H is for…..

Horneophyton
At a maximum size of about 20 cm Horneophyton was one of the commonest early land plants found in the Rhynie Chirt (a area of rock in Scotland with exquisitely preserved fossils from the early Devonian about 400+ million years ago).
Interestingly it showed an affinity to both bryophytes (mosses and liverworts) and the earliest vascular plants. It is considered a missing link by many paleobotanists.
They are the simplest known spore bearing plants and had neither true vascular tissue, true roots or leaves. Horneophyton may have led to the evolution of the modern plants known as Hornworts (Anthocerotophyta).