A pollinium is a coherent mass of pollen grains in a plant that are the product of only one anther, but are transferred, during pollination, as a single unit. This is regularly seen in orchids, which have a complex pollination system, and many species of milkweeds (Asclepiadoideae). Usage of the term differs: in some orchids two masses of pollen are well attached to one another, but in other orchids there are two halves each of which is sometimes referred to as a pollinium.