Some embryological stages of embryo are:
Stage # 1. T.S. Young (Developing) Anther:
1. It is a multicellular, four-cornered structure, surrounded by a layer of epidermis.
2. In each comer develops one or more archesporial initials.
3. These initials divide by a periclinal wall into outer primary parietal cell and inner primary sporogenous cell.
4. Primary parietal cell divides periclinally as well as anticlinally and form 3 to 5 concentric layers of cells.
5. Innermost wall layer is called tapetum which is nutritive in function.
6. From the sporogenous tissue develop the pollen grains.
7. Some cells form the procambial strand in the centre of die anther.
Stage # 2. T. S. Anther Showing Four Mature Pollen Sacs:
1. It is a four-cornered structure containing a pollen sac. (Fig. 56).
2. Anther is surrounded by a layer of epidermis throughout.
3. Each pollen sac is surrounded by epidermis, an endothecial layer, one to three middle layers or wall layers and innermost layer of tapetum.
4. In each pollen sac or chamber are present many pollen tetrads which on separation form microspores.
5. A joint in the form of connective is present in the centre.
Stage # 3. T. S. Mature Anther Showing Dehiscence:
1. Four-cornered, four-chambered, multicellular body surrounded by a layer of epidermis.
2. Partition wall between the two pollen sacs is dissolved (Fig. 57).
3. Many pollen grains or microspores are present in the pollen sacs in the form of fine, powdery or granular mass.
4. Endothecium, middle layers and tapetal layers are present below the epidermis.
5. Along the line of dehiscence of each lobe, thin walled cells of endothecium form the stomium.
6. A connective is very clear.
Stage # 4. Pollen Tetrads:
(A) Isobilateral Tetrad:
All the four spores are formed in one plane because the spindles of first and second meiotic division remain at right angle to one another, e.g., Zea mays.
(B) Decussate Tetrad:
Out of the two lower spores, only one is visible. Both the upper ones are clear; e.g., Magnolia.
(C) T-shaped Tetrad:
In meiosis II upper cell divides to form two cells present side by side and the lower cell forms two cells lying one above the other; e.g., Arislolochia.
(D) Linear Tetrad:
All the four spores are present one above the other in a linear fashion; e.g., Halophila.
(E) Compound Pollen Grain:
Sometimes microspore tetrads adhere to each other and form the compound pollen grain; e.g., Typha, Cryptostegia.
Pollen grains of a pollen sac sometimes remain together to form a single mass called pollinium. Each pollinium consists of carpusculum, caudicle and pollinia; e.g., Asclepiadaceae.
Stage # 5. Pollen Grain:
1. It is a unicellular, uninucleate structure (Fig. 59). But pollen grains are always 2- or 3 nucleate when shed.
2. It is surrounded by a double-layered wall, i.e., outer exine and inner intine.
3. Exine is thick, cutinized, pigmented, sculptured and perforated by germ pores.
4. Intine is thin, colorless, smooth and consists of cellulose.
5. In the cytoplasm are present water, protein, fats, carbohydrates, etc.
Stage # 6. Various Types of Ovules (Fig. 60):
(A) Orthotropous (Ortho, straight; tropous, turned):
When micropyle, chalaza and funicle lie in one straight line; e.g., Polygonaceae, Urticaceae.
(B) Anatropous (Ana, backwards; tropous, turned):
Here, the body of the ovule turns backwards by an angle of 180° and so the micropyle becomes close to the hylum and placenta; Sympetalae.
(C) Hemitropous (Hemi, half; tropous, turned):
Here the body of the ovule is placed transversely or somewhat at right angle to funicle. Chalaza and micropyle are present here in one straight line; e.g.; Ranunculus.
(D) Campylotropous. (Kampylos, curved):
Here the body of the ovule is curved so that the chalaza and the micropyle do not lie in the same straight line; e.g., Leguminosae.
