The inverted world-tree of this chapter draws directly on older scripture. The image of a cosmic tree “with roots above and branches below” appears in the Kaṭha and Śvetāśvatara Upaniṣads; the theme of the one Puruṣa pervading all echoes the Puruṣa-Sūkta of the Ṛgveda; and the two-birds-on-one-tree and the imperishable-syllable motifs run through the Muṇḍaka and Praśna. K is not inventing but gathering — condensing the Upaniṣadic vision into the Gītā’s compact idiom (as at 8.11, 13.4). The reader who knows those sources hears the resonances; the reader who does not still receives the essence.

The aśvattha — the sacred fig, Ficus religiosa (the pipal). But here it is described as no ordinary tree: “with roots above and branches below, imperishable, whose leaves are the (Vedic) hymns.” It is the world-tree of Saṃsāra, inverted — its root above in the unmanifest Brahman (the source), its branches spreading downward into the manifest world (the effects), nourished by the guṇas, its secondary roots (of attachment and action) striking downward into the soil of human life. The botanical name is only borrowed; the “species” found is the tree of conditioned existence itself, which the seeker is told to cut down with “the strong axe of non-attachment (asaṅga-śastreṇa)” (15.3).