FEMALE PROFESSOR: But coping with salt is not the only challenge for plants in a salt marsh. Soil there is dense and very low in oxygen.

FEMALE PROFESSOR: So spartinas have air tubes.Air enters through tiny openings on the leaves.

FEMALE PROFESSOR: The tubes provide a direct pipeline for the oxygen, carrying it down the leaves, through the stems and into the roots, where it's needed.

请问老师这里的down的用法，是along the course or path of (something)？

carrying it down the leaves, through the stems and into the roots翻译为 带着氧气沿着叶子，通过茎，进入根