(a) What are the different means of pollination in plants? Which natural devices ensure cross-pollination? (20 marks) (b) Using…

(a) Means of pollination and devices for cross-pollination

Pollination is the transfer of pollen from anther to stigma.

Means of pollination:

• Abiotic: anemophily (wind), with light, dry pollen and feathery stigmas, as in maize and sugarcane; hydrophily (water), as in Vallisneria (surface) and Zostera (submerged).
• Biotic: entomophily (insects), as bees on Brassica and Citrus and beetles on Magnolia; ornithophily (birds), as sunbirds on Erythrina and Callistemon; chiropterophily (bats), as on Kigelia and Adansonia; malacophily (snails) is rare.

Devices for cross-pollination:

• Unisexuality: dioecy, as in papaya, or monoecy, as in maize.
• Dichogamy: male and female parts mature at different times. Protandry, anthers first, as in sunflower and Salvia; protogyny, stigma first, as in Aristolochia and Annona.
• Herkogamy: a physical barrier or distance between anther and stigma, as in Calotropis.
• Heterostyly: flowers of different style lengths, as in Primula (dimorphic) and Lythrum (trimorphic).
• Self-incompatibility: genetic rejection of own pollen, sporophytic (arrest on the stigma, as in Brassica) or gametophytic (arrest in the style, as in Nicotiana).

(b) Magnoliaceae: features and primitiveness

Features: Trees or shrubs with aromatic oil cells and alternate simple leaves. Stipules enclose the bud and leave a ring scar (annular node). Flowers are large, solitary, bisexual and actinomorphic, with hypogynous insertion. Perianth is of undifferentiated tepals in whorls of three. Stamens are numerous, spiral, free and often laminar; carpels numerous, free (apocarpous) and spirally arranged on an elongated conical receptacle. Fruit is an etaerio of follicles, or samaras in Liriodendron. Seeds have a small embryo and copious endosperm, with a red fleshy sarcotesta in Magnolia.

Primitive characters: woody habit; undifferentiated tepals; numerous free spiral floral parts on an elongated axis; laminar stamens without a clear filament; apocarpous conduplicate carpels; scalariform vessel perforations and tracheid-like xylem; beetle pollination; and early fossils (Archaeanthus, Cretaceous).

(c) Groundnut and soybean: growth conditions and oil composition

Groundnut (Arachis hypogaea): a warm-season crop needing 25–30°C, about 50–100 cm rainfall, and well-drained, loose sandy loam of pH 6–7, rich in calcium in the pod zone. After fertilisation the gynophore (peg) bends down into the soil, so pods develop underground (geocarpy).

Soybean (Glycine max): a warm-season short-day crop needing 20–30°C, about 60–100 cm rainfall, and well-drained loam of pH 6–7. It is sensitive to waterlogging and photoperiod. Both fix nitrogen with root-nodule bacteria (Bradyrhizobium).

Oil composition: Groundnut seeds hold about 45–50% oil and 25% protein; the oil is about 40–55% oleic and 25–35% linoleic acid, with a little palmitic acid, so it is relatively stable for frying. Soybean seeds hold about 18–20% oil and 36–40% protein. Its oil is about 50–55% linoleic, 20–25% oleic and about 7–8% linolenic acid, so it oxidises more readily.