Tubes at Nematoplexus branch-knots  
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Nematoplexus
Since its discovery [1], new finds of this "enigmatic organism" [2] known from the Rhynie chert only have provided new problems in addition to a few answers to old questions. Apparently it is either unbelievably versatile as a species or it is nearly always associated with one or more other nematophyte species. Anyway, Nematoplexus is very peculiar, as indicated by this picture.


Fig.1: Nematoplexus tubes: spiralling ones (10-12Ám) near two obscure "knots" on the right, a bunch of straight ones below the middle, a long bow dimly seen in the depth, and broader slightly curved ones (18Ám) above. Image width 2mm.

The various aspects of Nematoplexus tubes evident from own chert finds have been described in previous contributions, as in Rhynie Chert News 29, 51, 102, 137.  Nematoplexus knot
The present contribution is meant to draw attention to the so-called branch knots. Usually they are surrounded by a tangle of tubes so the very "knot" is not seen. Since the conspicuous tubes are never seen branching, the simple idea came up that there might be profuse branching inside the knots. This seemed to be compatible with the questionable statement in [1], p.80, that "tubes can be seen entering and leaving them." Hence the term "branch knot" was accepted as if it were an explanation of how the tubes are generated.
There are two "branch knots" in Fig.1, which is a rare coincidence but is not relevant here. The upper one is divided by the big straight crack, which has provided the rare opportunity to illuminate half the knot with reflected light such that a rare sight has become visible in Fig.2: A kind of foot of a spiralling tube below left is seen connected to a kind of surface of the central body of the "branch knot". Another spiralling tube apparently begins on the dark side of the divide with a broad foot, too.

Fig.2: Nematoplexus "knot", detail of Fig.1 above right: other orientation, magnification, illumination, focus depth. 
        Image width 0.2mm.  
Photograph by G. Schmahl. 


Even though Fig.2 may seem rather confusing, it provides some information: The usually hidden central part of the "branch knot" is not simply a dense tangle of spiralling tubes. It is a separate clot with a rugged surface of granular appearance, where the spiralling tubes (11Ám) start from, without any branching. If there is some kind of branching unseen inside, it does not produce the emerging tubes.
Nematoplexus knot relic

Fig.3: Nematoplexus "knot" with 4Ám-tubes and only one stump of the spiralling 12Ám-tubes left.
        Image width 0.11mm.  Photograph by G. Schmahl. 

The globular shape of the central clot may become clearly visible when the spiralling tubes decay and vanish, as with the clot in Fig.3, seen amidst decaying 12Ám-tubes in Rhynie Chert News 133.  This clot seems to be similar to the one in Fig.2: no smooth surface, here with short (?) 4Ám-tubes poking out. Less conspicuous than in Fig.3, a few 4Ám-tubes are also seen in Fig.2. Probably the 4Ám-tubes have mostly been overlooked but on some clots they are absent: Rhynie Chert News 136.


The considerable diversity of Nematoplexus as indicated by Fig.1 is even topped by the big variant described in Rhynie Chert News 135, here Fig.4. It is a clot with a fuzzy surface where the big tubes seem to stick with a club-like broadened "foot". Nematopexus big clot
The big tubes are of the same type as the 18Ám-tubes in Fig.1. The picture plane apparently cuts through the clot near the surface, revealing circular sections of some tube "feet", up to 34Ám, but no big tubes branching. A few thin tubes, 4-5Ám, are seen emerging from the clot. Higher magnification reveals an irregular structure inside the clot but no branching tubes, as seen in Rhynie Chert News 135, there Fig.3.

Fig.4: Nematoplexus "knot" with slightly curved tubes of 17-22Ám and 4-5Ám,
        one separate 8Ám-tube of unknown affiliation; crack stained with hematite.
       
Image width 1.3mm, same scale as Fig.1.
  
Finally it appears that the statement in [3] that "Branching of the tubes occurs in ...very tightly coiled knots of tubes showing repeated and closely spaced branching" does not apply to the conspicuous tubes seen here, spiralling or not. They do not branch, neither inside nor elsewhere. Some of the clots are so small that there would not be enough room for "tightly coiled repeatedly branching" tubes: Rhynie Chert News 136,  there Fig.3.
The idea of "tubes ... entering and leaving" the knots [1] is misleading. Apparently the tubes come from the surface or near the surface of the clot, where they possibly are in contact with an internal structure possibly involving very thin tubes of a different type [1]. However, no tubular structures have been seen inside the above clots, not even with higher magnification in the clot of Fig.4.  
The question concerning the origin of the tubes has been partially answered here but other questions have remained unanswered: Why do the tubes of Nematoplexus vary tremendously even in one chert sample: straight, curved, and spiralling ones with several diameters, smooth-walled and patterned ones ?
Sample: Rh14/35: Fig.3;  Rh15/79: Figs.1,2,4.

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, E.L.Taylor, M. Krings: Paleobotany, Elsevier 2009.
[3]  www.abdn.ac.uk/rhynie/nemato.htm  

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