This continuous growth from a near-basal meristem is unique among plants to hornworts. As the sporophyte grows longer, it splits into two halves lengthwise, releasing the spores as they mature. Along with the spores are cells called pseudoelaters , which change shape as they dry out and thereby assist in scattering the spores. When a spore germinates, it produces a flat thalloid plant with a greasy blue-green color and odd morphology. This is an especially useful character for sterile material, but you must look at fresh material, since hornworts tend to dry up and fall apart if not kept moist, and will quickly rot if immersed.
Hornworts are a problematic group for evolutionary biologists.
Most hornwort fossils are Miocene spores found in Europe and assigned to the Anthocerotaceae. Such a late date for oldest fossils of hornworts is curious, considering that most cladistic analyses of living plants suggests that the group originated early in the history of land plants, probably before the Devonian , and should be one of the oldest lineages of plants. The delicate hornworts may not have been easily preserved, which might explain the absence of a Paleozoic record. However, it seems puzzling that their spores, which would be much more likely to be preserved, are not encountered either. Above, you can see pictures of the hornwort Phaeoceros .