soap plant

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

• noun Any of several bulbous plants of the genus Chlorogalum of western North America, having small white or purple flowers. The bulbs of several species, especially C. pomeridianum, produce a soapy lather when crushed.
• noun Any of various other plants having parts used as soap.

from The Century Dictionary.

• noun One of several plants whose bulbs serve the purpose of soap; particularly, the Californian Chlorogalum pomeridianum, of the lily family.