pollinium

Pollinium is a cohesive mass of pollen grains produced by a single anther or cluster of anthers and released or transported as a unit. This pollen unit occurs in several flowering plant groups, most notably in orchids (Orchidaceae) and in the milkweed lineage within Apocynaceae (often treated as Asclepiadoideae).

Structure and mechanism: In these plants, pollen grains are not shed individually. They are bound together to form a pollinium, typically attached to a viscidium (a sticky pad) by a stalk or caudicle, creating a pollinarium. When a pollinator visits, the pollinium adheres to the insect’s body and is carried to another flower, where it comes into contact with the stigma and fertilization may proceed. Orchid flowers often contain one or more pollinia, while milkweeds form pollinaria that link several pollinia into a single unit.

Ecology and evolution: The pollinium system enables efficient, targeted pollen transfer between flowers of the same species and often promotes cross-pollination through specialized pollinators. This mode of pollen packaging reduces pollen loss and can contribute to high pollination success even in plants with otherwise limited floral displays.

Taxonomic note: The singular form pollinium refers to one such unit, while pollinia is the standard plural. The broader concept of a pollinarium includes the pollinium or pollinia along with their attaching structures, which are adapted to specific pollinators in the respective plant groups.