Male reproduction in flowering plants:-
An angiospermic plant is a sporophyte. It is heterosporus and reproduces asexually. The flower is the main structure concerned with reproduction. It consists of two whorls of non-essential organs (calyx and corolla) and two whorls of essential organs (androecium and gynoecium). The androecium consists of stamens, each of which is distinguishable into filament, connective and anther. The anther or the microsporangium forms microspore mother cells or pollen mother cells (PMC). The PMC undergoes meiosis to form the tetrads of microspores. The microspore, as a result of microgametogenesis, forms the male gametophyte or the pollen. The anthers dehisce releasing the pollen grains.
The gynoecium consists of carpels, each of which is distinguishable into ovary, style and stigma. The ovary is the ovule bearing region of the carpel whereas the stigma is the pollen receiving region. The ovule or the megasporangium consists of nucellus covered by integuments. From the nucellar cells a megaspore mother cell is differentiated which undergoes meiosis to form tetrads of megaspores. The megaspore forms the female gametophyte or embryo sac by megagametogenesis.
As a result of pollination the pollen is transferred from anther to stigma. The pollination may be self (autogamy) or cross (allogamy). There are many contrivances for autogamy and allogamy. There are several agencies through which this process is carried out, such as, by animals (zoophily) by wind (anemophily) or by water (hydrophily). The pollen germinates on the stigma forming a pollen tube. The tube carries two male gametes into the ovule (Siphonogamy). Thus, the pollination leads to the phenomenon of fertilization involving fusion of male gamete with the egg and triple fusion involving fusion of second male gamete with the secondary nucleus. The two phenomenons jointly constitute a larger phenomenon called as double fertilization which is exclusive to angiosperms. As a result of fertilization the zygote is formed. It gives rise to embryo by segmentation. The triple fusion results in the formation of primary endosperm nucleus which gives rise to endosperm. The endosperm is the nutritive tissue for the developing embryo. The development of the two continues and as a result, the ovule is converted into a seed. Simultaneously, the ovary wall forms the pericarp and the ovary is converted into a fruit. The seed germinates to form a new angiospermic plant.
Anther Structure :-An anther is the broader terminal knob-like fertile part of the stamen. It consists of two lobes (anther lobes) which are separated in the anterior region by a deep groove but are attached to each other on the back side by a sterile tissue called connective. Connective possesses a vascular strand. Each anther lobe contains two long and cylindrical pollen sacs or microsporangia.
Thus a typical anther is tetrasporangiate. The two pollen sacs of an anther lobe are separated from each other by a shallow groove on the outside and a strip of sterile tissue internally. The four pollen sacs appear to lie in the four corners of an anther. They are covered on the outside by a well defined common epidermis of the anther. The pollen sacs or microsporangia develop hypodermally in anther from strips of archesporial cells. The archesporial cells divide periclinally to form outer parietal cells and inner sporogenous cells. The parietal cells divide further by periclinal walls to produce a 3-5 layered microsporangial wall. The sporogenous cells divide and form microspore mother cells.
MICROSPORANGIUM :- In most of the angiosperms the anther is tetrasporangiate i.e., consisting of four vertically elongated sporangia joined by a connective. The connective is vascularised. Each sporangium is covered by four wall layers namely epidermis, endothecium, middle layers, and tapetum. The wall layers enclose a mass of sporogenous tissue. In Malvaceae, the anthers are bisporangiate, whereas in Arceuthobium they are monosporangiate. The single sporangium may be horse-shoe shaped or ring-like. Here the central sterile portion is called columella.
ANTHER DEVELOPMENT :- To begin with, the anther is a homogenous mass of parenchymatous tissue. Soon it becomes four-angled. Now, four vertical rows of hypodermal archesporial cells are differentiated, one in each angle. Each archesporial cell undergoes a periclinal (tangential) division so as to form an outer primary parietal cell (PPC) and inner primary sporogenous cell (PSC). The primary parietal cell undergoes a few more periclinal divisions to form 3-5 layered anther wall. The primary sporogenous cell gives rise to the sporogenous tissue. The primary parietal cell, by a pericline forms two secondary parietal layers. Davis, 1966 on the basis of the behaviour of these layers differentiates four types of anther wall developments:
Types of anther wall development:-
A: - Basic type: It follows the pattern of development as given under :
PPC (primary parietal cell ) gives rise to outer secondary parietal layer & inner secondary parietal layer. The outer secondary parietal layer forms (i) Endothecium & (ii) Middle layer. The inner secondary parietal layer forms (i) Middle layer & (ii) Tapetum. e.g. Anacardiaceae (mango family).
