Aglaophyton branching
deutsche Version

Drawings showing the overall aspect of Aglaophyton (former name Rhynia major), including repeatedly re-drawn versions, are available in great variety. The early ones [1] are far from reality but are still offered occasionally as if they depicted the real thing. Others seem to have been inspired by the simplest thinkable option explaining the observed undulating growth mode, according to which the shoot grew upwards until it fell over under its own weight, touched the ground with its tip, grew rhizoids there, then turned upwards again [2]. However, the falling-over part of this alleged sequence of events is erroneous and has been replaced by the assumption of downward growth [3]. The idea proposed in [2] that essentially there is no morphological difference between undulating and upright axes, based on the observation that both have got stomata and look alike in cross-section, seems to come closer to reality than the differentiation between "rhizomatous" and upright axes in [4].
The branching in other published drawings of this species varies between narrowly V-shaped with very small angle of divergence and broadly U-shaped as it really is. Characterizing the U-shape as "dichotomous branching with an angle of dichotomy between 60 and 90 " [4] does not immediately make sense since it is not obvious what is meant by "angle of a U-shape", not counting the 180 at the bottom of the U and 0 farther up. In view of the confusingly different notions it seems appropriate to consider the subject once more.
Aglaophyton fork
Fig.1: In-plane section of forking upright axis of Aglaophyton, hollow inside, seen on a naturally smoothed fracture face of an old chert layer fragment.

A well-shaped fork is a rare sight among the abundant sections of Aglaophyton seen on the surface or on cut faces of Rhynie chert samples, the reason of which is obvious: In order that a fork is seen as such on a smooth face, the latter would have to approximately coincide with one of the two symmetry planes of the fork, which would be a rare coincidence.
The fork in Fig.1 is not quite perfectly seen since its prongs get out of the sample surface before they become parallel, so the section looks more like the letter Y. Also the forking xylem strand is not seen here. Incidentally the plant was partially hollow before silicification, hence the forking light-coloured streak inside.
Aglaophyton branching
Fig.2 (right): Two lateral branches on an upward-bent Aglaophyton axis seen on a naturally smoothed fracture face of an old fragment of a chert layer of 6cm thickness. Width of the picture 20mm.

It appears that forking and downward growth are not the only means of spreading which are available to Aglaophyton. The seldom seen lateral branching is another option [5]. An uncommon example with two lateral branches growing in the same direction, apparently from lateral buds positioned close together on the main axis, is shown in Fig.2.

Fig.3,4 (below): Aglaophyton branching site and related drawing, detail of Fig.2. Note the narrow attachment site without abscission tissue and the inclined lengthwise section of the central strand of the main axis without any indication of xylem branching off into the lateral branches. Width of the main axis 4mm. 
Aglaophyton branchingAg contours
The xylem strand in Figs.2-4 is seen to keep perfectly straight at the site of lateral branching, from which it can be concluded that there is no hidden part of xylem branching off and entering into the lateral branches. Hence, the lateral branches have to develop their xylem strand anew, which means that the lateral branching of Aglaophyton is fundamentally different from forking.


Small tufts of rhizoids are seen on the main axis in Fig.1 and on the lower branch. There are rhizoids on the upper branch, too, but not seen clearly. For lack of space they grow also along the gap between the branches (Fig.5). This peculiar arrangement with an evenly spaced gap is most probably incidental and would not be worth mentioning if it did not suggest a conclusion: Rhizoid growth is not necessarily triggered by contact with soil. It seems to be coupled to the onset of upward growth at the lowermost point of an axis or branch.
Aglaophyton rhizoids
Fig.5 (right): Aglaophyton rhizoids grown along the gap between closely spaced lateral branches,
     detail of Fig.3. Gap width 0.16mm.

The narrow attachment area between the main axis and the branches, the absence of a conducting strand leading into the branches, and the rhizoids at the beginning of every branch make the branches appear like propagules. The absence of an abscission tissue seems to indicate that the branches are not made for breaking off and being carried away by wind and water but for proliferation on the spot.
 
Conclusions drawn from the fossil evidence presented here and elsewhere partially confirm or contradict existing views. They can be summarized as follows:
  (1)  The idea of Aglaophyton growing upwards only, thereby forking, falling over, growing rhizoids when the tip touches the ground [2], is not true.
  (2)  
Aglaophyton does not fall over but can actively change growth from upward to downward [3].
  (3)  It may grow lateral branches from buds not connected to the central strand [5].
  (4)  Rhizoid growth is not necessarily triggered by contact with the ground.

The hollow fork in Fig.1 provides a connection to a quite different observation. The cavity inside Aglaophyton is obviously due to some kind of damage to the tissue. The damage has been ascribed to shrinkage or decay of the dead plant part [4]. This may sometimes or often really apply to the observed cavities but there must be other causes, too. Peculiar arrangements of cavities in Aglaophyton which cannot be explained by mere shrinkage or decay must have been present in the living plant, thus being manifestations of misguided growth [6]. Such conclusion is suggested by the existence of equal void patterns in the prongs of a fork, which could not reasonably have formed independently but must have been inherited from the stem below the fork [7]. Compatible with this idea although less convincing than the mentioned examples [6,7] is Fig.1 showing an apparently even-shaped cavity forking along with the stem.

H.-J. Weiss     2013   2015

[1]  R. Kidston, W.H. Lang : On Old Red Sandstone plants showing structure, from the Rhynie Chert bed, Part IV,
      Trans. Roy. Soc. Edinburgh 52(1921), 831-54.
[2]  David S. Edwards, Aglaophyton major, a non-vascular land-plant from the Devonian Rhynie Chert,
      Bot. J. Linn. Soc. 93(1986), 173-204.
[3]  www.chertnews.de , Rhynie Chert News 14
[4]  www.abdn.ac.uk/rhynie
[5]  W. Remy, H. Hass: New information on gametophytes and sporophytes of Aglaophyton ...,
      Rev. Palaeobot. Palyn. 90(1996), 175-93.
[6]  www.chertnews.de , Rhynie Chert News 4
[7]  www.chertnews.de , Rhynie Chert News 21
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