unicostate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Having a single main costa, rib, or riblike part.

from The Century Dictionary.

• Having but one rib; in botany, noting those leaves which have one large vein running down the center, called the midrib. Those having more than one great division are called multicostate.
• In zoology, having a single costa, rib, or nervure, as an insect's wing.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Bot.) Having a single rib or strong nerve running upward from the base; -- said of a leaf.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Having a single rib.