  We develop observational tests of the idea that dissipation in gas-rich
mergers produces the fundamental plane (FP) and related correlations obeyed by
ellipticals. The FP 'tilt' implies lower-mass ellipticals have a higher ratio
of stellar to dark matter within their stellar effective radii. Models argue
that mergers between more gas-rich (typically lower-mass) disks yield larger
mass fractions formed in compact starbursts, giving a smaller stellar R_e and
higher M_stellar/M_tot within that R_e. Such starbursts leave a characteristic
imprint in the surface brightness profile: a central excess above an outer
profile established by the dissipationless violent relaxation of disk stars. In
previous work, we developed empirical methods to decompose the observed
profiles of ellipticals and robustly estimate the amount of dissipation in the
original spheroid-forming merger(s). Applying this to a large sample of
observed ellipticals, we test whether or not their location on the FP and its
tilt are driven by dissipation. At fixed mass, ellipticals formed in more
dissipational events are smaller and have higher M_stellar/M_tot. At fixed
degree of dissipation, there is no tilt in the FP. We show that the dynamical
mass estimator R_e*sigma^2/G is a good estimator of the true mass: the observed
FP tilt cannot primarily owe to other forms of non-homology. Removing the
effects of dissipation, observed ellipticals obey the same FP correlations as
disks: unusual progenitors are not required to make typical ellipticals.
Dissipation appears to be both necessary and sufficient to explain the FP tilt.
