Plagiacantha = Having spines on the sides; πλάγιος, ἄκανθα.
Tetraplagia = Four-sided; τετραπλάγιος.
Plagoniscus = Side-windlass; πλᾶγος, ὀνίσκος.
Plagonidium = Small side-article; πλαγωνίδιον.
Plagiocarpa = Having its fruit (the central capsule) on the side; πλαγιόκαρπος.