The best way to find journal articles is to search one of the many scholarly databases. Below are listed some of the more relevant databases for medicinal plant topics.
Academic Search Ultimate (EBSCO) Tutorial
Anthropology Plus (EBSCO) Tutorial
Science & Engineering Help Desk
AGRICOLA is an agricultural sciences database that contains records from the U.S. Department of Agriculture's National Agricultural Library. Over 2,000 journals and publication series are indexed from 1979 to the present.
Agricultural & Environmental Science (ProQuest) Tutorial
International Plant Names Index (IPNI)
The International Plant Names Index (IPNI) is a database of the names and associated basic bibliographical details of seed plants, ferns and lycophytes.