B:- Dicotyledonous type: It follows the given pattern of development :
PPC (primary parietal cell ) gives rise to outer secondary parietal layer & inner secondary parietal layer. The outer secondary parietal layer forms (i) Endothecium & (ii) Middle layer. The inner secondary parietal layer forms (i) Tapetum. e.g., Acanthaceae, Malvaceae, Compositae, Solanaceae etc.
C:- Monocotyledonous type : The development of this type of anther wall takes place as under :
PPC (primary parietal cell ) gives rise to outer secondary parietal layer & inner secondary parietal layer. The outer secondary parietal layer forms (i) Endothecium.The inner secondary parietal layer forms (i) Middle layer & (ii) Tapetum .e.g. Cruciferae, Cyperaceae, Gramineae, Liliaceae.
D. Reduced type: This type of development occurs in the following manner:
PPC (primary parietal cell ) gives rise to outer secondary parietal layer & inner secondary parietal layer. The outer secondary parietal layer forms (i) Endothecium.The inner secondary parietal layer forms (i) Tapetum .(Thus, in these cases no middle layers are formed e.g., Lemnaceae, Najadaceae)
ANTHER WALL LAYERS
The hypodermal microsporangial wall consists of an outer layer of larger cells called endothecium, 1—3 middle layers and one layer of tapetum. In a typical anther the endothecial cells develop fibrous thickenings on the inner and radial walls and become dead. Because of the presence of fibrous thickenings, the endothecium is also called fibrous layer. In the shallow, groove present between the two microsporangia of an anther lobe the hypodermal cells lying at the level of endothecium remain thin walled. They constitute the stomium or line of dehiscence.
- Epidermis : It is 1 -cell thick outermost wall layer of the anther. It is protective in function. Thecells of epidermis may becomes binucleate in Zeuxine. In Arceuthobium, they develop a fibrousthickening. Such a layer is designated as exothecium. In Moms, the epidermis degenerates at maturity of the anther.
- Endothecium : It is the second wall layer of the anther. Usually it is 1 -cell thick but in Chelone itis 2-celIs thick. The endothecial cells develop a fibrous thickening before dehiscence of the anther.This thickening is made-up of α-cellulose with a little pectin and lignin in some cases. It appearsin the form of radial bands arising from the inner tangential wall. The endothecial thickening helpsin the dehiscence of the anther and dispersal of pollen grains due to its hygroscopic nature. However, no fibrous thickening has been observed in members of Hydrocharitaceae as also in cleistogamous flowers.
- Middle layers : It is the third wall layer of the anther. The number of middle layers generally ranges from 1-6 but rarely there are several middle layers. They are absent in the members of Lemnaceae and Najadaceae. The middle layers degenerate at maturity of the anther. They are nutritive in function and also participate in the formation of pollenkitt.
Tapetum : It is the innermost wall layer of the anther. Usually it is one cell thick. The tapetum developmentally has dual nature. While a part of it arises from the primary parietal cell (P-tapetum), a part develops from the cells of connective (C-tapetum).
The tapetum is of two types :
- Amoeboid or periplasmodial or invasive : The radial walls of tapetal cells break up releasingthe protoplast into the pollen chamber. All such protoplasts now fuse to form what is knownas periplasmodium.
