Thorns

the hard, sharp-pointed formations on plants, which are the result of the metamorphosis of the stem, leaf, or root (or their parts). The wood inside the thorns is united with the wood of the organ that produces them. In a thorn of leaf origin, the entire leaf (in barberry and citrus fruits), its middle vein (the pinnate leaf in some species of milk vetch and pea shrub), or the stipules (in black locust and christ’s-thorn) are transformed. If the thorns are modified shoots (in hawthorn, honey locust, blackthorn, crab apples, and pears), they are either leafless or bear a few poorly developed leaves. Thorns may also develop from adventitious roots at the lower part of the trunk (in palms of the genera Acanthorhiza and Iriartea). Thorns are most often found in plants in arid and hot regions. The biological significance of thorns, as well as of needles, is the protection of plants from animals. In addition the replacement of branches and leaves by thorns decreases the plant’s evaporative surface and, thereby, its transpiration.