외풀속 Lindernia ALL.

1. 잎에 톱니가 없고 꽃받침은 얕게 갈라진다 ····························································································321. 밭뚝외풀

1. 잎에 톱니가 있다.

2. 톱니는 낮아서 거의 없는 것 같고 꽃받침은 깊게 갈라진다 ·······································································322. 논뚝외풀

2. 톱니는 뚜렷하다.

3. 잎은 우상맥이고 지면으로 뻗어가는 가지가 있다 ···············································································323+ 미국외풀

3. 잎은 장상맥이고 꽃받침은 얕게 갈라지며 뻗어가는 가지가 없다  ··································································323. 외풀