Cacti and Succulents
Tubercles – a small rounded projection on cactus stem, formed from the vestigial remains of large leaf bases arranged around a stem in crisscrossing spirals.
Areoles – one of the small, spine-bearing areas on the stem of a cactus; highly specialized axillary or lateral bud or short shoot or branch.
Essentially, the areole is a compressed shoot made up of 2 perpendicular buds and the intervening internode. From the upper bud arises the flowers, fruit, and new growth of a cactus; from the lower bud arises the spines. Looking closely at an areole and tubercle, you can see that an areole is borne on the enlarged leaf base or tubercle. Unique to cacti.
Spines – a sharp, woody or hardened outgrowth from a leaf, sometimes representing the entire leaf, or from a fruit. Modified leaves; thorns are derived from branches.
Glochid – a barbed hair or bristle that is unique to the cactus genus, Opuntia. They emanate from the areoles on the cactus stem.
Ribbed or fluted stem – capable of an accordion-like expansion or contraction, depending on the amount of water within the plant. (Up to 90% of the fresh weight of a cactus may be water).
Succulence – derived from the Latin succus, juice, refers to a plant’s ability to store water. Cacti are succulent plants in which the stems are specialized to store water. Other succulents such as Aloe (Aloeaceae) and Agave (Agavaceae) have succulent leaves. Stapelia (Asclepiadaceae) and some members of the large genus Euphorbia (Euphorbiaceae), like cactus, are stem succulents.
Fasciation – where stem tissues grow together to form an irregular shape; joints may produce twin forms along the whole or most of its length, occasionally becoming distorted, opening out like fans, or bending over as they grow. Fasciation results from various physical or bacteriological causes. It is not hereditary; affected plants or their offspring may indeed regress. It occurs in other succulents and gives rise to so-called “monstrosities” that are highly prized by cactus collectors.
Convergent evolution – the process by which organisms of different evolutionary lineages
come to have similar form or behavior. A common example is the stem succulence of cacti and euphorbias.
Instead of homology (a condition in which a common trait possessed by different
species was derived from a common ancestor), this feature shows convergence,
which is one explanation for a similarity that does not come from a common
ancestor. In this case, descendants of a pre-cactus ancestor became adapted to
arid environments in the
Cacti are divided into 4 categories:
1. Plants that still produce leaves in spite of the special characteristics of the family. (Pereskia)
2. Plants that produce leaves, but in most cases lose them very soon. (Cylindropuntia, Platyopuntia)
3. Plants with rudimentary leaves similar to scales, or with no leaves at all. (Columnar cacti: Cactiodeae: Mammillaria, Echinocereus)
4. Plants with stems resembling leaves (cladodes), epiphytes or semi-epiphytes. (Opuntia, Schlumbergera, Rhipsalis)
Since cacti live in a wide range of habitats, they exhibit a wide range of shapes form simple to branched, from globose to columnar, from segmented joints to unsegmented stems, from flattened cladodes to terete, cylindrical stems, from having spines to the absence of spines, etc.
All members of Cactaceae are xerophytes – they are adapted for growth under dry conditions. In particular, they are designed to reduce moisture loss to a minimum, and they are capable of storing water in their tissues.
In cacti without leaves, transpiration and photosynthesis is carried out by the stems, and they are especially adapted to the job in several ways.
1. They are generally compact cylinders, semicylinders, or globes; therefore the surface area from which moisture can be transpired is reduced greater volume to surface area ratio). E.g. as a barrel cactus increases in size, its volume increases more rapidly than does its surface area. A larger spherical cactus can therefore better withstand drought than a smaller one can, because the water-storage volume per water-loss area is greater for the larger cactus. Small seedlings, therefore, have a far more limited ability to tolerate drought than older, larger adults.
2. The stem is covered with a thick, waxy skin through which moisture cannot pass.
3. The stomata are usually sunken which slows down moisture loss.
4. Transpiration is limited by the spines, bristles, and small hairs, which, if fairly dense, insulate the epidermis and protect it from excessive cold or overexposure to ultraviolet radiation. This protection mechanism is especially pronounced on the upper portion of the stem or at its apex, where the tissues are more delicate and flowers usually grow.
5. The characteristic roundness of stems and joints, no matter what their length, ensures that no one part is at the mercy of the sun’s rays for more than a short time in the course of the day, and only that a minimal part is permanently exposed to the north.
1. Leaves are persistent only in the category of cacti consisting solely of the genus Pereskia, which is considered a transitional form between normal plants and xerophytes.
