What is a cactus. Origin and characteristics of ‘Cactos’ [1].

The word ‘Cactus’ derives from the Greek word ‘Kàktos’.

The term was used by Theophrastus in his book HistoriaPlantarum to describe ‘unknown thorny plants’.

The term ‘Cactus’ was subsequently adopted by the renowned botanist Linneo to define the American thorny plant.

The term has since been discontinued and the term ‘Cactus’ is now used colloquially to define plants belonging to the cactus family, although there is confusion between cacti and succulents.

Succulent is a more general term for plants that have developed a water-storing system in a dry environment, while ‘cactus’ means ‘plants belonging to the cactus family’.

Cacti generally have flowers of unusual shapes, colours, forms and sizes, and are highly valued as ornamental plants due to their ability to adapt to extreme climates.

Distributions

Cactus are widely distributed, mainly in North and South America.

Exchanges with the USA introduced cacti to Europe and other continents.

The genus Rhipsalis is also present in tropical Africa and various species of Opuntia have been introduced into Africa, Australia and India.

Cactus habitats are generally deserts or very dry climates.

Few cacti grow in tropical forests as epiphytes.

Features

Cactaceae are weeds or trees with spiny stems, usually cylindrical, conical, spherical or flattened, commonly distinguished by the succulent macroblast.

They may or may not have photosynthetic leaves.

Shape

The most common forms of cacti are cylindrical (e.g. Espostoa, Cereus and the genus Cleistocactus), which are simple or branched, becoming cylindrical and tree-like over the years.

For example, the Mexican saguaro (Carnegieagigantea) is probably the most famous cactus in the world, growing up to 20 metres tall, and its flowers are the symbol of Arizona.

Some spherical seeds are also present.

This is a species adapted to dry conditions.

The shape of these cacti results in a low rate of surface area/volume.

This means that the surface area decreases in proportion to the volume.

This minimises water loss through transpiration (water always drains away from the surface of the plant).

The most famous spherical cactus is the ‘mother-in-law’s pillow’ (Echinocactusgrusonii), an almost spherical cactus, also known as the ‘golden ball’.

Another stem form is flattened, typical of Opuntias stems.

Opuntias have distinctive flat segments of different sizes depending on the species.

Leaf

Leaves deciduous as soon as the time is right.

The only exception is the genus Pereskia, which is considered the link between cacti and all other plants.

It is classified in the Cactaceae family because of its spinous loci.

Stalk

Cactaceae stems are generally grey-green and fleshy.

It performs the function of photosynthesis, normally retained by leaves, but is reduced or absent in the cactus family.

These stems are characterised by an accordion-like expansion and contraction in response to the amount of water stored.

Lack of water causes shrinkage, wrinkling and dryness.

The stem has spicules, a typical feature.

This is a type of ridge in which spines, hairs, bristles, buds and flowers develop.

The spinous locus carries out the function of a node in other plants.

Even though it looks like a tree and has a hard, woody trunk, its tissue is spongy like that of other cacti.

Thorns/others

Cactus spines differ from those of the Euphorbiaceae family in that they are easily dislodged when spined against the skin.

Spines are solitary or in groups and always develop from the spinous locus.