The Bryophyta and Pteridophyta have sprung from the higher Thallophyta, and together form the larger group Archegoniatae, so-called from the form of the organ (archegonium) in which the egg-cell is developed.
The Mosses and Liverworts include forms with a more or less leaf-like thallus, such as many of the liverworts, and forms in which the plant shows a differentiation into a stem bearing remarkably simple leaves, as in the true mosses.
For instance, all the leaves of the Bryophyta are generally homologous inasmuch as they are all developments of the gametophyte.
Thus in the series Bryophyta, Pteridophyta, Phanerogamia, whilst the sporophyte presents progressive development, the gametophyte presents continuous reduction.
In illustration of the indirect response, the evolution of the Bryophyta and of more highly organized plants may be briefly considered.