 |
 |
The Cactaceae family is sub-divided into three tribes - Pereskieae, Opuntieae, and Cacteae. These were first proposed by Nathaniel Lord Britton and Joseph Nelson Rose in their four-volume "Cactaceae" which was published between 1919 and 1923. This formed the basis for the classification of cacti as it is today.
Cactaceae is just one of the many families of plants that are known as succulents - plants that store water in their leaves, stems, and roots - and is in fact the biggest with over 2,000 species. Other succulent plant families include euphorbias, milkweeds, and lilies.
With only a very few exceptions, all members of the Cactaceae family are native to the Americas - being found as far North as Canada and as far South as Patagonia.
|
|
It is thought that early Cactaceae developed very early on in the history of plants, and as such, the botanical classification of the species in the family is based on the supposed age as well as apparent similarities.
Essentially the Cactaceae are divided into four chronological categories:
| Plants that still produce leaves. This category consists almost entirely of the genera Pereskia, for example Pereskia nemorosa. | Plants that produce leaves but then lose them very quickly. A good example of such a cacti is Opuntia subulata. | Plants with rudimentary leaves akin to scales, or no leaves at all. An example of a cacti with scales is Obregonia denegrii. | Plants with stems that resemble leaves, epiphytes, or semi-epiphytes. Such as the Christmas Cactus Zygocactus truncatus. |
 |
 |
 |
 |
These categories assume that succulents lost their leaves in order to adapt to dry climates, but do not preclude that the Cactaceae themselves developed in some other way.
The distinguishing feature of the Cactaceae - apart from their obvious adaption to dry conditions (i.e. low moisture loss and succulent tissue) - is that they all produce areoles. These are round or elliptical buds from which any flowers, fruits, new segments, spines, bristles, or wool emerge. Typical areoles are shown clearly here on Weberbauerocereus johnsonii.
Areoles are unique to the Cactaceae - not being found in any other plant family. There are several other features that set the Cactaceae apart from other species, but they are not common to all in the family.
The stems of the various cacti vary depending on their type, from woody (in those with persistent leaves) to fleshy. The majority of cacti have no leaves as such and so have to rely on their stems to photosynthesise, their shapes having evolved to reduce the amount of moisture lost.
The stems also reduce moisture by having a waxy skin covering, and in some species with a dense coat of spines or wool. There are many species that have a covering of wool such as Espostoa lanata.
Occasionally some succulent plants' stem tissues can grow in a distorted manner - often a result of bacteria or other physical causes - giving rise to "Monstrous" forms as shown by this Rebutia senilis which is normally a globular cacti.
These can later revert back to their more normal forms unless grafted.
Though the majority of cacti produce flowers, it may not always seem that way. It is always difficult to get certain species to flower in captivity and others only flower once they are quite old. It has a lot to do with the environment they are in.
Cacti are not really suitable as house-plants, their natural habitat does not compare with modern centrally-heated houses, thus it can be quite a task to coax some into flower. Those genera that are more keen to flower on a window-sill are Mamillaria, Rebutia, Gymnocalycium, Rhipsalidopsis, and Zygocactus. A good example is the Rebutia miniscula.