Anyone who's ever seen a cactus knows that one of the plant's defining characteristics is pointed, sharp spines on leaves and other parts of their foliage. Scientists have long speculated over the evolutionary purposes of these spines, and new evidence and research in this area points to some intriguing conclusions.
Spines as Modified Leaves or Bud Scales
The traditional view held by biologists is that the spines on a cactus are modified version of leaves or bud scales -- layers of protective coating on top of flower buds, according to research conducted by the University of Texas at Austin. This conclusion was based primarily on the discovery that spines are produced in the same part of the plant that produces new leaves and because developing spines closely resemble developing leaves in their cellular structure. The distinctly unleaflike shape of spines was thought to be a product of evolutionary changes that allowed spines to retain more water in desert climates than leaves are capable of retaining.
Spines vs. Leaves
University of Texas researcher John Mauseth, however, have called into question the definition of spines as evolutionarily modified leaves. For one thing, spines do not contain any of the cellular structures that are characteristic of leaves in other plants. Spines in fact contain mostly dead cells; the live cells that propel their growth are found only at the base of the spine. From a more technical standpoint, the microscopic origins of spines, called "primordia" are created in a different part of the the plant than are leaf primordia.
Spines as Extra-Floral Nectaries or Sources of Shade and Protection
Summarizing research he published from 1975 to 2002 in the "American Journal of Botany" and the "The Cactus and Succulent Journal," Mauseth postulates that spines may act as "extra-floral nectaries," specialized glands that secrete a sugar solution in order to attract ants. Though these extra-floral glands only occur in specific species of cacti. Another possibility is that the spines act as protection from sun damage. Cacti that occur in partially shady or rainy areas, for example, usually only have a few spines while those that occur in desert climates are covered with dense layers of spines over their entire surface. Scientists have therefore theorized that spines may provide protection to a cacti from sun damage and other heat and sun-related injuries or from thirsty animals looking to extract water from cacti leaves.
The jury is still out on the evolutionary purpose of cactus spines. There is no doubt that spines help the cacti retain water and are thus part of the evolutionary adaptations that all succulent plants have made in order to survive in desert climates. But the conventional wisdom of spines simply acting as modified leaf structures has been challenged by biological evidence. Theories abound as to the possible purposes of cacti spines, but further research is needed before scientists can definitively say with any certainty why cacti have developed spines.