From Prakriti to Purusha -- in fact, as the title of Canto XV says, to 'Purushottama', the 'Highest Purusha'.
One of Hinduism's most beautiful, memorable and meaningful religious symbols is presented in the four opening shlokas of this Canto. It is the symbol of the Kalpa-Taru, the Imagination Tree or the Wish-Fulfilling Tree.
Why does Krishna introduce the cosmic fig-tree Ashvattha with roots above, shoots mid-space, and fruits below ? 'Cut down this tree, Arjuna,' he says, 'with the sword of detachment. And stand up !' The suggestion, of course, is that the network of ruthless Karma, produced by Prakriti, embraces sky, earth, and underworld; affects gods, men and anti-gods (as Prajapati explains in the Brihadaranyaka Upanishad); involves pre-life, this life, and after life (Kala in Sanskrit is Time for yesterday, today and tomorrow).
But there is another triad presented in this Canto which is of profound significance : the Purusha Trinity. One Purusha is embedded in the body -- embodied. This is the perishable Spirit. The second Purusha is the imperishable atman which enters the body but leaves it, taking with it the subtle impressions of the senses, like a breeze carrying the fragrance of flowers. But there is a third, the Paramatma Purusha, the 'deathless Lord', who is honoured in the key shloka 19 of Canto XV : 'I am above the perishable and the imperishable; therefore the world and the Vedas call me the Highest Purusha, Purushottama'. When the tree of Karma is sliced with non-detachment, Purushottama is attained.