flat part of the leaf

例句：

1. 1. The floor is very flat.
   地板非常平坦。
2. 2. The table is too flat for my liking.
   桌子太平坦了，我不喜欢。
3. 3. The roof of the house is flat and smooth.
   房子的屋顶是平坦光滑的。