凡是表示生长“在树上”，都用介词 on，所以如果是表示树上长有叶子，也应用 on ，例如：

There aren't many leaves on the tree. 那棵树上的叶子不多。

In autumn the leaves on the trees begin to fall. 秋天，树叶纷纷落下。

There are some new leaves on the plant. 植物生长出了一些新叶。

There are several leaves on each stem. 每一根茎上都有几片叶子。

The leaves on the trees are bright green in spring. 春天树上的叶子是翠绿的。

Last fall we traveled to the mountains to see the leaves on the trees change color. 去年秋天我们到山里去看树叶染上秋色。