I think that every Corinthian capital (1st photo below) is decorated with the leaves (2nd photo below) of the acanthus plant. Scrolls made from the vines of acanthus plants are also common (3rd and 4th photos below).
A Corinthian capitas in Santa Maria degli Angeli in the Baths of Diocletian See all Acanthus Plant photos.
A scorpion and a worm on the acanthus leaves at the bottom of the lower-left panel on the Public Approach Front of the Ara Pacis See all Acanthus Plant photos.
Red Porphyry Sarcophagus of Constantia, daughter of Constantine the Great, who died in 354 AD, and was buried in a mausoleum beside the Basilica of St. Agnes, now in the Greek Cross Hall of the Vatican Museum. The coffin is decorated on all four sides with garlands and grape vines, large acanthus scrolls and cupids treading grapes. Below there are two peacocks, a ram and a cupid with garland. The Dionysian decoration of the grape harvest also appears in the exquisitely refined mosaic decoration of the vault of the mausoleum of Constantia. See all Acanthus Plant photos.
Lower-right corner (Scrolling Acanthus Relief) of the Ceremonial Front of the Ara Pacis (Altar of Peace) See all Acanthus Plant photos.
Below are photos of a few real acanthus plants I happened to see while walking around Rome. I didn't find one wrapped in a circle like the vines above, but the leaves are easy to spot.
The two plants below fooled me into thinking they were acanthus plants. They looked like acanthus to me, but I've been informed that they're different species entirely. So beware, and don't get fooled like I was! Thanks to all who pass corrections on to me.