Conformation

Cactus are dicotyledoneae fanerogamae plants. This means its embryo shows two visible primary leaves after germination. Like other dicotyledoneae, if they are cut cross-sectionally, they show a generative ring tissue (cambium), and this is how the plant grows.

Shapes
There are cactus with globular body, cylindrical shape, with segments and with stems similar to leaves. The size also varies according to the genera and the species. In some cases they are very big (Cereus, Carnegiea), whereas in others they do not have more than 1 cm. wide (Blossfeldia)

Ribbed conformation of Cereus aethiops

Cereus reaches arborescent shapes from a main trunk, from which ramifications are born. But ramifications can also born from the base of the initial stem (Trichocereus). They can grow turgid, inclined or crawling (Machaerocereus). However some cactus do not sent branches at all, and its body remains solitary all its life. In the stems of columnar cactus, longitudinal protuberances or ribs appear. Their number and prominence changes according to species. Thus, some Cereus has only four, several centimeters prominent, whereas in Cleistocactus they are many and little prominent. The utility of the ribs is related to the resistance drought, since when losing water the plant during the dry time, the stem falls back conserving its ribbed structure. When rains arrive this structure help the plant protecting the stem for cracking due to the fast absorption of water.

Clumped specimen of Mammillaria columbiana

Many species are globular (spherical). This feature allows them to absorb the maximum amount of water with a minimum outer surface (smaller evaporation). Among globular species, some have ribs (Echinocactus, Melocactus) and others have their body covered with protuberances (Mammillaria, Coryphantha). Sprouts or offsets are born from some cactus forming dense clumps in some cases. This sprouts or offsets can be separated from the mother plant and rooted for propagation (Echinopsis, Mammillaria). Another characteristic shape is the one of Opuntia. From their shovel shaped stems, leaves, flowers and fruits can arise. Some Opuntia has cylindrical segments instead of flat ones. Epiphytic cactus, that live in the tropical wooded zones hanging between rocks or branches, have very particular shapes. In presence of some of this plants nobody would say that it is a cactus. Their stems have form of leaves (Rhipsalis, Epiphyllum) and sometimes they are segmented like a succession of small leaves (Zygocactus, Schlumbergera).

Ferocactus diguetti areola close-up

Areolas
Areolas are peculiar organs that characterize cactus. They give rise to spines and flowers, but also hairs, bristles, glochids and spines arise from them. They have the shape of small pads, sometimes felted, and can be located throughout the ribs (columnar cactus and some globular ones) or on tubercles (several globular species). In areolas two points of vegetation exist: one that generate flowers, and other that generate spines. They can be together in a same areola (Cereus, Echinopsis, Opuntia) or separated, one on the tubercle, place of growth of spines, and the other displaced towards the armpit of the same one, where flowers and buds take place (Mammillaria, Coryphantha).

Leaves
Just a few cactus (Pereskia, Pereskiopsis) have perfect and permanent leaves. Generally leaves are not permanent or rudimentary. Aside from the tiny foliar elements that appear in some species, there are vestiges of the true leaves in the body of cactus: the spines and tubercles. Spines are portions of the foliar limb modified during millennia, whereas the tubercles correspond to bases of leaves. In some Opuntia, the leaf is reduced to small extended remainders, of fleshy texture.

Epithelantha micromeris dense and close spination

Spines
Spines are modified leaves, produced by the epidemical layer of the stem and they are not united to internal tissues, unlike the stings of other succulent plants. Cactus from places which are very sunny, usually have dense and strong spines so, with its shade, the effect of solar rays on the body of the plant diminished. On the other hand, some cactus lack totally of spines (Lophophora, Aztekium). The shape of spines changes considerably according to species. In some cases they are very small and in others they can reach 20 cm. long (Stetsonia coryne). There are acicular, prism shaped and cylindrical spines; its end can be very acute, rectilinear or hooked. Some species (Cephalocereus, Espostoa) have been transformed its spines into hairs (criniform spines).

Matucana madisoniorum spines

Two types of spines exist on each areola: radial ones that are numerous and born on the periphery, and central ones, of different shape, but hard and resistant. It is good to clarify that when the number of spines is indicated in a cactus description, it usually refers to the ones on each areola although it is not explicitly mentioned. In the areolas of Opuntia are small called spines called glochids. They are tiny thorns grouped in pads. You have to be careful with this spines since it is very hard to take them out of the skin. In some species of Ferocactus, Opuntia and Mammillaria, there are glandular spines that exude nectar.

Notocactus uebelmannianus typical flowers

Flowers
Cactus flowers do not have peduncle, they are born directly from areolas. Is difficult to tell the number of segments (petals and sepals) since there is not a clear differentiation among them. The external ones are small and sometimes of greenish color (sepals), however the internal ones offer living colors (petals). Also there are colored sepals (petaloid sepals). On the other hand, the calix of some species is extended in such a way that it can be confused with a peduncle. Petals and sepals can be separated in all their length (Pereskia, Opuntia), or can be together (Cleistocactus, Oreocereus). The flowers are generally regular or actinomorfic, although in some sorts (Zygocactus, Cleistocactus) they are irregular or zigomorfic. They can be white, yellow, greenish color, orange, rose, red, purple and violet. Stamens are inserted on the base of the calix and are numerous. In some species (Coryphantha, Opuntia), these shows irritability. At the moment of its maximum opening, when they are touched by an insect or an object, they close over the style and open again a few minutes after. Cactus flowers have masculine organs (stamens) and feminine (stigma), except in Mammillaria dioica and some Opuntia. Another characteristic of cactus flowers is that the fruit grows below the flower.

Napiform root of Neoporteria napina

Roots
They can be quite superficial, extending widely so they have easy access to water fallen in rains, or can be napiform (conical shape), making reserve functions (Lophophora, Ariocarpus). In certain species of Pereskia and Pereskiopsis, the root is heavy and nodulous.

Fruits
Generally fruits are fleshy berries like the ones in some Opuntia species, although in some cactus they appear dry (Echinocactus). The analogy that shows the external cover of the fruits with the stems of the cactus (presence of areolas, leaves, etc.) causes that flowers or new growths can arise from the fruits of some species.

Epithelantha micromeris fruits

Other feature of the fruit of some species like Opuntia microdasys, is that they can root and produce buds, so they are organ forming segments too.

Seeds
Seeds width is generally smaller than 1 mm., except in Opuntia and some other genera whose seed is big and has a hard cover. They appear oval or rounded shaped.