  We show that the isotropic to nematic liquid crystalline phase transition of
F-actin can be either continuous or discontinuous, depending critically on the
filament length. For F-actin with average filament length > 3 um, we confirm
that the transition is continuous in both filament alignment and local
concentration. In contrast, for filament length < 2 um the F-actin solution
undergoes a first order transition. Tactoidal droplets of co-existing isotropic
and nematic domains were observed. Phenomena of nucleation-and-growth and
spinodal decomposition both occur, depending sensitively on the exact
concentration and average filament length of F-actin. In the late stage, the
tactoidal droplets continually grow and occasionally coalesce to form larger
granules.
