Sexual Reproduction in Flowering Plants, practice questions

C, The stamen, the male whorl of a flower, consists of an anther and a filament.

B, Meiosis halves the chromosome number and produces four haploid microspores from one pollen mother cell.

B, The mature embryo sac is a seven-celled, eight-nucleate structure with an egg cell, two synergids, three antipodals and two polar nuclei.

C, Cross-pollination specifically requires two different plants of the same species; transfer within one flower or plant is self-pollination.

C, This fusion produces a triploid nucleus that develops into the endosperm, the food store for the embryo.

D, The zygote, formed by the fusion of a male gamete with the egg cell, develops into the embryo.

C, A large, feathery, exposed stigma helps a wind-pollinated flower catch drifting pollen grains from the air.

B, A monocotyledonous seed, such as maize, contains a single cotyledon, unlike the two cotyledons of a dicotyledonous seed.

D, Germination itself needs only water, oxygen and a suitable temperature; light is not required until the seedling begins photosynthesis.

C, Hooks or spines that catch onto fur are a structural adaptation for external animal dispersal.

• (a) The flower is likely pollinated by an insect.
• (b) The bright petals and scent, together with the nectar, attract the insect to visit the flower and pick up or deposit pollen.
• (b) The small, sticky, enclosed stigma is positioned to receive pollen directly from an insect's body as it brushes past.
• (c) The pollen is likely to be sticky or spiky, so that it clings to the insect's body during a visit.

• (a) One male gamete fuses with the egg cell to produce a diploid zygote, which develops into the embryo.
• (a) The other male gamete fuses with the two polar nuclei to produce a triploid nucleus, which develops into the endosperm.
• (b) It is unique to flowering plants because two separate fusion events occur from the same pollen tube in the same event, producing both an embryo and its own food store (the endosperm) at once.

• (a) The bean seed has two cotyledons, while the maize grain has only one.
• (b) In the bean, the two cotyledons absorb the endosperm's food before germination and become the main food store; in the maize grain, the endosperm itself remains as the food store.
• (c) Water is needed to soften the seed coat (testa) and activate the enzymes that break down the stored food.

The stamen, made up of the anther and filament, is the male reproductive part of a flower, while the carpel, made up of the stigma, style and ovary, is the female part; a single flower may contain one or more of each, and the number and arrangement of these parts is often used to identify a species. Some flowers, described as incomplete, lack one or more of these parts, such as petals or stamens, yet can still reproduce successfully.

Pollen grains, each containing a male gamete, form inside the anther, while the embryo sac, containing the female gamete, forms inside the ovule within the ovary; both are produced by meiosis, which creates genetic variation among the gametes even before fertilisation combines two parents' genes.

Pollination is the transfer of pollen grains from the anther to the stigma, either of the same flower (self-pollination) or of a different flower on a different plant of the same species (cross-pollination); the transfer is carried out by an external agent such as wind or an insect, and floral features often reveal which agent a flower relies on. A flower can receive its own pollen and pollen from another plant at the same time, so both types of pollination are not always mutually exclusive events.

After pollination, a pollen tube grows down through the style towards the ovule, carrying two male gametes; in double fertilisation, one male gamete fuses with the female gamete to form a diploid zygote and the other fuses with two nuclei in the embryo sac to form a triploid tissue called the endosperm, which stores food for the developing embryo. Because two separate fusions occur from the same pollen tube, double fertilisation is unique to flowering plants and is not seen in animals or in non-flowering plants such as ferns.

After fertilisation, the ovule develops into a seed containing the embryo, its food store and a protective seed coat, while the ovary wall develops into the fruit, which protects the seed or seeds inside and, in many species, also assists with dispersal.

Seeds allow a plant species to survive conditions unfavourable for growth, such as drought or cold, by remaining dormant until conditions improve, and to be carried away from the parent plant to colonise new areas with less competition for resources.

Seeds and fruits are dispersed by wind, using light weight and structures such as wings or feathery plumes; by water, using a buoyant, waterproof fruit wall; by animals, either through hooks and spines that cling to fur or through a fleshy, edible fruit that is eaten and the seeds later deposited elsewhere; or explosively, when a dry pod twists and splits suddenly, flinging seeds away from the parent plant. A single fruit sometimes combines more than one feature, such as being both lightweight and having a hook, so a real specimen may not fit neatly into only one dispersal category.

A seed generally needs three conditions to germinate: water, to soften the seed coat and activate enzymes; oxygen, to allow aerobic respiration that releases the energy needed for growth; and a suitable temperature, at which these enzymes work efficiently; light is not usually required for germination itself, although the seedling needs it soon afterwards for photosynthesis. These three conditions are often tested through a controlled experiment comparing seeds given different combinations of water, air and temperature to show which factor is limiting.

Self-pollination reliably produces seed even when no pollinating agent is available and preserves a combination of genes already well suited to the environment, but it produces offspring with little genetic variation and can lead to weaker offspring over many generations; cross-pollination increases genetic variation, generally producing hardier offspring, but it depends on an external agent and on the pollen source being compatible. Structural features that reduce self-pollination, such as the anther and stigma maturing at different times, are themselves valid exam answers when asked how a flower favours cross-pollination.