Nematophyte tubes and "branch knots"
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Among the questions arising in connection with the enigmatic nematophytes, two of them seem to be most intriguing: Where do the tube-like filaments come from, and what is hidden inside the lumps known as branch-knots ? It has been tempting to answer the two questions by reducing them to one phenomenon: The tubes have been supposed to branch profusely [2] inside the branch-knots, then come out. The idea that the tubes seen outside the knots are also present within has been supported by the questionable statement in [1], p.80, that "tubes can be seen entering and leaving them.".
It is emphasized here that there is evidence from several nematophytes that the clots called branch-knots are not mere dense tangles of tubes, branching or not, but objects of their own. They can be so small that there would not be enough space inside for "very tightly coiled ... tubes showing repeated and closely spaced branching" [3], as seen in Rhynie Chert News 136, there Fig.3.
The big clots of the hitherto unknown nematophyte in Rhynie Chert News 92 are distinguished by a definite boundary and an apparently mainly structureless interior. The absence of tubes emerging radially from the clots enables the tubes to form a parallel texture. Probably they emerge from the clots in such a way that they fit into the texture immediately. small knot in nematophyteA smaller clot among the big ones in Rhynie Chert News 92 , there on Fig.2, seems to be different: Tiny reflecting crystals, perhaps pyrite, indicate a deviating chemical composition, possibly due to another stage of growth at the time of silicification: here Fig.1. The vaguely seen undefined structure inside the clot could possibly provide the expected but elusive connection between interior and surroundings. Shrinkage cracks in the gel outside the clot may mislead to the illusion of cell walls, which is not relevant here.

Fig.1: Small clot with vaguely seen structure of obscure significance, surrounded with nematophyte tube cross-sections up to 60Ám. Image width 0.4mm. Sample: Rh2/81.3.

Obviously, the big tubes in Fig.1 are unlikely to branch within the small clot but the hardly seen much smaller tubes might do so.

nematophyte clot
Unexpected support for the idea of "normal" tubes being somehow connected to small branching tubes inside clots comes from another unknown nematophyte (in Rhynie Chert News 13 , 156 , there Fig.4), which does not grow lots of clots with definite outline as the one in Rhynie Chert News 92 and Fig.1. The clot in Fig.2 is the only one seen in this specimen, and it has not got any visible boundary but a strangely shaped object inside which could possibly connect the poorly seen thin branches inside to the big tubes outside.

Fig.2: Nematophyte clot with branching object and small tube parts inside, surrounded by nematophyte tubes with cross-sections up to 60Ám. Image width 0.6mm. inside nematophyte knotSample: Rh13/7.1.

Fig.3, detail of Fig.2: Connecting piece (?). Image width 0.15mm.

Figs.2,3 are compatible with the less conventional interpretation that the 60Ám-tubes seen outside do not emerge from inside the clots but are formed on their surface by fusion of smaller tubes: about 20-25Ám with this specimen.
Nematoplexus tube formed at clot surface
Fig.4 (right): Small Nematoplexus clot with the onset of an 11Ám-tube. Image width 0.1mm. Sample: Rh15/79.4.

The phenomenon of tube formation from two or more smaller ones is apparently rather common. It has been observed recently, on a smaller scale, with Nematoplexus:
Fig.4, which is a detail of Fig.2 in Rhynie Chert News 134, shows one of the rare occasions where a tube is seen being formed near the surface of a clot. Here, the onset of an 11Ám-tube is made up of two very short converging parts, 6Ám and 8Ám wide, possibly connecting the tube with some internal structure. There are no clearly seen tubes inside the clot.
Note the largely differing tube sizes among the nematophytes: up to 60Ám and more in the undescribed species (Figs.1-3), and mostly 7-15Ám in Nematoplexus.

Samples: All photographs taken by Gerd Schmahl, Dresden.

Fig.1: Rh2/81 (0.63kg), obtained from Shanks in 2003, Part 3.
Figs.2,3: Rh13/7 (0.25kg), found by S. Weiss in 2005, Part 1.
Fig.4: Rh15/79 (0.27kg), obtained from Barron in 2014, Part 4.

H.-J. Weiss       2020

[1]  A.G. Lyon: On the fragmentary remains of an organism referable to the nematophytales, from the Rhynie chert, Nematoplexus rhyniensis.  
      Trans. Roy. Soc. Edinburgh 65(1961-62), 79-87, 2 tables.
[2]  T.N. Taylor et al.: Paleobotany. Elsevier 2009, p.180.
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