Damage control in an early land plant
deutsche Version

damage 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 living plant, similar as with hollow trees. Elusive herbivores gnawed holes into the shoots or sporangia 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. (See Rhynie Chert News 117, Fig.6.)
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.
The response of the plant to herbivore attack can be more or less intense. Obviously, the response had been more 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 a 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 grown in 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, other cortex tissue subsequently vanished owing to rot, resulting cavities clad with agate-like linings; lower hollow straw squeezed flat, apparently with similar damage as above, incidentally touching the upper one. Width of the picture 7mm.  damage response

The confusing structure in Fig.2 reveals some response of the living plant to herbivore attack. Apparently the vicinity of the tunnel eaten into the plant had been made rot resistant to avoid the spreading of rot through the tissue. This is similar as with Fig.1 where the newly grown big cells seem to have precluded early rotting.
While most 
of the tissue in Fig.2 decayed, the resistant vicinity of the tunnel remained as an odd tube and became silicified later.
new cells
Cell 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 2mm.   

As a confusing and not yet explained complication,
Aglaophyton may or may not have got a resistant peripheral layer of cortex tissue (Fig.2) below the often inconspicuous or decayed epidermis. The dark or black appearance of the walls of the resistant cells seems to be a secondary phenomenon, perhaps a microbial cladding which can chip off.

damage repaired
Fig.4 (below right): Hole in the rot-resistant peripheral layer repaired with a rot-resistant dome-shaped cap of 2mm width.

A peculiar case of damage control is shown in Rhynie 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 still been there and living 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 [1]. That explanation has already been rejected.
Figs.2,4 suggest 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, thus conspicuous in advanced stages of decay when 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 hole with a cap of elegant design. Hence, the plant had been able to respond to impending danger or attack in flexible ways, which is by modification of cortex tissue into special layers (Figs.2,4) or by activation of cell growth (Figs.1,3).
The term "hollow straw" used in connection
with Aglaophyton preserved as a persistent tube might suggest the idea of retaining some strength while most of the tissue being decayed. A small hole in the wall of the straw would not much affect strength so that elaborate repair as in Fig.4 suggests a different explanation: The persistent tubes, in Aglaophyton peripheral only but in Ventarura always amidst the cortex tissue, are less for strength than for protection. They possibly delayed the decay. It remains obscure whether tissue decay in Figs.2-4 proceeded in the live or dead plant.   
Samples: Fig.1: Rh6/9.2 (2002), Fig.2: Rh15/82.4 (2014 obtained from Barron), Fig.3: Rh2/68.1 (2002 obtained from J. Shanks), Fig.4: Rh12/162.2 (2007).

H.-J. Weiss    2017

[1]  C.L. Powell, N.H. Trewin, D. Edwards: Palaeoecology and plant succession in a borehole through the Rhynie cherts, ...
      Geological Society, London, Special Publications 180 (2000), 439-457.


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