On Earth, plants are active, aggressive, and sometimes they fight to the death for sunlight. They employ cunning weapons and strategies, both offensive and defensive.
For example, strangler figs (several varieties of Ficus) start as seedlings that germinate up on tree branches and trunks in jungles. As they grow, their roots wrap around the host tree and eventually strangle and kill it. The fig starts halfway up to sunshine, which is an advantage.
But how do the seeds get up there? Birds eat fig fruit, and the seeds have a gluey covering that sticks to a bird’s feathers when it defecates. The bird wipes off its vent on tree branches and trunks, where the seeds adhere and germinate.
To combat this, the kapok or silk cotton tree (Bombax ceiba) grows spike-like thorns on its trunk and branches. Birds would risk injury to clean themselves there.
The photo shows just how risky. This is a white-flowered silk cotton tree in Gambia, and it’s well-armed against strangler fig seed deposits.
Do roses have thorns for the same reason? No. Wild roses clamber over other plants to pursue light, and they anchor their thorns into them to facilitate climbing. Like the strangler fig, they might mercilessly kill other plants in the process, in this case by starving them of sunlight.
In the plant kingdom, thorns are an offensive and defensive weapons in the battle for survival.
Semiosis 