Final answer:
Touching thorns causes more injury compared to other parts of the plant because the thorn's design concentrates force over a tiny area, resulting in higher pressure that can break the skin.
Step-by-step explanation:
In terms of pressure, touching the thorns on a stem is more likely to cause injury than touching other parts because thorns are designed to concentrate force over a very small area.
This is related to the concept that a given force exerted on a smaller area result in greater pressure. In simpler terms, the smaller point of a thorn means that the same amount of force applied by your hand is concentrated over a smaller spot, increasing the pressure dramatically compared to touching the wider areas of the plant's stem.
Rigid and pointed, thorns have evolved as protective structures to deter herbivores and others from touching or eating the plant. The increased pressure exerted at the tip of a thorn can break the skin, which could explain why they are considered to be potentially deadly.
This is akin to a knife being more effective when sharp because its ability to cut is enhanced by the high pressure exerted at its edge, or why a sharp hypodermic needle causes less pain than a dull one; in both cases, sharpness minimizes the contact area and maximizes pressure.