Damage control in an early land plant

damageThere 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 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 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 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 7mm.  damage response

Although 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. This 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.
new cellsCell 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 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. 

damage repairedFig.4 (below right): Hole in the rot-resistant peripheral layer repaired with a rot-resistant cap.

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 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 [1]. That explanation has already been rejected.
Fig.4 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 hole 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 wall of 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.   

H.-J. Weiss    2017

[1]  www.abdn.ac.uk/rhynie, Chapter Taphonomy.

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