- Secretory or glandular or parietal : Here the tapetal cells remain in situ all through thedevelopment of microspores and finally they degenerate.The tapetal cell prepare pro-ubisch bodies. They pass into the space between cell wall and plasma membrane. Here they get surrounded by sporopollenin. Now they are called ubisch bodies or orbicules. Due to destruction of tapetal cell wall, the ubisch bodies come to lie in the anther loculus. They are involved in the formation of a part of exine.
|Important notes by AKB sir:-
The tapetal cells, secretory or amoeboid show increase in DNA contents by mitosis, restitution nucleus formation, endomitosis or polyteny. The mitosis involves DNA replication, splitting of choromosomes and the nucleus through prophase, metaphase, anaphase and telophase. Sometimes, there is normal mitosis upto anaphase. Then the chromosomes at two poles get surrounded by a nuclear membrane so as to form a restitution nucleus. The endomitosis involves DNA replication and splitting of chromosomes through endoprophase, endometaphase, endoanaphase and endotelophase. There is no splitting of the nucleus. When DNA replication is not accompanied by splitting of chromosomes, polytenic chromosomes are formed. The tapetum is concerned with transportation of nutrient, contribution of sporopollenin, transport of pollenkitt substances and storage of reserve food which is used by the developing pollen grains.
|Important notes by AKB sir:-
The pollen grains of a tetrad grow and separate from one another. However, they remain attached forming compound pollen grains in Typha. In Calotropis and related plants all the pollen grains of an anther lobe remain united in a single sac called pollinium. Two pollinia of adjacent anthers are attached to produce a translator.
The primary sporogenous cells divide and re-divide miotically to form a mass of sporogenous tissue. The last mitosis results in the formation of microspore mother cells or pollen mother cells (PMC) which undergo meiosis. Thus, the meiotically dividing sporogenous cell is the pollen mother cell. As meiosis begins, the plasmodesmata in between the PMCs and tapetum are broken and the walls of PMCs become thicker due to deposition of callose. The plasmodesmatal connections in between PMCs are now replaced by massive channels. The PMCs of an anther locule thus form a meiotic syncytium. These channels help in maintaining synchrony in meiosis within an anther locule. The karyokinesis is followed by cytokinesis and the latter is of two types:
a.Successive : The PMC, as a result of heterotypic division, forms a dyad and then a tetrad. The wall formation is centrifugal e.g., monocots.
b.Simultaneous : The PMC, as a result of heterotypic division forms two nuclei i.e., no dyad is formed. The two nuclei then divide to form a tetrad e.g., dicots.
The tetrads are of five types namely:-
(i)tetrahedral (most common), (ii) isobilateral, (iii)T-shaped, (iv)decussate and (v)linear.
In Zostera marina, The microspores are as long as 2-55 mm. Compound pollen grains are found in Drimys, Drosera, etc. In Asclepiadaceae and Orchidaceae, the microspores of a sporangium remain together to form a pollinium. In Mimosaceae several such groups are found, each having more then four spores. Occurrence of more than four spores in a tetrad is called polyspory e.g., Cuscuta.
Pollen Grain:- It is commonly globular in outline, though several other shapes are also found. Its cytoplasm is rich in starch and unsaturated oils. The latter protect the chromosomes from radiation damage. Pollen grain protoplast is uninucleate in the beginning but at the time of liberation it becomes 2-3 celled. Wall or covering of pollen grain is called sporoderm. It has two layers, outer exine and inner intine. Intine is pecto-cellulosic in nature. At places it contains enzymatic proteins. Exine is made of a highly resistant fatty substance called sporopollenin. In addition it possesses proteins for enzymatic and compatability reactions.
Exine is differentiated into outer extexine and inner endexine. Extexine is further made up of a continuous foot layer, a discontinuous baculate layer and outermost discontinuous tectum.Tectum provides a characteristic sculpturing or designs over the surface of pollen grain. It can help experts to identify the pollen grains and refer them to their family, genus or species. The branch of study is called palynology.
|Important notes by AKB sir:-
In insect pollinated pollen grains the exine is covered over by a yellowish sticky and oily layer called pollenkitt. At certain places the exine is thin or absent. The areas may have thickened intine or deposition of callose. They are called germ pores (if rounded) or germinal furrows (if elongated). Pollen grains are tricolpate (with three germ pores) in dicots and monocolpate (with single germinal furrow) in monocots.