Which preposition to use with involucres

of Occurrences 2%

An evergreen shrub, with oblong, light green leaves and terminal inconspicuous greenish flowers, surrounded by an involucre of four large, pinky-yellow bracts.

after Occurrences 1%

What he hoped for was, roughly speaking, the preservation of the ancient and beautiful husk after the kernel had been withered up and discarded; what Patmore looked forward to was the expansion of the kernel bursting one involucre after another, and ever clamouring for fairer and more adequate covering.

Which preposition to use with  involucres