They say that Ignorance is the soil. Acts form the seed that is placed in that soil. Desire is the water that causes that seed to grow. In this manner they explained re-birth.
अविद्यां क्षेत्रमाहुर्हि कर्म बीजं तथा कृतम्। तृष्णा संजननं स्नेह एष तेषां पुनर्भवः॥