Palaeonitella
whorls
-- young and old
On the occasion of the recent centenary
of the Devonian charophyte alga Palaeonitella
[1] it may be appropriate to cast
a brief look on its whorls of branches once more, although a
lot of images has been offered before.
Compared to the recent Nitella
with spacings between the whorls of branches up to several centimeters,
Palaeonitella
is tiny, with spacings of 0.5...3mm.
Fig.1:
Palaeonitella
top, branches of upper whorls incurved, protecting
the growing tip, filled with bluish chalcedony.
Fig.2: Palaeonitella
whorl, hollow or filled with mineral grains.
Image sizes 1.6mm,
same chert sample.
The different aspect in these pictures can be
ascribed to age in this case. Fig.1 seems
to show "young" whorls with incurved branches
protecting the growing alga top, filled with freshly formed silica gel
turned into bluish
chalcedony now. The whorl in Fig.2 looks old with the yellowish stain
whose deposition on the walls and
inside the hollow tubes must have taken some time. Also this picture serves as evidence of the alga cell wall acting as a
diffusion barrier separating spaces of different silicification regimes
within and without. This is compatible with the observation that silica
gel may turn into chalcedony as in Fig.1 or dissolve, leaving behind
mineral grains as in Fig.2.
Sample:
Rh5/3, Part 2 , found in 2001 near Milton of Noth by S. Weiss.
H.-J. Weiss 2022
[1] R. Kidston, W.H. Lang:
On Old Red Sandstone plants showing structure ... Part V...
Trans. Roy. Soc. Edinburgh 52
(1921), 855-902.
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