Nothia sporangia and their peculiar wall structure
deutsche Version

The early land plant Nothia aphylla found in the Rhynie chert below the hill with the obscure name Tap o' Noth shows a unique structural component: aligned tube-like cavities called "giant cells" in [1], where it is claimed "that they are true cells". Observations [2] have led to the conclusion that the big tubes most probably are of a more complex origin involving the dissolution of numerous cell walls. Independent of this feature it seems appropriate here to draw attention to the fact that the spore capsules of Nothia are seen in a variety of shapes on the surface of the chert samples and on cut faces, hence it may be difficult to tell them apart from capsules of Horneophyton and Asteroxylon which are often found together with Nothia in the same sample. (Sporangia of Asteroxylon are rare.) While the more abundant spindle-shaped sporangia of Aglaophyton are easily recognized by their mostly elliptical or circular sections, the irregularly shaped more or less bulky bags which are the Nothia sporangia may provide odd-shaped sections beside occasional circular ones. 
sporangia and xylem strands of Nothia seen on a fracture face of Rhynie chert14 Nothia sporangia contours on less than 2 square centimeters

Xylem strands and sporangia with spores are obviously more resistant to rot than the soft tissue of the plant so that they are closely stacked in some places in the chert.


Fig.1 with drawing: Nothia sporangia and xylem strands on an old fracture face of a chert layer smoothed by weathering. 14 sporangia are visible on this area of less than 2cm2. Note also the numerous persistent xylem strands below.


Apparently the shapes and sizes of the sporangia vary within wide bounds, which is obvious from the three pictures below, with equal scale. Some are nearly globular, others are flat like the uncommonly wide one in Fig.3.


3 Nothia sporangia of different aspect extremely wide Nothia sporangiumNothia sporangium sections big and small
Figs. 2,3,4:
Width 4.8mm, 6mm, 3mm.
Nothia
sporangia with spores
varying between black
 (below left in Fig.2)
and completely bleached.
Note the coherent bag of 6mm width
 in Fig.3.





Beside the variability of the capsules, the tube-like cavities in their walls are worth looking at closely. Although the sporangia seen in the chert can be numerous, the tubes in the wall are seldom preserved such that they are clearly visible, and if preserved well, they are not well seen when tilted with respect to the cut plane. Hence, the below pictures taken with incident light on arbitrarily cut faces have been brought about by lucky coincidences.

Fig.5 (left): Three cross-sections of wide tubes below the surface of a Nothia sporangium, with pale fill.
Fig.6: Two sporangia of Nothia, the left one being in the depth and thus seen from outside.
Fig.7: Detail of the capsule wall in Fig.6: wide tubes hidden behind the long narrow strips between the cell files.
Nothia sporangium wall section with big tubes 2 Nothia sporangia, one beneath the cut faceNothia sporangium wall detail with long narrow strips  
  Widths of Figs.5,6,7:
  1.4mm, 2.5mm, 0.38mm.





The rare case of contrast by the white chalcedony fill of two wide tubes in Fig.5 shows that the tubes are essentially enclosed within the sporangium wall. A narrow ridge extends towards the surface where it appears, when looked at from outside, as a narrow long strip, about 10Ám wide, as seen between cell files in Fig.7. The tubes themselves are about 100Ám across in Fig.5. (According to [1], they can be up to 1.6mm long and 0.2mm across. Hence the volume of one tube equals that of hundreds of normal cells of the capsule wall.) There is at least one more cross-section of a big tube in Fig.5, vaguely seen on the left, with a pale bluish spot inside. (The bigger white spots inside the capsule are not relevant.)
The epidermis cells arrange themselves in files along the strips, which makes the "textile" aspect of the capsule in Figs.6 left and in Fig.7. (The brown "egg" in Fig.7 is a separate spore incidentally placed in front of the capsule.)
From the complex structure of the capsule wall as seen here and in [2] it can be concluded that the evolution of Nothia involved quite a number of developmental steps, which distinguish it from other "early land plants" and suggest a closer relationship with the more advanced zosterophylls, which has been suspected [1] but apparently never confirmed.
As a well-known fact, tiny contaminations in the silica gel can largely affect the aspect of the chalcedony owing to differential silica grain sizes formed in silicification. Hence, the white fill in some of the tubes possibly indicates a content which had been chemically different from the surrounding tissue before silicification, which would be compatible with the idea of tubes releasing poison or glue if tapped by creatures trying to get at the tasty spores. This idea differs essentially from others proposed in [1], including "large water reservoirs".
Since the surface tension of liquids is a dangerous force for small creatures, an earlier statement [2] may be repeated here:  "If tapped by a spore eater, a tube under pressure would spill its content profusely and soil the biting creature with sticky liquid with possibly fatal effect."
All pictures have been taken from one chert sample of about 0.25kg found in 2003.

H.-J. Weiss    2013,
  emended 2014

[1] H. Kerp, H. Hass, V. Mosbrugger: New data on Nothia aphylla,
     in: P.G. Gensel, D. Edwards (eds.): Plants Invade the Land, N.Y. 2001.
[2] Rhynie Chert News 33
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