(a) Describe the development of female gametophyte in angiosperms with the help of suitable illustrations. (20 marks) (b) Give…

Development of Female Gametophyte

In angiosperms, the monosporic *Polygonum*-type development represents the most common pathway of female gametophyte (embryo sac) formation. The process comprises two distinct phases: megasporogenesis and megagametogenesis.

During megasporogenesis, a hypodermal archesporial cell in the ovule's nucellus differentiates into a diploid megaspore mother cell (MMC). The MMC undergoes meiosis, producing a linear tetrad of four unilocular haploid megaspores. Typically, the three micropylar megaspores degenerate, while the chalazal megaspore remains functional and enlarges.

During megagametogenesis, the functional megaspore undergoes three successive mitotic nuclear divisions without immediate cytokinesis. The first division forms a 2-nucleate stage, where nuclei migrate to opposite poles. The second and third divisions produce four nuclei at the micropylar end and four at the chalazal end (8-nucleate stage).

Cytokinesis then reorganizes the nuclei into a 7-celled, 8-nucleate structure:

• Micropylar end: An egg apparatus consisting of two synergids characterized by filiform apparatus (guiding pollen tube entry) and one central egg cell (female gamete).
• Chalazal end: Three antipodal cells involved in nutritional transfer.
• Center: A large central cell containing two polar nuclei, which typically fuse prior to fertilization to form a diploid secondary nucleus.

Schematic of Development: MMC (2n) → Meiosis → Linear Tetrad of Megaspores (n) → Functional Chalazal Megaspore → 3 Mitotic divisions → 8-Nucleate Coenocyte → Cellularization → Mature Embryo Sac [Egg Apparatus (3 cells) + Central Cell (2 polar nuclei) + Antipodals (3 cells)].

Ethnomedicinal Species of Fabaceae, Euphorbiaceae, and Liliaceae

Fabaceae:

1. Glycyrrhiza glabra: Dried roots and stolons are used ethnomedicinally as a demulcent and expectorant for respiratory tract infections, cough, and peptic ulcers.
2. Cassia fistula: The pulp of mature pods and bark are utilized as a safe laxative and for treating inflammatory skin diseases.
3. Butea monosperma: Dried seeds and gum are employed as potent anthelmintics against intestinal parasites and as an astringent in chronic diarrhea.

Euphorbiaceae:

1. Euphorbia hirta: The whole aerial plant is used in decoctions to treat bronchial asthma, cough, and amoebic dysentery.
2. Ricinus communis: Fixed oil extracted from seeds serves as a cathartic and purgative, while poultices of leaves relieve rheumatic joint pain and inflammation.
3. Phyllanthus niruri: The whole plant is administered widely for jaundice, hepatitis, and urolithiasis due to its hepatoprotective and diuretic actions.

Liliaceae:

1. Aloe vera: The mucilaginous parenchyma of leaves is used topically for burns, wound healing, and dermatitis, and orally for gastrointestinal soothing.
2. Asparagus racemosus: The tuberous roots are prized as a galactagogue, adaptogen, and restorative tonic in female reproductive disorders.
3. Gloriosa superba: Tuberous rhizomes (containing colchicine) are traditionally applied externally in paste form to alleviate acute gout, joint inflammation, and parasitic skin infections.

Secondary Metabolites and Chemotaxonomy

Secondary metabolites are organic compounds synthesized by plants via specialized pathways (e.g., shikimic acid, acetate-mevalonate, and malonate pathways) that are not directly involved in basic growth, development, or reproduction. Unlike ubiquitous primary metabolites, secondary metabolites (such as alkaloids, flavonoids, terpenoids, and glucosinolates) show restricted and specific distribution across plant taxa.

In taxonomy, secondary metabolites serve as valuable chemical characters (chemotaxonomy) to classify taxa, resolve phylogenetic ambiguities, and establish evolutionary lineages:

• Diagnostic markers: Specific classes characterize particular families, such as benzylisoquinoline alkaloids in Papaveraceae and Magnoliaceae, and betalains in Caryophyllales (which replace anthocyanins).
• Resolving affinities: The presence of mustard oil glucosides (glucosinolates) links Brassicaceae, Capparaceae, and Cleomaceae, demonstrating their close phylogenetic affinity.
• Species identification: Cardenolides (cardiac glycosides) in Apocynaceae and sesquiterpene lactones in Asteraceae provide micromorphological clarity at lower taxonomic ranks, confirming that secondary metabolic distribution reflects underlying genomic and enzymatic divergence.