pick up leave能不能换成是pick leaves?

pick up leave能不能换成是pick leaves?
小学英语中“摘树叶”教科书的翻译是“pick up leaves”,练习里面遇到“摘苹果”却是“pick apples.”为什么不能换成是为pick leaves,pick up apples啊?pick up跟pick到底如用法如何区别啊?
可是图片是爬到树上摘树叶啊？