The phenomenon of thorns on plants is an interesting one. It invariably comes about when the soil has become severely depleted. So, for instance, the thistle typically comes up when a paddock has been too heavily grazed, as was pointed out to us in an initial course of plantecology. When I realized that, it seemed to me that the thorns on roses must similarly be the result of some nutrient deficiency.
It is almost as if the rosebush is trying to say: “Stay away from me, you have already plucked so many of my flowers and you never give me anything in return.”