Green algae-like moss gametophyte .

Think green but lacking cellular differentiation.

Protonema are part of the gametophyte generation, which in turn is the more of the plant generations. The protonema thus represents pretty much as as a plant gets

Protonema thus can be viewed as indicating from whence plants came or, at least, what -like green algae develop from (which, indeed, also go through a protonema stage during their development).