Editing Cactus (section)

===Stems===
[[File:Starr 030202-0037 Cereus uruguayanus.jpg|thumb|Stem of young ''[[Cereus hildmannianus]]'' subsp. ''uruguayanus'', showing ribbing and waxy coating]]

The majority of cacti are [[stem succulent]]s, i.e., plants in which the stem is the main organ used to store water. Water may form up to 90% of the total mass of a cactus. Stem shapes vary considerably among cacti. The cylindrical shape of columnar cacti and the spherical shape of globular cacti produce a low surface area-to-volume ratio, thus reducing water loss, as well as minimizing the heating effects of sunlight. The ribbed or fluted stems of many cacti allow the stem to shrink during periods of drought and then swell as it fills with water during periods of availability.{{sfnp|ps=none|Anderson|2001|pp=15–37}} A mature saguaro (''[[Carnegiea gigantea]]'') is said to be able to absorb as much as {{convert|200|USgal|l impgal|sp=us}} of water during a rainstorm.<ref name=nps/> The outer layer of the stem usually has a tough [[Plant cuticle|cuticle]], reinforced with waxy layers, which reduce water loss. These layers are responsible for the grayish or bluish tinge to the stem color of many cacti.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

The stems of most cacti have adaptations to allow them to conduct photosynthesis in the absence of leaves. This is discussed further below under [[#Metabolism|Metabolism]].