Which one of the following does NOT promote cross pollination?

Cleistogamy refers to the condition where flowers do not open at all. In such flowers, the anthers and stigma lie close to each other, and when anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, cleistogamous flowers are invariably autogamous (self-pollinating) as there is no chance of cross-pollen landing on the stigma. Therefore, cleistogamy does not promote cross-pollination.

Herkogamy is a physical barrier between the anther and stigma that prevents self-pollination.

Self-sterility (or self-incompatibility) is a genetic mechanism that prevents self-pollen from fertilizing the ovules.

Heteromorphy involves different forms of flowers (such as different lengths of stamens and styles) in the same species, which promotes cross-pollination.

Answer: Cleistogamy