Q.(a) Explain how it is ensured that the orchid Ophrys is pollinated by a specific species of bee.

Pollination by water — hydrophily — is a relatively rare strategy in the plant kingdom, adopted mainly by species that live fully or partially submerged. The mechanism is straightforward: pollen grains are released into the water and drift with currents until they encounter a receptive stigma. This mode of pollination demands specific adaptations, because pollen must remain viable in an aquatic environment and the flowers must be positioned to facilitate contact.

Vallisneria is perhaps the most celebrated example taught in textbooks. This submerged freshwater plant has a fascinating pollination story. The female flowers rise to the water surface on long stalks and form a small depression in the surface film. Male flowers, meanwhile, break free from the plant and float upward. When they reach the surface, they drift along until they encounter a female flower's depression, slide down into it, and pollination occurs. The entire process is a beautiful demonstration of how plants exploit water currents and surface tension.

Hydrilla, another fully submerged aquatic plant, also depends on water for pollination. Its pollen is released directly into the water column, where it drifts until it contacts the stigma of a female flower. Both Vallisneria and Hydrilla are adapted to life underwater, and their reproductive strategies reflect that commitment.

Now consider the other plants mentioned in the options:

• Water lily has showy, emergent flowers that sit above the water surface. These flowers are pollinated by insects — beetles and bees are common visitors — not by water. The flowers are too exposed and structurally designed for insect pollination. …

Pollination is the crucial process by which pollen grains are transferred from the anther to the stigma, a prerequisite for fertilization in flowering plants. While the majority of flowering plants rely on animals (like insects, birds, and bats) or wind for pollination, a small fraction has adapted to use water as their primary pollinating agent. This mechanism, known as hydrophily, is particularly interesting due to the unique challenges and adaptations it presents.

Let us examine the given statements regarding water pollination:

• Statement (i): In Vallisneria, the female flowers are pollinated by pollen grains inside the water.

  • Vallisneria, often called tape grass, is a dioecious aquatic plant. Its female flowers have long stalks that allow them to reach the surface of the water. The male flowers, or pollen grains, are released onto the water surface. These pollen grains are then passively carried by water currents to the floating female flowers. Therefore, pollination in Vallisneria occurs on the surface of the water, not inside it. This statement is incorrect.

• Statement (ii): In Zostera, pollen grains are released on the surface of water.

  • Zostera, commonly known as seagrass, is an example of a submerged aquatic plant. In such plants, both the male and female flowers remain entirely underwater. The pollen grains are released inside the water and are carried passively by water currents to the submerged stigmas. These pollen grains are often long and ribbon-like, which helps them to be carried effectively within the water. This statement is incorrect.

• Statement (iii): In most of the water-pollinated species, pollen grains are covered by a mucilaginous coating. …

Concept First: Why This Question Is About Plant Sex Systems

Pollination modes depend entirely on where the male and female flowers are located.

• Geitonogamy is pollination between two flowers on the same plant. It’s functionally cross-pollination (pollen moves between flowers) but genetically self-pollination (same parent).
• Xenogamy is true cross-pollination between flowers on different plants.

For geitonogamy to even be possible, the plant must have both male and female flowers on the same individual. That’s called monoecy (Greek: one house). If male and female flowers are on separate plants (dioecy), geitonogamy is impossible — you can’t move pollen between flowers that don’t exist on the same plant.

So the question is really: which of these four plants is monoecious (bisexual in terms of flowers, but unisexual flowers on the same plant)?

Step-by-Step Reasoning

1. Papaya — This is a classic dioecious plant. Male flowers and female flowers grow on separate plants. A papaya tree is either male or female. So geitonogamy is impossible here. Xenogamy is the only option (and it happens via insects or wind).

   → Eliminate (A).

2. Date Palm — Also dioecious. Male and female flowers are on different trees. Farmers often plant one male tree for many female trees to ensure pollination. Again, no geitonogamy possible.

   → Eliminate (B). …

The question tests your understanding of a fascinating example of coevolution and deceptive pollination in plants. The Mediterranean orchid Ophrys has evolved flowers that mimic the appearance and scent of a female bee. This is a classic case of sexual deceit — the male bee is tricked into attempting to mate with the flower (a process called pseudocopulation), and in doing so, picks up or deposits pollen. The reason provided, however, is about female bees changing colour based on temperature. That is a completely unrelated biological phenomenon (some insects do show temperature-dependent colour changes, but it is not relevant to this orchid's pollination strategy). So the assertion is correct, but the reason is incorrect.

Let’s break it down step by step.

1. Understanding the Assertion (A)

   The Mediterranean orchid Ophrys is well-known for its sexual deception strategy. Its flower not only looks like a female bee but also releases pheromones that mimic the female’s sex attractant. When a male bee lands on the flower, it tries to copulate, and during this process, pollen sacs (pollinia) attach to its body. When the bee visits another Ophrys flower, the pollen is transferred. This is a highly specific adaptation — each Ophrys species often mimics a particular bee species. So Assertion (A) is true.

2. Understanding the Reason (R) …

Part (a)

For self-pollination to be fully ensured, there must be no chance of foreign pollen reaching the stigma. Checking the options:

• (A) Bisexual — necessary for autogamy but does not guarantee it (the flower may still cross-pollinate).
• (B) Style longer than the filament — a form of herkogamy that actually favours cross-pollination.
• (C) Cleistogamous — the flower never opens; anthers and stigma lie enclosed together, so the flower's own pollen must land on its own stigma. This makes self-pollination obligate and fully assured, even without pollinators. (Examples: Viola, Oxalis, Commelina.) …

Part (a)

Cleistogamy (Greek: 'closed marriage') describes flowers that remain permanently closed. Examining the options:

• (A) Bisexual flowers which do not open at all — correct: they possess both stamens and pistil, and because the flower never opens, the anthers dehisce inside the closed bud and the pollen is deposited on the stigma of the same flower, guaranteeing self-pollination (autogamy) even without pollinators.
• (B) Bisexual and open flowers — these are chasmogamous (normal open) flowers, not cleistogamous.
• (C) Unisexual — wrong; a unisexual flower cannot self-pollinate at all. Cleistogamous flowers are always bisexual.
• (D) Stigma matures before the anthers dehisce — this is protogyny, a device that promotes cross-pollination. …