What Makes a Cactus?

By Joe Merkelbach (July 2009)

Those of us who collect, grow and enjoy cacti know one when we see one and can often give the genus and species. But what is the trait that places a plant inside or outside the order Cactales and its single family, Cactaceae. This is actually a question that is more obscure than we might think.

Is it succulence, thickened and juicy form for the purpose of water retention in arid habitats? No, because this trait is common in several other orders. Succulence is an analogous trait in evolution, one that has developed independently in several hereditary lines in various parts of the world.

Is it spines or sharp, hard edges? No, this trait also has evolved independently all over the world in relation to severe habitats. Spines on cacti have many diverse forms, from sharp and stiff to feathery and soft. However, the points on agaves and some euphorbias can make them as painful to bump as any cactus.

Most cacti lack any leaves at all, but some, such as pereskias, have typical-looking if fleshy ones throughout their lives. Opuntias and their relatives have small tubular leaves during initial growth, but these are deciduous and fall away in dry conditions.

Most cacti are from dry, hot habitats, but deserts are not their exclusive home. Many species are found in other areas – grasslands, shrub savannas, even jungle forests.

Although all but one species of cacti are native only to the new world, this is not the defining trait. Spread over the old world tropics, Rhipsalis buciffera is the exception.

The defining characteristic of cacti is the areole, an organ specific to the Cactales. It is a sort of pit at the axil or base when leaves are present and the source of all branching growth. Branches, flowers, spines, hairs, glochids – even specialized glands all form in areoles. In a few genera, there are two types of areoles, sometimes connected by a groove. These second areoles are the location of nectar glands that attract insects that aid in pollination.

So in a word, what makes a cactus is an areole, the beginning of all sorts of things.

Source:
The Cactaceae, Volume I – Britton and Rose – 1963