Bhagavad Gita: Chapter 15, Verse 2

अधश्चोर्ध्वं प्रसृतास्तस्य शाखा
गुणप्रवृद्धा विषयप्रवाला: |
अधश्च मूलान्यनुसन्ततानि
कर्मानुबन्धीनि मनुष्यलोके || 2||

adhaśh chordhvaṁ prasṛitās tasya śhākhā
guṇa-pravṛiddhā viṣhaya-pravālāḥ
adhaśh cha mūlāny anusantatāni
karmānubandhīni manuṣhya-loke

adhaḥdownward; chaand; ūrdhvamupward; prasṛitāḥextended; tasyaits; śhākhāḥbranches; guṇamodes of material nature; pravṛiddhāḥnourished; viṣhayaobjects of the senses; pravālāḥbuds; adhaḥdownward; chaand; mūlāniroots; anusantatānikeep growing; karmaactions; anubandhīnibound; manuṣhya-lokein the world of humans


BG 15.2: The branches of the tree extend upward and downward, nourished by the three guṇas, with the objects of the senses as tender buds. The roots of the tree hang downward, causing the flow of karma in the human form. Below, its roots branch out causing (karmic) actions in the world of humans.


Shree Krishna continues comparing the material creation with the aśhvatth tree.  The main trunk of the tree is the human form, in which the soul performs karmas.  The branches (śhākhās) of the tree extend both downward (adhaḥ) and upward (ūrddhva).  If the soul commits sinful activities, it is reborn either in the animal species or in the nether regions.  These are the downward branches.  If the soul performs virtuous acts, it is reborn in the celestial abodes as a gandharva, devatā, etc.  These are the upward branches.

As a tree is irrigated by water, this tree of material existence is irrigated by the three modes of material nature.  These three modes generate sense objects that are like the buds on the tree (viṣhaya-pravālāḥ).  The function of buds is to sprout and cause further growth.  The buds on this aśhvatth tree sprout and create material desires that are like the aerial roots of the tree.  The specialty of banyan trees is that they send down aerial roots from the branches to the ground.  Hence, the aerial roots become secondary trunks, enabling banyan trees to grow to vast sizes.  The biggest known banyan tree is “The Great Banyan” in the Botanical Garden of Kolkata.  The area occupied by the tree is about four acres.  The crown of the tree has a circumference of about 1,100 yards, and there are about 3,300 aerial roots reaching down to the ground.  Similarly, in the analogy of the aśhvatth tree, in the material world the sense objects are like the buds on the tree.  They sprout and evoke desires for sensual enjoyment in the individual.  These desires are compared to the aerial roots of the tree.  They provide juice for the tree to keep growing.  Impelled by desires for material enjoyment, the living being engages in karma.  But sensual desires are never fulfilled; rather they only multiply as we try to satiate them.  So karmas performed to fulfill desires only serve to increase them further.  Thus, the aerial roots of this metaphorical tree keep expanding in size and growing unlimitedly.  In this way, they entangle the soul further in material consciousness.