Krishna is the root of the Pandavas, Partha is their risen trunk. The other sons of Pritha and the Panchalas are like their branches and leaves respectively.
कृष्णो हि मूलं पाण्डूनां पार्थः स्कन्ध इवोद्गतः। शाखा इवेतरे पार्थाः पञ्चालाः पत्रसंज्ञिताः॥