Yama, while instructing Naciketa describes the eternal Asvattha tree with its root upwards, and branches downwards, which is the pure immortal Brahman, in which all these worlds are situated, and beyond which there is nothing else (Katha Upanishad Verse II.iii.1).
Yama, while guiding Naciketa explains the eternal Asvattha tree with its root pointing upwards and branches extending downwards, symbolizing the pure, immortal Brahman that encompasses all these worlds, transcending everything else (Katha Upanishad Verse II.iii.1).