Sentence:  A naturally sweet herb called <WSD>stevia</WSD> is gaining in popularity as a sugar substitute, especially in beverages. Food additives must provide evidence of their safety and effectiveness before receiving FDA approval, and <WSD>stevia</WSD> lacks this approval for use as a sweetener because so little is known about its effects on human health, save that it can be absorbed by the human digestive tract. 
Meaning of the word marked with <WSD> tags:  Any of the sweet herbs of genus "Stevia", native to tropical and subtropical regions of South America and western North America. (from 19th c.)