Embryology (Botany): asexual and sexual reproduction in angiosperms, gametophytes, embryo, endosperm

a. Identification and labelling

The given figure is a germinating pollen grain (pollen tube) — the male gametophyte of an angiosperm, drawn after the pollen grain has landed on a compatible stigma.

• A — Pollen tube (the elongated tube growing out of the pollen grain, carrying the male gametes to the ovule).
• B — Generative cell / male gametes (the cell inside the pollen tube that divides to form the two male gametes; in the figure it lies just behind the tube tip).

b. Development of the male gametophyte from a microspore

The microspore (pollen grain) is the first cell of the male gametophyte. Its development proceeds as follows:

1. Microspore (uninucleate stage): The microspore just released from the tetrad is a single haploid cell with a prominent nucleus and dense cytoplasm. It is surrounded by a two-layered wall — the outer exine (thick, sculptured, made of sporopollenin) and the inner intine (thin, made of cellulose and pectin).

2. First mitotic division (unequal): The nucleus of the microspore divides mitotically to give two unequal cells:

   • a large vegetative cell (tube cell) with abundant food reserve and a large nucleus, and
   • a small generative cell (B) that lies at one side, often lens-shaped, with little cytoplasm.

   This two-celled stage is the mature pollen grain that is shed at anthesis in most angiosperms.

3. Formation of the pollen tube: After pollination, the pollen grain germinates on the stigma. The intine grows out through a germ pore as a tubular extension — the pollen tube (A). The tube secretes enzymes (pectinases, cellulases) that digest the stigma and style tissues, and it grows down the style towards the ovule.

4. Second mitotic division: The generative cell (B) migrates into the pollen tube and divides mitotically to form two male gametes (sperm cells). The vegetative nucleus usually degenerates. Thus the mature male gametophyte is three-celled (one tube cell + two male gametes).

5. Delivery of gametes: The pollen tube enters the ovule through the micropyle (porogamy) and discharges the two male gametes into the embryo sac — one fuses with the egg (syngamy) and the other with the secondary nucleus (triple fusion), completing double fertilisation.

Labelled diagrams (described):

• Stage 1 — Microspore: a rounded cell showing outer exine, inner intine, and a single central nucleus.
• Stage 2 — Two-celled pollen grain: the same grain with a large vegetative cell and a small generative cell (B) at one pole.
• Stage 3 — Germinating pollen grain: the intine bulging out as a pollen tube (A); the generative cell (B) has moved into the tube.
• Stage 4 — Mature male gametophyte: the pollen tube containing the tube nucleus and two male gametes at its tip.

(d) Porogamy

Porogamy is the entry of the pollen tube into the embryo sac through the micropyle. Mesogamy is entry through the integuments, chalazogamy through the chalaza, and syngamy is the fusion of a male gamete with the egg nucleus.

(a) The type of propagation is layering, specifically air layering (gootee). P is the girdled part of the branch wrapped in moist moss or soil and covered with polythene; Q is the mass of adventitious roots that develops from the girdled region while the branch is still attached to the parent plant.

(b) In air layering, a ring of bark is removed (girdled) from a healthy branch that is still attached to the parent plant. The exposed area is covered with moist sphagnum moss or soil and wrapped in polythene to retain moisture. After a few weeks adventitious roots grow from the girdled part. The branch is then cut below the roots and planted as a new, independent plant. Example: litchi, guava, mango, lemon and pomegranate.

(c) Applications:

1. It gives plants identical to the parent (clones), preserving desirable characters.
2. It propagates plants whose seeds are non-viable or slow to germinate, and the new plants flower and fruit early.

(c) The male and female gametes fuse to form a zygote.

In fertilization, the male gamete (from the pollen tube) fuses with the female gamete (egg cell) inside the embryo sac, forming a diploid zygote. Option (a) describes pollination, (b) describes post-fertilization change of the ovule into a seed (the ovary becomes the fruit), and (d) describes microsporogenesis in the anther.

(a) Labelling (1 mark)

• A: Pollen tube, which carries the two male gametes into the embryo sac.
• B: Embryo sac (female gametophyte), containing the egg apparatus, the polar nuclei and the antipodal cells.

(b) Double fertilization and triple fusion (2 marks)

The pollen tube grows through the style and enters the ovule through the micropyle. It pierces one synergid (guided by the filiform apparatus) and releases its two male gametes into the embryo sac.

