placentiform

from The Century Dictionary.

• In zoology, having the form, structure, or character of a placenta.
• In botany, shaped like a placenta; having a thick circular disk, concave in the middle on both upper and lower sides. The root of Cyclamen is an example.

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

• adjective (Bot.) Having the shape of a placenta, or circular thickened disk somewhat thinner about the middle.

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

• adjective botany Having the shape of a placenta, or thickened disk somewhat thinner about the middle.