Damage control in an early land plant
There are a
few causes of damage in
early land plant tissues. The tissue between epidermis and
central strand, called cortex, could have partially decayed in the
plant, similar as with hollow trees. Elusive herbivores gnawed holes
into the shoots
to eat the content. Holes in the tissue of
living plants may even arise
not by partial decay of tissue but by pushing it aside. As a rare coincidence,
these two types of cavities are seen together in one cross-section.
Chert News 117,
Some early land plants had already developed precautions against
herbivore attack: bristles on the outside, tubes with deterrent
liquid (?) along the epidermis, or a protective
wall (?) in the cortex.
response of the plant to herbivore attack can be more or less intense.
Obviously, the response had been intense with the early land plant in
Fig.1 where big cells had grown around the gnawed hole.
Fig.1: Cross-section of Aglaophyton
with big cavity due to some herbivore, big cells around, a
few dimly seen sections of coated fungus hyphae grown later in the
water-filled cavity, levels
of watery suspensions, agate-like lining and quartz crystals finally
the cavity of reducred size. Width of the picture 6mm.
Fig.2 (below): Cross-sections of Aglaophyton with "hollow
straw" aspect, upper one with access hole
in the wall and adjoining tunnel
with surrounding tissue made rot-resistant as a
response to herbivore
attack, other cortex tissue subsequently vanished owing to rot,
resulting water-filled cavities clad with agate-like
linings; lower hollow straw squeezed flat, with "scab" incidentally
touching the upper one. Width of the picture
most of the tissue in Fig.2 had decayed before the cross-section had
got its present aspect, one can assume that the plant responded to the
attack by making the adjoining tissue rot-resistant for the purpose of
containing the rot which otherwise would quickly spread throughout.
is similar as with Fig.1 where the newly grown big cells seem to have
precluded early rotting. If the structure had not been fixed by
silicification, rot would probably have consumed the cortex tissue
later, except the resistant cavity walls.
growth for the repair of the boundaries of a hole is seen with slightly
higher magnification in Fig.3.
Fig.3 (right): Cell growth with the aim to fill a hole. Width
As a confusing and not yet explained complication, Aglaophyton may or
may not have got a peripheral layer of
cortex tissue (Fig.2) which is rot-resistant independent of an attack
from outside. Furthermore, the dark or black appearance of the walls of
the resistant cells seems to be a secondary phenomenon, not always
coupled to rot resistance.
(below right): Hole in the rot-resistant
peripheral layer repaired with a rot-resistant cap.
A peculiar case of damage control is shown in
Chert News 60,
Fig.4, which has been reproduced here (Fig.4). This dome-shaped cap
must have been formed when the now vanished cortex tissue had been
still there since it consists of cortex cells made rot-resistant. This
allows conclusions to be drawn concerning the "hollow straw"
phenomenon. It had been explained as a mere diffusion effect in the
sense that dissolved silica entering from outside into a limited depth
preserved only a narrow seam of tissue . That explanation has
already been rejected.
suggests another scenario: The plant made a peripheral seam of modified
cortex tissue, perhaps as a protection of the tissue below. This seam
is rot-resistant and therefore well seen in
advanced stages where most of the cortex tissue is no more there. This
protective seam had got a hole for reasons unknown. The plant covered
this potentially dangerous
with a cap of elegant design. Hence, the plant had been able to protect itself by making
a barrier against intrusion by modification of cortex cells.
The term "hollow straw" occasionally used in connection with
a persistent seam might
suggest the idea of retaining some strength while most of the tissue is
decaying. This may be true but there must be another purpose since the small hole in the
the straw would have a negligible effect on strength so that elaborate repair would
not be appropriate. Hence, Fig.4 leads to the assumption that the
persistent tubes, often peripheral in Aglaophyton and always inside Ventarura, are less for strength than for protection.
 www.abdn.ac.uk/rhynie, Chapter Taphonomy.