Here the curvature of ovule is more pronounced and embryo sac becomes horse-shoe shaped; e.g., Butomaceae.
Here the funicle is very long and the ovule rotates by an angle of 360° in such a fashion that it is completely circled around by the funicle. Micropyle faces upward; e.g., Cactaceae.
Stage # 7. L. S. Anatropous Ovule (Fig. 61):
1. It is attached to the placenta with a stalk called funicle.
2. The point of attachment of funicle with the body of the ovule is known as hilum which extends above in the form of a ridge i.e., raphe.
3. Nucellus consists of parenchymatous cells.
4. Nucellus remains covered by one or two coverings called integuments.
5. Integuments remain disconnected at one point forming a passage called micropyle.
6. Embryo sac consists of three antipodals, two synergids, one egg cell and one secondary nucleus.
7. Antipodals are located near the chalaza end, and the egg cell and synergids towards the micropylar end.
Stage # 8. Archesporial Initial (Fig. 62):
1. It is hypodermal in origin.
2. Archesporial initial is bigger than that of its surrounding cells.
3. A conspicuous nucleus and dense cytoplasm is present in it.
4. In its later stages, it divides into two cells forming an outer parietal cell which form the parietal tissue and inner megaspore mother cell.
Stage # 9. Two-celled Stage of Megaspore Mother Cell:
1. Two cell are present one above the other.
2. These are formed after reduction division and so each cell contains haploid set of chromosomes.
3. From these two cells, tetrad is formed.
Stage # 10. Linear Tetrad of Megaspores:
1. Four megaspores are arranged in linear fashion.
2. These are haploid in nature.
3. Out of the four, only one remains functional which is near the chalazal end. Remaining three degenerate (Fig. 64).
4. Functional megaspore is the First cell of the female gametophyte and develops into the embryo sac.
Stage # 11. Ovule with Binucleate Embryo-Sac:
1. Two nuclei are present in the embryo sac.
2. These two nuclei are formed by the division of the nucleus of the functional megaspore.
3. After some time two nuclei are separated by a large vacuole and they reach at the corners.
Stage # 12. Ovule with 4-Nucleate Embryo-Sac:
1. Four nuclei are present in the embryo sac (Fig. 66).
2. Out of the four, two nuclei are present near the chalazal end and the rest two near the micropylar end.
3. In the centre is present a large central vacuole.
4. Traces of degenerated megaspores are also seen at the micropylar end.
Stage # 13. OvuIe with 8 – Nucleate, Polygonum type Embryo-sac:
1. Near the micropylar end is present the egg apparatus.
2. Egg apparatus consists of an egg and two synergids.
3. Near the chalazal end are present three antipodals (Fig. 67).
4. In the centre are present two polar nuclei which ultimately fuse and form a secondary nucleus.
5. Many small vacuoles are present throughout.
Stage # 14. Endosperm:
1. Endosperm is formed because of the fusion of two polar nuclei and one of the male gametes.
2. It has triploid number of chromosomes.
3. It is of following three different types (Fig. 68).
Different kinds of endosperm:
(A) Nuclear Type:
Endosperm nucleus divides many times thus forming many free nuclei which in the later stages may be separated by walls.
(B) Cellular Type:
In this type all the nuclear divisions are accompanied by wall formation.
(C) Helobial Type:
In this type, first the nuclear divisions are accompanied by wall formation but later on there is no wall formation and nuclei remain free. So it is an intermediate stage between nuclear and cellular.
Stage # 15. Monocot Embryo:
1. Only one cotyledon is present (Fig. 69).
2. Plumule forms the stem and radicle forms the root.
3. Hypocotyle and a small suspensor are also present.
Stage # 16. Dicot Embryo:
1. Two large cotyledones are present
2. Both the cotyledones covet a small stem apex.
3. Suspensor is swollen.
4. Near the suspensor is present the root cap.
5. Central region forms the procambium which is present in between root cap and stem apex (Fig. 70).