Cactus ribs often come in Fibonacci numbers, and there's a reason
Count the ribs on a cactus and you will often land on a Fibonacci number: 3, 5, 8, 13, 21. Those pleats let the stem swell after rain and shrink during drought, like an accordion. A grown saguaro can soak up around 760 litres in a single storm.
The family Cactaceae has some 1,750 species in about 127 genera, native to the Americas from Patagonia to western Canada. The lone exception, Rhipsalis baccifera, also grows in Africa and Sri Lanka. The name goes back through Latin to a Greek word Theophrastus used for some spiny plant nobody can now identify. Many spiny euphorbias of Africa look similar but are not cacti at all.
Almost every cactus stores water in its stem, and up to 90 percent of a plant's mass can be water. Most have abandoned true leaves; the spines are modified leaves that fend off grazers and also cut water loss by shading the surface and slowing air movement. The swollen green stem takes over photosynthesis. Like many succulents, cacti use CAM photosynthesis, opening their pores to take in carbon dioxide during the cool night and storing it as malic acid until daylight. Shallow, spreading roots grab water the moment rain hits the surface.
The defining feature is the areole, a tiny, compressed side branch that sprouts spines, shoots and flowers. It even identifies oddballs such as Leuenbergeria and Pereskia, which look like ordinary trees, with bark and lasting leaves; Leuenbergeria is thought to resemble the ancestor of all cacti. At the other extreme are tropical climbers and epiphytes with flat, nearly spineless stems, such as the Christmas cactus. One climbing Hylocereus was reported at 100 metres from root to farthest stem.
Size spans three orders of magnitude. Pachycereus pringlei holds the height record at 19.2 metres, while Blossfeldia liliputiana matures at about 1 centimetre across. Small species tend toward globes, which pack the most storage into the least surface. People grow cacti as ornamentals, fodder and fruit, and some host the insect that yields cochineal dye.
Source: Cactus
