orientation of the pollinium is correct and the concentration of nectar surrounding a pollinium is optimal, pollination will take place. Pollen tubes grow out of a germination pore on the convex surface of the pollen mass on pollinium into the stigma and down one of the 2 styles to the ovary where the male genetic material fertilizes the (female) ovules. Each ovule has the potential to form one seed. The image "Antelope Horns pollen tube" shows the route pollen tubes take to transfer genetic material into an ovary. Asclepias asperula does not self-pollinate so they are dependent on attracting pollinators that will transfer pollen between plants. Taxonomy Asclepias asperula is divided into two subspecies: A. asperula. ssp. asperula and A. asperula ssp. capricornu. Ssp. capricornu occurs in comparatively more mesic conditions and has comparatively broader leaves, floral crowns that are more white, and a more prostrate habit. Ssp. asperula occurs in comparatively more arid conditions and has comparatively narrower leaves, floral crowns that are more purple, and a more upright habit. Ecology Like several other species of milkweed, A. asperula is a food for monarch butterfly caterpillars. Along with being food for monarchs, the plants also contain toxic cardiac glycosides (cardenolides) that the monarchs retain, making them unpalatable and poisonous to predators. For the same reason, A. asperula can be poisonous to livestock and other animals, including humans. In addition to the monarch, it is a larval host to the dogbane tiger moth, the queen butterfly, and the unexpected cycnia. Cultivation The plant is difficult to cultivate and does not grow well in containers. In cultivation, this species favors quickly draining soil that is high in inorganic matter, such as sand and rock chips. It can grow in loam and clay, if provided with adequate drainage and frequent dryness. Moisture level demands and tolerance depend upon the subspecies and possibly ecotype. The plant has a deep taproot, so it needs to have the deepest-possible pot if grown in a container and should not be waterlogged. References Further reading External links Native Plant Information Network asperula Butterfly food plants Flora of the Southwestern United States Flora of Northwestern Mexico Flora of Sonora Taxa named by Joseph Decaisne