With a neat, labelled diagram, describe the parts of a mature angiosperm embryo sac. Mention the role of synergids.
After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
Six of the eight nuclei are surrounded by cell walls and organised into cells. Three cells present towards the micropylar end grouped together, constitute the egg apparatus. The egg apparatus, in turn consists of two synergids and one egg cell.
Three cells of the chalazal end are called the antipodals. The large central cell is formed by the fusion of 2-polar nuclei. Thus, a typical angiospermic embryo sac, at maturity consists of eight nuclei and seven cells. This embryo sac is formed from, the single megaspore, so it is called 'monosporic embryo sac.
Role of Synergids
The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid.
Draw the diagram of a microsporangium and label its wall layers. Write briefly about the wall layers?
$$ \text { The diagram representation of a microsporangium is shown below } $$
(a) Transverse section of a young anther (b) Enlarged view of one microsporangium showing wall layers (c) Mature dehisced anther showing pollen grain
In a transverse section, a typical microsporangium is circular in outline and is surrounded by four wall layers.
(a) Epidermis The epidermis is the outermost protective layer. It is composed of tangentially flattened cells. The cells are closely fitted and have thick walls which is helpful in the dehiscence of anther.
(b) Endothecium It is present below the epidermis and expands radically with fibrous thickenings, at maturity these cells loose water, at contract and help in dehiscence of pollen sac.
(c) Wall Layers It is present between well marked endothecium and tapetum. These are thin walled layers, arranged in one to five layers, which also help in dehiscence of anther.
(d) Tapetum It is the innermost wall layer with large cells, thin cell walls, abundant cytoplasm and have more than one nuclei. Tapetum is a nutritive tissue which nourishes the developing pollen grains.
The centre of the microsporangium consists of sporogenous tissue, which undergoes meiotic divisions to form microspore tetrads. This process is known as microsporogenesis.
Embryo sacs of some apomictic species appear normal, but contain diploid cells. Suggest a suitable explanation for the condition.
Replacement of the normal sexual reproduction by asexual reproduction without fertilisation is called apomixis. e.g., replacement of the flower by bulbils and replacement of the seed by a plant.
Apomitically produced offsprings are genetically identical to the parent plant. In flowering plants, apomixis is used in a restricted sense to mean angiosperm, i.e., asexual reproduction through seeds.
In some plant species it is common, e.g., Asteraceae, Poaceae. In some species, the diploid egg cell is formed without reduction division and develops into embryo without fertilisation. It is an asexual reproduction in the absence of pollinators such as in extreme environments.
In some species like citrus, some of the nucellar cells surrounding the embryo sac start dividing and develop into embryo. It occurs in the megaspore mother cell does not undergo meiosis, thus produces diploid embryo sac through mitotic divisions.
Thus, it explains that, embryo sacs of some apomictic species appear normal, but produce diploid cells.