Horneophyton sporangia as odd-shaped bags

Horneophyton had been one of the four land plants discovered early in the Rhynie chert [1]. Nevertheless it has defied any attempt to group it with other fossil or extant plants, mainly because of its unique branched sporangia. Not only their shape is unique but also their interior: There is a central “tree” inside whose branches extend into the lobes of the sporangium. A central column is known from the sporangia of the hornworts, where it is called columella. (The similarity of the names is incidental.)

The peculiar build of the Horneophyton sporangium had been pictured in [2] as an obviously simplified schematic drawing (Fig.1). Disregarding evidence for more irregular shapes of the sporangia [2,3], that drawing was redrawn many times, with slight variations for copyright reasons, apparently by people who never saw a sporangium in reality, and has been repeatedly published in palaeobotany monographs and on websites as if it were the real thing.

						Fig.1: Horneophyton sporangium as usually pictured
						Fig.2: cut chert face with Horneophyton sporangia
						Fig.3: variety of Horneophyton sporangium

Although it is difficult to reconstruct a typical shape from the sections seen on cut chert faces in random orientation as in Fig.2, it became obvious soon that shape and size of the sporangia were distinctly variable and different from Fig.1. They can be rather simple with only one(?) or two lobes but never as regular as in Fig.1, and possibly as odd-shaped as pictured in Fig.3. With the combination of bulky bag and narrow pipe some specimens may even resemble a bagpipe.

						Fig.4: cross-sections of shoot and sporangium
						Fig.5: section of an apparently two-lobed sporangium
						Fig.6: cross-section of sporangium near its base

The principle of dichotomy or forking as it is known from the shoots (Fig.4) is still there with the sporangia, as seen in sections near the base (Fig.4, Fig.5 & Fig.6). However, a tendency towards asymmetry takes over soon. By the way, the transition from dichotomous to asymmetrical branching is an early phenomenon in many clades of the plant system. Whether the odd-shaped sporangia evolved by the fusion of simpler ones or else by an increasing complexity of shape is still an unsolved problem.

text, photographs & drawings: H.-J. WEISS/Rabenau