  We report that the properties of the isotropic to nematic liquid crystalline
phase transition of F-actin depend critically on the average filament length.
For average filament lengths longer than 2 $\mu$m, we confirm previous findings
that the phase transition is continuous in both alignment and concentration.
For average filament lengths shorter than 2 $\mu$m, we show for the first time
a first order transition with a clear discontinuity in both alignment and
concentration. Tactoidal droplets of coexisting isotropic and nematic phases,
differing in concentration by approximately 30%, form over the course of hours
and appear to settle into near equilibrium metastable states.

Biological Physics