (now called central cell). They meet at the center and may remain separate until the discharge of male gametes take place and then fuse to form secondary nucleus of central cell.
(ix) Thus, a typical angiosperm embryo sac is 8-nucleate and 7-celled.