What is special about Voynich plants?

[Pierre Dumont Himself]

The fat symmetrical loop is probably impressionistic, but there are multiple causes for smaller splits in nature:
excessive water flow through vascular tissue, heat, cold,  or wind damage, bacterial  or fungal infections, etc. It's more common to see cracks, but splits are absolutely possible.
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Tomato stem splitting due to environmental conditions.

It could also be an exaggeraated hole caused by beetles or various larvae:

Tomato stem damaged by a borer.

It's also possible that there's no splitting, and what we see is an artifact of how the dried out plant was pressed in a herbarium:

Malva parviflora from Florida

Malva sylvestris from Kent.