  We consider semi-algebraic subsets of the Grassmannian of lines in
three-space called tree amplituhedra. These arise in the study of scattering
amplitudes from particle physics. Our main result states that tree amplituhedra
in ${\rm Gr}(2,4)$ are positive geometries. The numerator of their canonical
form plays the role of the adjoint in Wachspress geometry, and is uniquely
determined by explicit interpolation conditions.
