수국속 Hydrangea LINN.

1. 곧추 자란다.

2. 산방상 화서이다.

3. 하위씨방이며 삭과는 절두이고 잎에 털이 없거나 뒷면에 다소 있다   ·····················································672. 미국수국

3. 상위씨방이며 삭과는 난형이다.

4. 잎은 난형 내지 넓은 난형이며 털이 없거나 뒷면에 약간 있다   ·······························································673. 수국

4. 잎은 난형， 타원형 내지 피침형이며 양면에 다소 털이 있다   ·····························································674. 산수국

2. 원추화서이다 ··························································································································676. 나무수국

1. 덩굴식물이다   ·····························································································································675. 등수국