长在树上用on the tree还是in the tree

　　Most of the apples on the tree have ripened.

　　树上的苹果大多已经成熟。

　　Day after day, there were less and less leaves on the tree.

　　日复一日,树上的叶子越来越少。

　　His speciality was psychology--one important branch on the great tree of knowledge.

　　他的专业是心理学，这是知识之树上的一个重要分枝。

　　The woodpecker pecked a hole in the tree.

　　啄木鸟在树上啄了一个洞。

　　There is a bird sitting in the tree.

　　树上卧着一只鸟。

　　A little bird was singing happily in the tree.

　　小鸟在树上快乐地歌唱。