2. The second category, comprising the tribe Opuntieae, have leaves that are usually small, very deciduous, and perform no real functions, the later having been taken over by the stem. In all the genera in this tribe the areoles have groups of minute barbed bristles (glochids), that are not found in any other members of Cactaceae.
3. The cacti of the other 2 categories have rudimentary leaves often reduced to minute scales, or leaves may be absent altogether (in which case, there are enlarged leaf bases that are fused together in various arrangements to form what are known as ribs or tubercles. Areoles grow on ribs, often at the apex.
1. Cactus flowers are generally solitary, and there is no clear distinction between calyx and corolla in the perianth
2. Borne on or near the areoles. Flowers are mostly regular, and the perianth is inserted above the ovary (inferior ovary).
3. Stamens, which are always numerous, have long filaments.
4. The pistil may be even longer, and the stigma is often stellate. Three – many carpellate, unilocular, commonly covered with hairs, spines, or bristles.
5. The fruit of almost all cacti consists of a berry containing several to many seeds.
Similarity of cactus and agave
1. Biologists tend to divide things into mutually exclusive categories, a process that helps classification but requires a huge scientific vocabulary. For instance, is something animate or inanimate, plant or animal, monocotyledonous or dicotyledonous?
a. Agaves, grasses, lilies, and palms are monocotyledonous – they have a single leaf axis with parallel veins.
b. Cacti are dicotyledonous, initially producing two leaf axes, and have leaf veins with netlike pattern.
2. Taxonomically speaking, these two families are very far apart. Yet, physiologically speaking, these two taxa (groups) have converged on many of the same solutions to the problems of growing in regions of low rainfall.
3. There are about 420 recognized families of flowering plants: 70 are monocots, 350 are dicots.
4. The event separating monocots from dicots probably occurred about 140 million years ago.
The line that led
to agaves apparently originated about 60 million years ago; its major radiation
(evolution of new species) probably about 30-35 million years ago, in southern
6. Based on flower structure, tissue properties, and specific chemical compounds, close taxonomic relatives of agaves include amaryllis and onion family, and the asparagus family.
divergence of cacti from other flowering plants probably occurred 70-90 million
years ago, also in the
relatives of Cactaceae are most likely families Portulacaceae, which contains
the portulacas, and Didieraceae, which contain the didiereas, and is confined
10. Cacti diversity (as compared with agaves)
a. Agaves all have prominent leaves and a single vertical stem that is usually hidden by the leaves.
b. Agaves are most often monocarpic – flowering once in its lifetime.
c. Cacti can range from primitive species with large leaves and thin stems (Pereskia) to rather short shrubs with readily detachable stem segments (cylindropuntias like Opuntia bigelovii – jumping or teddy-bear cholla) and the platyopuntia with flattened stem segments (O. ficus-indica, a prickly pear cactus), to tall tree-like plants with prominent columnar trunks, such as the saguaro (Carnegiea gigantea), which can be found in the Sonoran Desert, and Trichcereus chilensis, widely distributed in Chile.
1. Although agaves have the potential to propagate sexually, many are propagated asexually by bulbils. Bulbils are young plantlets that occur on the inflorescence (flowering stalk) when the flowers remain unfertilized. These plantlets can be harvested and placed directly into the soil.
2. Most older agaves produce multiple underground stems, rhizomes, from the base of their stems. A new plant, or ramet, emerges from the end of the rhizome. The ramets are initially dependent on the mother plant.
3. When a mature agave established by seed eventually flowers and dies, its descendants produced on rhizomes will have radiated out from its base. Subsequent rhizomes on the descendant plants cause a ring of vegetatively-produced plants to expand with time, creating a pattern similar to the famous “fairy rings” of various mushrooms.
4. Large prickly pear cacti, like Opuntia ficus-indica, are propagated by removing individual flattened stem segments, termed cladodes, when they are about 20 cm (8”) or more in length. The cladodes are allowed to dry for a few days to discourage fungal or bacterial infections. Although this procedure would kill the stems of nearly any other plant, for these cacti it merely allows calluses to form on the cut surfaces. The cladodes are then placed vertically in the ground with about one-quarter of their surface area in the soil. The same can easily be down with cylindropuntia segments, like the chollas.
a. Cactus fruits are high in sugar (70-80% by dry weight). About 1/3 of the sugar content is fructose, which is better tolerated than glucose and sucrose by those with diabetes mellitus. Cactus fruits are also high in Vitamin C (ascorbic acid), and low in fats.