1. Syngamy: one male gamete fuses with the egg cell to form the diploid zygote (2n).
2. Triple fusion: the other male gamete fuses with the two polar nuclei (often already fused as the secondary nucleus) to form the triploid primary endosperm nucleus (3n). Three haploid nuclei take part, hence "triple fusion".

Since two fusions take place in the same embryo sac, the event is called double fertilization; it is found only in angiosperms.

(c) Role (1 mark)

Syngamy forms the zygote, which develops into the embryo. Triple fusion forms the primary endosperm nucleus, which develops into the endosperm that nourishes the embryo. Together they ensure the endosperm develops only after fertilization, so no food is wasted on an unfertilized ovule, and embryo and food store develop together.

The pollen grain is produced in the anther (specifically in the pollen sacs of the anther) during microsporogenesis, when the diploid microspore mother cell undergoes meiosis to form haploid microspores. This occurs at the time of maturation of the anther, i.e., just before or at the time of anthesis (flower opening), when the flower is ready for pollination.

(ii)

• A — Exine (the outer, thick, sculptured layer of the pollen wall, made of sporopollenin).
• C — Generative cell (the smaller cell inside the pollen grain that later divides to form the two male gametes).

(iii) In angiosperms the pollen tube delivers two male gametes into the embryo sac, and this is because of the phenomenon of double fertilization, which is unique to angiosperms:

1. One male gamete fuses with the egg cell (syngamy) to form the diploid zygote, which develops into the embryo.
2. The second male gamete fuses with the two polar nuclei (or the secondary nucleus) of the central cell (triple fusion) to form the triploid primary endosperm nucleus (PEN), which develops into the nutritive endosperm tissue.

Thus the second gamete is not "extra" — it is essential for forming the endosperm that nourishes the developing embryo. A single gamete could fertilize the egg, but then no endosperm would be formed and the embryo would lack its food reserve. Hence two male gametes are carried so that both fertilization events occur, ensuring proper seed development.

Practical importance of seed dormancy in agriculture and horticulture

1. Safe storage and transport. Dormant seeds are metabolically inactive, so they can be stored for long periods and carried over long distances without germinating or spoiling; seed banks and seed companies depend on this.
2. Survival through unfavourable seasons. Dormancy prevents germination during drought, severe cold or waterlogging, so seeds germinate only when conditions suit the seedling and the crop stand is not lost.
3. Prevents pre-harvest sprouting. Dormancy stops grains germinating on the mother plant (vivipary) in the ear or fruit during wet weather, saving the harvest of crops such as wheat, rice and maize.
4. Spread of germination. Not all seeds germinate at once, so if one batch of seedlings is destroyed by frost or pests, the dormant seeds act as a reserve that germinates later.
5. Planning of sowing in horticulture. Dormancy can be broken artificially (scarification, stratification, soaking, growth regulators such as gibberellin) so that nurseries and growers can time germination and planting to market demand.

(Any four points.)

(b) Autogamy

Pollination between the anther and stigma of the same flower is autogamy (self-pollination within the same flower). Geitonogamy is pollination between two flowers of the same plant, allogamy is cross-pollination between different plants, and xenogamy is cross-pollination between flowers of different plants.

Primary function of endosperm: The endosperm is the nutritive tissue of the seed. It provides nourishment (stored food in the form of starch, protein and oils) to the developing embryo during seed formation and to the germinating seedling until it becomes independent and can photosynthesise.

Types of endosperm: Based on how the endosperm develops, it is of three types:

1. Nuclear endosperm — The primary endosperm nucleus divides repeatedly by free nuclear division without wall formation, so a multinucleate mass of cytoplasm is formed. Wall formation occurs later, and the tissue may remain partly cellular. Example: coconut (the liquid coconut water is nuclear endosperm), maize, wheat, rice.

2. Cellular endosperm — Every nuclear division is followed immediately by wall formation (cytokinesis), so the endosperm is cellular from the beginning. Example: coconut (the white kernel), Balsam, Petunia.

3. Helobial endosperm — The first division of the primary endosperm nucleus is followed by wall formation, dividing the embryo sac into a large micropylar chamber and a small chalazal chamber. The micropylar chamber then divides freely (nuclear type) while the chalazal chamber usually remains uninucleate. Example: members of Helobiae (e.g. Alisma, Sagittaria), some monocots.

Note: In many seeds (e.g. pea, groundnut) the endosperm is completely consumed by the developing embryo, giving non-endospermic (exalbuminous) seeds; where it persists, the seeds are endospermic (albuminous).

