Erroneous and other views on a Rhynie
chert plant
In the first paper on Rhynie chert plants [1], the two species now
known as Aglaophyton
major and Rhynia
gwynne-vaughani were treated as
one and given the latter name. When it became apparent that there were
two, the bigger one was named Rhynia
major. It was thought to be bigger
than it really was, and its reconstruction was rather different from
the present ones. (See drawing below left.)
It is one of the crazy things happening sometimes that the poor
reconstruction raised a great idea in the mind of the German
paleobotanist Zimmernann
[2]. The result was his Telome Theory of plant
evolution, which became, and still is, widely accepted but also much
disputed. His construct of an archetypal plant was meant to represent
the simplest early land plants. So it is surprising to see that the
real plant is even simpler than the model plant derived from it.
According to D.S.
Edwards
it grows in only one direction, which is
upward, until it falls over, grows rhizoids where it touches the
ground, and turns upwards again [3]. Falling over is not quite a bad
idea as the plant would finally do so if it had not invented a more
clever option which consists in turning downwards intentionally. The
latter is obvious from the observation that the radius of curvature of
the downward bend is as small as about 1.5cm or even smaller.
As indicated in the drawing on the right, the
overall shape may simply
be dominated by repeated forking*, with one or two prongs of the lower forks
growing downwards on purpose, touching the ground, etc. This reminds
one strongly of the habit of roots. Hence, what we see here might be a
glimpse at roots in the making.
As pointed out by Dianne Edwards [4], the change of name into Aglaophyton by David
S. Edwards [3] was not well justified and the placement outside the tracheophytes by others is questionable.
The reason why it took a long time to find out the overall shape
of the most abundant Rhynie plant is the preservation in chert, which
provides tiny detail even inside cells but does not easily reveal the
large-scale structure. The chert is essentially isotropic and hence
does not show cleavage planes with compressions of whole plants.
Reconstruction by cutting thin slabs or by successive removal of chert
is laborious since fresh and more or less decayed specimens are
usually silicified in a confusing tangle.
In
view of this, it would not be surprising if the
reconstruction of Aglaophyton
had to be further modified.
H.-J.
Weiss
2007, emended 2014
*According to other and own observations,
this plant is able to branch not only by forking but in the lower parts
also by lateral buds growing into shoots with rhizoids. (See Rhynie
Chert News 52). Hence,
this plant may be more tangled near the ground than suggested by the
drawing on the right which is to visualize the principle of downward
growth. As another observation, the upward bend is usually not nicely
U-shaped, as already indicated in the 1986 drawing.
[1] R.
Kidston, W.H. Lang
: On Old Red Sandstone plants
showing structure from the Rhynie Chert bed, Part I,
Trans. Roy. Soc. Edinburgh 51(1917),
761-84.
[2] W.
Zimmermann
: Die Phylogenie der Pflanzen, 1930, 2.
Aufl. 1959.
[3] David
S. Edwards, Aglaophyton
major, a non-vascular land-plant from the Devonian Rhynie
Chert,
Bot. J. Linn. Soc. 93(1986), 173-204.
[4] Dianne Edwards: Embryophytic sporophytes in the Rhynie and Windyfied cherts,
Trans. Roy. Soc. Edinburgh, Earth Sci. 94(2004 for 2003), 397-410.
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