Why Plants Grow Thorns, Spines

While a rose by any other name may smell sweet, it will still hurt you if you are not careful of the thorns. Plants around the world have developed these sharp, pointed and painful extensions on their stems, leaves, and fruit. These evolutionary weapons of the plant world serve several very important functions for the vegetation that produces them.

Plants are well known for not being able to run away from danger. Being rooted to the ground can leave one at the mercy of any hungry animal that might wander by its plot of soil. Thorns and spines are a way to help protect the plant from being eaten. Whether through brushing against the plant or taking a bite, the pain experienced by the animal will make it very wary of getting close or biting it again. As a further hedge against being eaten, many of these sharp projections are also toxic. Others will damage the digestive tract of the animal so badly that they will die and thus not eat any more thorny plants.

Basic survival from animal consumption is not the only reason that plants have developed thorns and spines. Many species have utilized their thorns as a way to help hook onto other plants and trees so they can better position themselves in the sunlight. Vines such as the climbing hydrangea use their thorns this way.

Many plants use thorns to help disperse their seed. Some plants which do not grow thorns on themselves will still develop seeds that have a variety of barbs, thorns, prickles and other sharp extensions on the shell that will catch in animal fur as it brushes by. The seed is then carried far from its parent plant before being brushed or pulled off. Other plants like datura, will grow a thorny pods around their seed to protect them from consumption before they ripen and the pod splits to release them. There are some thorny plants that do not use their barbs to spread seed but are able to have sections of themselves ripped off from the parent plant and reroot themselves elsewhere.

Thorns are a woody extension of the stem of a plant and are made up of the same material. Spines on the other hand are modified leaves. While they are also very effective at keeping some animals from eating them, spines generally offer a different type of protection. Prevalent in hot, arid climates, spiny plants like the cactus need to conserve what moisture they can hold on to. Spiny leaves offer a minimum of exposed surface area to lose water through. Cacti keep their chlorophyll for photosynthesis in their stems rather than the spiny leaves. The leaves are stiffened with calcium carbonate and pectin. Thick spines also help provide a bit of shade for the stem of the plant.