Plant 385

Loasa species (Loasaceae)

Rock nettle

The genus Loasa comprises approximately 36 species of herbs and small shrubs, naturally distributed in montane and rocky areas of Mexico and Central and South America. The entire rock-nettle family itself only contains approximately 300 species in 20 genera; most of these species are distributed in the Americas from the Great Lakes to Patagonia. Species diversity is particularly high at high elevations in tropical America. The origin of the name Loasa is unclear but may be a reference to an indigenous Chilean name for these plants.

Two features make many plants in the rock-nettle family stand out: their intricate flowers; and the stinging, and sometimes grapple-shaped, hairs that may cover all parts of the plant. The distinctive flowers of Loasa are hermaphrodite, but in addition to fertile stamens, each flower contains a complex arrangement of sterile stamens (staminodes). The staminodes form elaborate nectar scales, where nectar is produced and stored. To access nectar, pollinators must move the nectar scale, which in the process triggers pollen release – the mechanism by which this happens in unique to Loasa and related genera.

Although the process of pollen release associated with nectar access is similar across the genus Loasa, there are differences among species in flower size and colour, the shape and size of the nectar scales and in the composition and quantity of the nectar produced. Loasa species with small, star-shaped flowers that produce nectar containing a high percentage of sucrose tend to be pollinated by short-tongued bees. Species with bowl- or saucer-shaped flowers, and nectar containing a moderate percentage of sucrose, are pollinated by long-tongued bees. Across the entire family, the diversity of recorded pollinators is much greater, ranging from insects through birds to small mammals.

Some rock nettles are cultivated as horticultural novelties but, despite their attractive and unusual flowers, it is unlike they will ever be widely grown because of their stinging hairs. Stinging hairs –botanical equivalents of hypodermic syringes – are physical and chemical defences that inject chemical irritants into herbivores. The Loasaceae is one of five flowering-plant families with true stinging hairs. The most familiar of these is the nettle family (Urticaceae), whose stinging hairs have attracted scientific attention for centuries; magnified images were included in Robert Hooke’s Micrographia (1665). The ‘needle’ of the stinging hairs of Loasa differs from those of the nettle (Urtica dioica) in having tips strengthened with calcium carbonate and calcium phosphate rather than silica.

Further reading

Ackermann M and Weigend M 2006. Nectar, floral morphology and pollination syndrome in Loasaceae subfam. Loasoideae (Cornales). Annals of Botany 98: 503-514

Henning T and Weigend M 2013. Beautiful, complicated—and intelligent? Novel aspects of the thigmonastic stamen movement in Loasaceae. Plant Signaling & Behavior 8: e24605.

Mustafa A et al. 2018. Stinging hair morphology and wall biomineralization across five plant families: conserved morphology versus divergent cell wall composition. American Journal of Botany 105: 1109-1112.

Stephen Harris