15.1 The Lord said  The Vedas speak of the imperishable 'Asvattha tree' called Samsara, which has its 'roots above and branches below', in such passages as the following:  This Asvattha tree with its roots above and branches below is eternal' (Ka. U., 6.1), and 'He who knows the tree with its roots above and branches below' (Tai. A., 1.11.5). It has its roots above since it has its roots in Brahma (the Creator otherwise known as Hiranyagarbha) who is seated above the seven worlds. It has 'branches below' ending with denizens like men, animals, beasts, worms, insects, birds and immovables. It is 'immutable' since it cannot be felled, being of the form of a continual flow. It can be felled only at the dawn of perfect knowledge which causes detachment. They say that the leaves of this Asvattha tree constitute the Vedas. 'The Vedas are said to be the leaves', since this tree of Samsara increases by actions prompted by worldly desires as taught in certain Srutis as, 'He who desires prosperity should sacrifice a white animal to Vayu' (Taitt. Sam., 2.1.1) and 'The desirer of offspring shall offer to Indra and Agni a sacrifice with eleven cups of rice-cakes' (Ibid., 2.2.1). Indeed the tree flourishes with the help of leaves. He who knows the Asvattha of such a nature 'knows the Vedas'. The Vedas also set forth the means of felling this tree of Samsara. He who understands this is called the knower of the Vedas, since knowledge of the nature of the tree to be cut off is helpful to the knowledge concerning the means of felling the tree.

This tree spreads downward with men etc., who are the products of their Karma, as branches. It again spreads above into Gandharvas, Yaksas, gods, etc. They are nourished by the Gunas of Sattva etc. They have tender shoots augmented by sense-objects.

How does this happen?  Sri Krsna explains: