Verse 2.3.1

ऊर्ध्वमूलोऽवाक्शाख एषोऽश्वत्थः सनातनः ।
तदेव शुक्रं तद्ब्रह्म तदेवामृतमुच्यते ।
तस्मिंल्लोकाः श्रिताः सर्वे तदु नात्येति कश्चन । एतद्वैतत् ॥ १ ॥

ūrdhvamūlo'vākśākha eṣo'śvatthaḥ sanātanaḥ |
tadeva śukraṃ tadbrahma tadevāmṛtamucyate |
tasmiṃllokāḥ śritāḥ sarve tadu nātyeti kaścana | etadvaitat || 1 ||

1. Root up and branches down is this ancient asvattha tree, that (its source) is pure. That is Brahman and that alone is called immortal. On that, do all worlds depend and none passes beyond that. This verily is that.

Shankara’s Commentary:

Com.—As in the world, the ascertainment of the root of a tree is made by ascertaining the nature of the tula (panicle of the flower), this sixth valli (part) is begun with the object of ascertaining the nature of the Brahman, the source (of the samsara tree,) by ascertaining the nature of the effect, the tree of samsara. Root up, having its root up, i.e., ‘that highest place of Vishnu’ is its root, this tree of samsara, extending from the avyakta to the immoveable, has its root up, i.e., in Brahman. It is vriksha (tree), so called, because it is felled; this tree consisting in manifold miseries of birth, decay, death and grief, etc., changing its nature every moment, like jugglery, waters of the mirage, a city formed by the clouds in the sky, etc.; because like these perceived only to vanish ultimately, non-existent like a tree, sapless like the stem of the plantain tree, the subject of several doubtful alternatives in the intellects of many hundreds of sceptics, not ascertained to be what it really is by seekers after truth, receiving its sap from its source, i.e., the highest Brahman ascertained by Vedanta, growing from the seed of ignorance, desire, karma and avyaktam, having for its sprout hiranyagarbha—the combination of the power of knowledge and activity of the lower Brahman, having for its skandha (trunk), the various subtle bodies of all living things, possessed of the pride of stature from the sprinkling of the waters of desire, having for its tender buds the objects of intelligence and the senses, having for its leaves the srutis, the smritis, logic, learning and instruction, filled with the lovely flowers of sacrifice, gift, penance and many other deeds, having various tastes such as the experience of joy and sorrow, having endless fruits on which living beings subsist, with its roots well grown, i.e., (tendencies of the mind) entwined and fastened firm by the sprinkling of the waters of desire for the fruits, with the nests built by birds, i.e., all living beings from Brahma downwards in the seven worlds beginning with that called satya, reverberating with the tumultuous noise arising from dancing, singing, instrumental music, joking, clapping on the shoulders, laughing, pulling, crying, exclaiming ‘leave me,’ ‘leave me,’ etc., induced by mirth and grief, produced by the happiness and misery of living beings and felled by the unresisted sword of the realisation of the Paramatman proved by the Vedanta, this tree of Samsara, always shaking by its nature to the wind of desire and karma, like the asvattha tree, having its branches, i.e., heaven, hell, the world of beasts and pretas, etc., downwards, existing from time immemorial, because having no beginning. That which is the root of this tree of Samsara is indeed pure, bright, i.e., resplendent, the intelligence of atman; that indeed is Brahman, being greater than all; that indeed is described as immortal in nature, being true; any other than that is a mere matter of speech, modification, name and falsehood and therefore subject to death. On that, i. e., on the Brahman absolutely true, do all the worlds, false like the city of clouds in the sky, waters of the mirage and jugglery (maya) and perceived as non-existent by the knowledge of the absolute truth, depend during their birth, stay and absorption. None, i.e., no modification passes beyond that, i.e., Brahman, as the thing made, such as pot, etc., does not pass beyond the mud, etc. This verily is that.