We investigate the asympotic behaviour of the moduli space of morphisms from
the rational curve to a given variety when the degree becomes large. One of the
crucial tools is the homogeneous coordinate ring of the variey. First we
explain in details what happens in the toric case. Then we examine the general
case.Comment: This is a revised and slightly expanded version of notes for a course
delivered during the summer school on rational curves held in June 2010 at
Institut Fourier, Grenobl