Krishna starts Chapter 15 with an image of a tree.

Urdhvamulam adhahshakham
ashvattham prahuravyayam ||
Chandamsi yasya parnani yastam
veda sa vedavit ||15.1||

They say the imperishable ashvatha tree has its roots above, its branches below and the Vedas are its leaves. The one who knows that is the knower of the Vedas.

Adhaschaordhvam prasritastasya
shakhah gunapravriddha
vishayapravalah ||
Adhascha mulanyanusantatani
karmanubandhini manushyaloke
||15.2||