피속 Echinochloa BEAUV.

1. 소수는 장타원상 난형이며 거센 털의 밑은 쟁반처럼 굳어지는 것이 많다.

2. 짙은 녹색이며 소수는 길이 2.5-4mm로서 까락이 길고 잎가장자리는 두꺼워지지 않는다  ······································ 97. 돌피

2. 황록색 또는 담녹색이며 소수는 길이 5mm로서 잎가장자리가 두껍다······························································ 97+. 물피

1. 소수는 넓은 난원형이고 거센 털의 밑은 굳어지는 일이 드물다   ·········································································· 98. 피