“Dimidiate Involucre. Divided unequally into halves, so that
one half is so reduced as to appear lacking (Dimidiate).
A whorl of bracts subtending a flower or flower cluster (Involucre).”

Figure 388 from Plant Identification Terminology: An Illustrated Glossary
by James G. Harris / Melinda Woolf Harris

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Dimidiate Involucre