Editing Cactus (section)

===Leaves and spines===

The absence of visible leaves is one of the most striking features of most cacti. ''[[Pereskia]]'' (which is close to the ancestral species from which all cacti evolved) does have long-lasting leaves, which are, however, thickened and [[Succulent plant|succulent]] in many species.<ref name=EdwaDono06/>  Other species of cactus with long-lasting leaves, such as the opuntioid ''[[Pereskiopsis]]'', also have succulent leaves.{{sfnp|ps=none|Anderson|2001|p=572}} A key issue in retaining water is the ratio of surface area to volume. Water loss is proportional to surface area, whereas the amount of water present is proportional to volume. Structures with a high surface area-to-volume ratio, such as thin leaves, necessarily lose water at a higher rate than structures with a low area-to-volume ratio, such as thickened stems.

[[Thorns, spines, and prickles|Spines]], which are modified leaves, are present on even those cacti with true leaves, showing the evolution of spines preceded the loss of leaves. Although spines have a high surface area-to-volume ratio, at maturity they contain little or no water, being composed of fibers made up of dead cells.{{sfnp|ps=none|Mauseth|2007|p=845}} Spines provide protection from [[herbivore]]s and camouflage in some species, and assist in water conservation in several ways. They trap air near the surface of the cactus, creating a moister layer that reduces evaporation and [[transpiration]]. They can provide some shade, which lowers the temperature of the surface of the cactus, also reducing water loss. When sufficiently moist air is present, such as during fog or early morning mist, spines can condense moisture, which then drips onto the ground and is absorbed by the roots.{{sfnp|ps=none|Anderson|2001|pp=15–37}}