Acanthaceae

Defining Characteristics

• Opposite , decussate leaves (each pair at 90° to the pair below)
• Flowers bilaterally symmetrical, tubular, often 2-lipped; typically 2 or 4 stamens
• Seeds borne on hook-like structures (retinacula/jaculators) that explosively eject seeds from the capsule
• Cystoliths (calcium carbonate concretions) often visible as streaks in leaves
• Many species with showy bracts subtending the flowers
• Predominantly tropical herbs and shrubs

Notable Genera

• Acanthus (bear's breeches)
• Hygrophila (hygrophilas — aquatic)
• Justicia (water willows, shrimp plants)
• Thunbergia (clock vine, black-eyed Susan vine)
• Ruellia (wild petunias)
• Strobilanthes (Persian shield)

Notes

Predominantly a tropical family with high diversity in the Indo-Malayan and Neotropical regions. Several Hygrophila and Justicia species are important aquarium plants. The explosive seed dispersal mechanism (via jaculators) is a distinctive family-level character. The acanthus leaf motif from Acanthus mollis has been a dominant decorative element in Western architecture since ancient Greece (Corinthian column capitals).