fruits are often called “cactus pears” in the
c. Before eating cactus pears, the glochids are usually brushed off.
d. Fruits from the columnar cacti are called pitayas.
segments (cladodes) of prickly pear are used as green vegetable throughout
b. Young stem segments have little oxalic acid, which can be concentrated in older cacti. Oxalic acid in large amounts and dense concentrations can be toxic to humans and livestock.
c. Flower buds of some chollas have been eaten by Native and desert dwelling peoples for a long time. The buds may be pit-roasted for a day, and either eaten immediately or dried and pickled for later consumption. Cholla buds have significant amounts of protein, but may be more important for their high calcium content and soluble fiber.
d. Cladodes are commonly called joints, pads, or nopales.
e. Nopales have a consistency similar to okra, very mucilaginous. They have been used for thickening soups and broths. The mucilage also helps control blood-sugar levels associated with adult onset diabetes ( a common affliction among Native Amercans who adopt Western high fat, low fiber diets). Nopales may also reduce blood cholesterol.
a. The juice expressed from prickly pears has been used for centuries to strengthen adobe mortar.
b. More recently, it has been used to stabilize dirt footpaths and erosion-prone slopes.
6. Cochineal Story
a. The robes of Aztec emperors were royal red – to create this color, the emperors demanded that their subjects pay a tax in insects. The vibrant red robes of European royalty, as well as the striking red jackets of the British Regulars, referred to by Paul Revere when he warned in 1775 that “the redcoats are coming”, and the redcoats of the Royal Canadian Mounted Police all derive from the same insect.
b. The insects for commercial production of the red dye feed on the cladodes of certain prickly pears, including Opuntia ficus-indica. The dye cones from the female cochineal insects in the genus Dactyopius, which surround themselves with telltale cottony webs on the cladodes. (The females are considerably larger than the males, and remain in a prolonged larval stage and can draw nutrients from the cladodes for up to 3 years via tubular parts).
c. In 1856, cheap synthetic aniline dyes were developed from coal tar, revolutionizing the dyeing industry and stealing the market from the cochineal dye.
d. To produce the dye, cochineal insects can be incubated on individual detached cladodes. The insects can be removed by jets of air and collected mechanically before extracting the dye. Most cochineal insects are presently collected by hand.
are raised for carmine production in
f. Expensive: About 130,000 adult females must be harvested to obtain 1 kilogram (2.2 lbs) dry wt. Webs and insects are dried in the sun after harvesting. Dried insects are ground to produce a purplish powder, the most commonly marketed form. The dye may also be extracted in boiling water and marketed in liquid form.
Cacti native to
Escobaria vivipara (Beehive cactus) – small globular cactus with flowers at the very apex of the globose stem. Stem clearly not jointed.
Mammillaria tetranscistra (Fish-hook cactus) – small, globular cactus that flowers around the periphery of the stem apex. Stem clearly not jointed. The central spines of this plant are generally hooked. Fruits of Mammillaria are edible, but small. Called “coyote’s paws” by O’odham people.
Ferocactus cylindraceus (Barrel cactus) – spherical or columnar cactus, not jointed; may reach to 2 meters in height. Crowns of flowers are crowded among the dense spines at the stem tips and are almost always yellow.
Echinocereus engelmannii (Hedgehog cactus) – grows in clusters of stems, up to 60 or so in number, each up to 1’ tall. Brilliant red to lavender flowers open in April.
Echinocereus triglochidiatus (Mojave mound cactus) – brilliant crimson red flowers.
Opuntias (O. acanthocarpa: buckhorn cholla, O. echinocarpa: silver cholla, O. bigelovii: teddy-bear cholla, O.basilaris: beavertail cactus, O. littoralis: coastal prickly pear, O. erinacea: old man cactus.
Opuntias are distinguished from other cacti by 4 characteristics:
1. The stems grow in distinctly jointed segments. The elongation of joints is permanently terminated by the onset of the dry season (determinate growth); subsequent growth of the plant occurs by the initiation of new joints by branching from the areoles. (Other cacti, like saguaro have indeterminate growth).
2. Whether the stems have spines or not, Opuntia stems always have glochids borne in the areoles.
3. Rudimentary leaves are present on new joints.
4. Seeds have a pale covering called an aril; most other cacti have black, shiny seeds.
Vernacular names for Opuntia are based on growth form. The chollas (Cylindropuntias) have cylindrical stem segments; prickly pears (Platyopuntia) have flattened pads.