Appello, I call; appeal, appellation.
Aptus, fit; apt, aptitude.
Aqua, water; aqueduct, aquatic, aqueous.
Arbiter, a judge; arbitration, arbitrary.
Arbor, a tree; arboraceous, arbour.
Ardeo, I burn; ardent, arson.
Arduus, steep [with the idea of difficulty of attainment]; arduous.
Arma, weapons; arms, armistice, disarm, army.
Aro, I plough; arable.
Ars (art-is), art; artificial, inertia, artisan.