Heterostyly is a reproductive mechanism found in certain plant species that promotes cross-pollination. It involves the presence of two or more distinct floral morphs within a single species, each with a different arrangement of reproductive organs. This mechanism ensures that pollen from one morph is transferred to the stigma of another morph, increasing the chances of outcrossing and genetic diversity.

Among the options provided, the plant species that exhibits heterostyly as a contrivance for cross-pollination is Primula vulgaris, also known as the common primrose.

Explanation:

Heterostyly in Primula vulgaris:
Primula vulgaris is a typical example of a heterostylous plant species. It possesses two floral morphs, known as "pin" and "thrum" flowers. The morphological differences between these floral forms involve the relative positions of the reproductive organs, namely the stigma and anthers.

1. Pin flowers:
- Pin flowers have a long style, which positions the stigma well above the anthers.
- The anthers are located towards the top of the flower tube, away from the stigma.
- This arrangement prevents self-pollination and promotes cross-pollination.

2. Thrum flowers:
- Thrum flowers have a short style, positioning the stigma at the same level as the anthers.
- The anthers are located at the bottom of the flower tube, in close proximity to the stigma.
- This arrangement also prevents self-pollination and promotes cross-pollination.

Pollination mechanism in Primula vulgaris:
The distinct floral morphs of Primula vulgaris promote cross-pollination through a specialized pollination mechanism called "pin-and-thrum" pollination.

- Insects, such as bees, are the primary pollinators of Primula vulgaris. When an insect visits a pin flower, it will come into contact with the anthers and pick up pollen grains.
- The insect will then move to another flower, which could be a thrum flower, and when it probes for nectar, it will deposit the pollen on the stigma of the second flower.
- Since the anthers and stigma are positioned at different heights in pin and thrum flowers, the insect must visit flowers of both morphs to achieve successful cross-pollination.

Advantages of heterostyly in cross-pollination:
1. Outcrossing: Heterostyly promotes cross-pollination between different individuals, enhancing genetic diversity within a population.
2. Preventing self-pollination: The spatial separation of anthers and stigmas in pin and thrum flowers prevents self-pollination, reducing the chances of inbreeding depression and promoting outbreeding.
3. Enhancing pollen transfer: By requiring insect pollinators to visit flowers of both morphs, heterostyly increases the efficiency of pollen transfer between different individuals.

In conclusion, heterostyly is a reproductive mechanism found in Primula vulgaris, the common primrose, which promotes cross-pollination. The presence of pin and thrum flowers with different arrangements of reproductive organs ensures that pollen from one morph is transferred to the stigma of another morph, increasing genetic diversity and preventing self-pollination.