(c) Soil layering

In soil layering a low, flexible branch is bent down and part of it is buried in soil while the tip stays above ground. Roots form at the buried part while it is still attached to the parent plant, and the rooted branch is then cut off. Air layering is done on stiff, aerial branches, grafting joins a scion to a stock, and a cutting is separated from the parent before rooting.

The female gametophyte (embryo sac) develops inside the ovule by the monosporic, Polygonum type of development. It has two phases: megasporogenesis and megagametogenesis.

Phase 1: Megasporogenesis (formation of megaspores)

• A single cell of the nucellus near the micropyle becomes the megaspore mother cell (diploid, $2n$).
• It undergoes meiosis to form four haploid megaspores ($n$) in a linear row.
• Three megaspores towards the micropyle degenerate; the one at the chalazal end remains functional.

Phase 2: Megagametogenesis (development of the embryo sac)

1. The functional megaspore enlarges and its nucleus divides by mitosis to give two nuclei, which move to opposite poles.
2. Each nucleus divides twice more, giving four nuclei at each pole, a total of eight nuclei.
3. One nucleus from each pole moves to the centre; these are the two polar nuclei, which may fuse to form the secondary nucleus.
4. Cell walls form around the rest:
   • At the micropylar end: egg apparatus (one egg cell and two synergids).
   • At the chalazal end: three antipodal cells.
   • The large central cell contains the two polar nuclei.

The mature embryo sac is therefore 7-celled and 8-nucleate.

Diagram: draw one sketch for each stage: megaspore mother cell, tetrad (3 degenerating, 1 functional), 2-, 4- and 8-nucleate stages, and the mature embryo sac labelled with the egg, synergids, polar nuclei and antipodals.

(c) Tube nucleus and sperm

The pollen grain has a tube nucleus and a generative cell. The generative cell divides to form two male gametes (sperm nuclei). At the tip of the growing pollen tube, the tube nucleus and the two sperm nuclei are present, so "tube nucleus and sperm" is the best option.

Diagram: L.S. of a pistil showing pollen germination

Draw the pistil with its three parts and label: stigma (with a pollen grain on it), style (with the pollen tube growing down through it), ovary wall, and an ovule inside the ovary showing the micropyle, integuments, nucellus and the embryo sac. Show the pollen tube growing from the stigma, through the style and ovary, and entering the ovule at the micropyle.

Events in the embryo sac during fertilization (double fertilization)

The mature embryo sac is 7-celled and 8-nucleate: egg apparatus (one egg and two synergids) at the micropylar end, three antipodals at the chalazal end, and a large central cell with two polar nuclei.

1. The pollen tube enters through the micropyle (porogamy) and penetrates one synergid, which degenerates and the tube bursts to release two male gametes.
2. Syngamy: one male gamete (n) fuses with the egg (n) to form the diploid zygote (2n), which develops into the embryo.
3. Triple fusion: the second male gamete (n) fuses with the two polar nuclei (n + n) of the central cell to form the triploid primary endosperm nucleus (3n), which develops into the endosperm.
4. The synergids and antipodals degenerate afterwards.

Since two fusions take place, the process is called double fertilization.

(c) Polar nuclei

In double fertilization one male gamete fuses with the egg to form the zygote, and the second fuses with the two polar nuclei of the central cell (triple fusion) to form the triploid primary endosperm nucleus.

Development of a monocot embryo (e.g. maize, grasses)

1. The zygote divides transversely into an upper suspensor cell and a lower terminal cell.
2. The terminal cell divides to form a proembryo (globular stage) and the suspensor anchors it.
3. Differentiation into a single cotyledon, which becomes the scutellum lying to one side, and the plumule and radicle lying at the other end.
4. The plumule is covered by a protective sheath, the coleoptile, and the radicle by the coleorhiza.

Salient features

• Only one cotyledon (scutellum), which absorbs food from the endosperm and passes it to the embryo.
• Embryonal axis has the plumule with coleoptile and radicle with coleorhiza.
• The embryo lies in a groove at one end of the endospermic seed.

Diagram: draw a longitudinal section of a maize grain embryo and label scutellum, plumule, coleoptile, radicle, coleorhiza and endosperm.

(b) Parthenogenesis

Parthenogenesis is the development of an embryo from an unfertilized egg without fusion with a sperm (e.g. in honey bee drones). Parthenocarpy is fruit formation without fertilization, polyembryogeny is the formation of many embryos from one zygote or ovule, and polycarpy means fruiting many times in a life.