Excerpts from Jim Conrad's
from the February 2, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
Right outside the window of the tiny guardhouse in which I live there's a chain-link fence with ornamental vines twining up through it. Nowadays the star of the fence is the Bengal Clock Vine, also called Sky Flower, THUNBERGIA GRANDIFLORA. Below, you can see the vine's three-inch broad, sky-blue flowers and its opposite leaves.
Thunbergias are members of the mostly tropical Acanthus Family, and in the photo you can see one thing that makes a Thunbergia a Thunbergia: The four, upward arching stamens in the flowers' throats. Not seen are the two large bracts subtending the corollas, looking like huge, two-pointed calices. The actual calyx is a hardly noticeable rim at the corolla's base.
Several Thunbergias are widely planted in the tropics. They're mostly African and Asian in origin. Our Bengal Clock Vine is from Indian Bengal. The flowers wilt quickly when cut, but the two pictured above blossomed on the fence for over a week.