That's a Cactus??
What is a cactus? Cacti are New World succulent plants, often, but not always, with spines and usually, but not always, leafless, and that have well-developed flowers complete with stamens and petals. At least that is one definition. Another was any plant member of the family Cactaceae, which I found fairly useless in a circular sort of way. Cacti can often be confused with other plants, particularly other spiny plants that are columnar- or barrel- shaped. Many Euphorbia species fit this description and are erroneously referred to as cacti. But most Euphorbia are old world species and have very different flowers. All cacti, with the exception of one species of African Rhipsalis, are from the New World. But not all cacti look like cacti.
Echinopsis terscheckii, example of a typical columnar cactus
Ferocactus pillosus, a typical spiny barrel cactus; and Opuntia gosseliana in the second photo- these are what most people think of when they think of cacti
The following are some of the less typical cacti.
Ariocarpus. I am not even sure how these plants were identified as cacti as they are very ‘un-cactus-like.' They all have large, tuberous roots and only the thick leaf-like tubercles extend barely above ground. These plants have no spines whatsoever. In fact, they look more like some Aloe relative (like Haworthias perhaps?) than cacti. But their flowers are much more cacti-like, being large, well-developed and colorful. These mostly Mexican succulents are some of the most sought after and prized cacti in collections as they are very slow-growing; large specimens are worth a fortune.
Ariocarpus varieties can look a bit like some other completely unrelated succulents. First photo is of a few of my seedlings; second photo is of Ariocarpus agavoides, and third photo shows the flowers on one of my Ariocarpus sp.
Ariocarpus retusus varieties
My own Ariocarpus kotschoubeyanus barely 1/2" tall, with a huge tuberous root below the gravel
Astrophytum. This genus has some species that are not obviously related to cacti at first glance, even despite their cactus-like shape (globoid and leafless). Some are mostly spineless and a few completely spineless. They have Euphorbia-like bodies, but definitely cactus-like flowers. These are among some of the easiest and most popular of all the pot-grown cacti there are.
Astrophytum myriostigma and Astrophytum asterias both flowering; third photo are some seedling Astrophytum myriostigmas for sale
Very old Astrophytum myriostigma (true collector's item) and a weird sport of the same species called 'Lotusland'.
Disocactus. The most common species of this genus, Disocactus flagelliformis, looks at least somewhat like a cactus in that it has lots of itty bitty spines. But the plant itself is made up of many rat tail-like drooping tubes that develop large, flamboyant pink flowers in spring. The other Disocactus species look even less cactus like in that many have no spines at all and are completely flattened, trailing, epiphytic plants (also known as orchid cacti, though there are other orchid cacti genera).
Disocactus flagelliformis (aka Rat Tail Cactus); and a bit more 'typical' Disocactus phyllanthoides (photo by boojum)
Epiphyllums. These epiphytic, flattened, trailing plants are very similar to the orchid cacti genera and though familiar to most people who have any plant experience, they are not always identified correctly as cacti. These produce flowers that have been hybridized into many dozens if not hundreds of color shapes and sizes, most spectacular and large. The non-hybrids are virtually all white, night-blooming species. Few have any but a hint of spines on them.
Epiphyllum 'Fruhlingsgold'; Epiphyllum oxypetalum (photo by Happenstance); and Epiphyllum phyllanthus (atypical form)
examples of some of the fantastic flowers of some Epiphyllum hybirds: Epiphyllum 'Block Party'; Epiphyllum 'French Gold'; and Epiphyllum 'Mondore Bell'
Hatioras are also epiphytic cacti that don't look all that cactus-like. These are highly segmented plants made up of flattened or tubular sections and are closely related to the Rhipsalis cacti (see below).
Two tubular species: Hatiora salicorniodes and (in second photo) Hatiora epiphylloides
These two are often referred to as Easter Cacti: Hatiora rosea (photo by PotEmUp) and Hatiora gaertneri because they always seem to bloom conveniently around Easter time
Hylocereus species are climbing rather than epiphytic plants, but they are similar to Epiphyllums in that they are largely spineless, flattened trailing plants with large, white, night-blooming flowers.
Hylocereus undatus in southern California (plant and flower); Hylocereus ocamponis fruit (also known as Dragon Fruit)
Lepismium species are another flattened, trailing spineless genus that has few onlookers thinking ‘cactus' when they see these plants. I have a Lepismium houlletianum and it definitely does not look like a typical cactus.
two shots of my Lepismium houlletianum
Leuchtenbergia principis is the only species in this genus of ‘Agave cactus'. And it is well named as it looks more like an Agave than a cactus. It does have spines, but they are modified to look like dead, grass-like appendages rather than spines. It is not until this plant flowers that you can see it is probably a cactus.
two shots of my Leuchtenbergia and one of an older specimen in a nusery with a bit of a woody stem
Pereskias. Though most cacti can be identified easily by their numerous spines on a leafless, relatively featureless body, some cacti do have leaves and even quite substantial leaves at that. The Pereskias are a notably profoundly leafy species of cactus native to various islands of the Caribbean down to Argentina. This is a variable group of plants but most look like your average shrub or tree, only with succulent leaves and some of the most intensely sharp, long spines in the entire family. I have grown Pereskia grandiflora, one of the more common species in cultivation, and it certainly looks very little like a cactus despite its lethal spines.
Pereskia grandiflora in my yard (amongst the more 'typical cacti'), and a large 'tree' specimen in Huntington Botanical gardens, and shot of mine in winter when leafless showing the spines
leaves, flower and fruits of Pereskia grandiflora
Rhipsalis is a large group of epiphytic, trailing cactus that look more like some species of mistletoe than members of the cactus family. Not only are these not very cactus like in appearance, but they also tend to be tropical, thirsty plants that do not like a lot of hot, sun exposure nor are all very drought-tolerant. Some are spiny but most are not. Even their flowers are not very cactus-like being small or nearly absent. I am not even sure why these are included in the cactus family.
Rhipsalis species of mine that is actually spiny; Rhipsalis baccifera middle photo, and some hybrid called Rhipsalis 'Dreadlocks'
Rhipsalis grandiflora growing in the ground (first photo) and as epiphyte (second photo)
This fine, almost thread-like species, Rhipsalis clavatum, is about as uncactus-like as you can get
more Rhipsalis species
Schlumbergera are a group of non-cactus-like succulents that are popular holiday houseplants (sometimes known as Christmas cacti). These are flattened, segmented trailing plants that, like the Rhipsalis species, act more like a tropical exotic than a drought-tolerant, sun-loving cactus.
Schlumbergeras blooming right around Christmas (hence the name)
Tephrocactus articulatus var. diadematus (pine cone cactus). This is a fantastic plant for both pot and garden use, but is not always recognized by all growers as a cactus, having no spines. It does look just like a bunch of pinecones growing together. The other Tephrocactus species are more obviously cacti as they all have at least some spines.
Tephrocactus articulatus var. diadematus (first photo by Xenomorf); third photo is another weird Tephrocactus: T. geometricus that also is sort of un-cactus-like
So as you can see, not all cacti are cactus-like. In fact, from this group of plants it is not any clearer what makes a cactus a cactus. I leave that to the professional taxonomists. But they are certainly an interesting group of plants and great fun to grow and show.
A few more odd cacti: Lophophora williamsii, aka Peyote; grafted cacti that look more like candy than cacti; Lophocereus schotti monstrose looking like the right shape for a cactus, but smooth as can be