(a) Describe the androecium of Orchidaceae and Asclepiadaceae. Highlight the differences between them. (10 marks) (b) Discuss…

(a) Androecium of Orchidaceae and Asclepiadaceae, and the differences (10 marks)

Description (8 marks):

• Orchidaceae: one fertile stamen in most orchids (monandrous), two in slipper orchids, the rest being staminodes. The stamen is fused with the style and stigma into a column, the gynostemium. Pollen is bound into masses, the pollinia (powdery or sectile), each with a stalk (caudicle) ending in a sticky disc (viscidium) that sticks to an insect.
• Asclepiadaceae (now Apocynaceae, subfamily Asclepiadoideae): five stamens, with filaments united into a tube, and anthers adhering to the stigmatic head to form a gynostegium, with a corona of five hoods on the tube. Each anther lobe has a waxy pollinium, and pollinia of adjacent anthers are joined by two arms to a sticky gland (corpusculum), the translator.

Differences (2 marks):

• Stamens: one or two in orchids and five in Asclepiadaceae.
• Pollinia: powdery or sectile with a viscidium in orchids, and waxy with a translator in Asclepiadaceae.
• Orchids have a gynostemium (column) and no corona; Asclepiadaceae have a gynostegium with a corona.

(b) Types of polyembryony and its importance (10 marks)

Polyembryony is the occurrence of more than one embryo in a seed. Its types (8 marks):

• Cleavage polyembryony: the zygote or young embryo splits into several embryos, as in Pinus and other gymnosperms and in some angiosperms such as orchids.
• Adventive (nucellar or integumentary) embryony: embryos arise directly from diploid nucellus or integument cells, without fertilisation, and grow beside the zygotic embryo, as in Citrus and mango.
• Multiple embryo sacs: more than one embryo sac in an ovule, each producing an embryo, as in Casuarina and some Rosaceae.
• Other cells of the embryo sac: synergids or antipodals develop into embryos, as reported in Argemone and Ulmus.
• Simple polyembryony in gymnosperms: several archegonia in an ovule are fertilised.

Importance (2 marks):

• Nucellar seedlings are genetically identical to the mother, so they give uniform, true-to-type and often virus-free rootstocks in Citrus and mango.
• Twin seedlings are a source of haploids and polyploids in breeding, but polyembryony makes it hard to recover hybrids.

(c) Botanical names and families (10 marks)

• (i) Pearl millet (bajra): Pennisetum glaucum (accepted name now Cenchrus americanus), family Poaceae.
• (ii) Flax (sun or alsi): Linum usitatissimum, family Linaceae.
• (iii) Pomegranate (anar): Punica granatum, family Lythraceae (formerly Punicaceae).
• (iv) Bottle gourd (lauki): Lagenaria siceraria, family Cucurbitaceae.
• (v) Chickpea (chana): Cicer arietinum, family Fabaceae (Leguminosae).

(d) Cell suspension culture and its significance (10 marks)

Cell suspension culture is the growth of single cells and small cell aggregates dispersed in a liquid medium.

Procedure (8 marks):

• A friable callus is raised on solid MS medium with 2,4-D and a cytokinin.
• About 2 to 3 g of callus goes into 50 to 100 ml of liquid medium in a conical flask.
• The flasks are shaken at 100 to 120 rpm at about 25 °C in dim light, to aerate and break up clumps.
• Large clumps are removed by nylon mesh, and the suspension is subcultured every 1 to 2 weeks at about 1 in 5 to 1 in 10 dilution.
• Growth is followed by cell count, packed cell volume and dry weight (lag, exponential, linear, stationary phases), and viability by fluorescein diacetate or Evans blue; cultures may be batch or continuous.

Significance (2 marks). It is a source of secondary metabolites (shikonin, berberine), of protoplasts and somatic embryos, and a system for selecting mutant or stress-tolerant cells and for biotransformation.

(e) History of plant domestication (10 marks)

Domestication is the genetic change of wild plants under human selection so that they suit cultivation. It began about 10,000 to 12,000 years ago at the end of the last Ice Age, when people shifted from gathering to farming. Vavilov identified centres of origin, such as the Fertile Crescent (wheat, barley, peas, lentil, flax), Mesoamerica (maize from teosinte, beans, squash), the Andes (potato), China (rice, millets, soybean), and the Indian region (rice, mung, cotton, brinjal). Early farmers at sites such as Mehrgarh in the Indus region grew wheat and barley by about 7000 BCE.

Selection, first unconscious and then deliberate, produced the domestication syndrome: loss of seed shattering, larger seeds and fruits, reduced dormancy, uniform ripening, less branching and less bitterness or toxin. Hybridisation and polyploidy created new crops, such as bread wheat. Crops then spread along trade routes, and the Columbian exchange after 1492 moved potato, maize and tomato worldwide. Modern breeding after Mendel and the Green Revolution continued the process.