There are about 1,750 known cactus species in the order Cactaceae. Cacti, cactus, cactuses, and less commonly, cactus) are members of the plant family Cactaceae. Originally named káktos by Theophrastus for a spiny plant of unknown identity, cactus comes from the Greek word káktos. Cacti can be found in a variety of shapes and sizes through Latin. Most are native to the Americas, with Rhipsalis baccifera being found in Africa and Sri Lanka as well. They are adapted to live in very dry environments, including the Atacama Desert. This causes cacti to show many adaptations to conserve water. Most cacti are succulents, meaning they have fleshy, thickened parts that store water. This vital process takes place only in the stem of most cacti, unlike many other succulents. Cacti are mostly without true leaves, retaining only spines, which are highly modified leaves. By reducing air flow close to the plant, spines help prevent water loss and protect it from herbivores. Despite lack of leaves, cacti’s enlarged stems activate photosynthesis.
Stems
A leafless, spiny stem characterizes most cacti (all members of the Cactoideae subfamily). Stems are typically succulent, which means they store water. Tubercles are protuberances of various kinds on the stem (as in some Opuntia species). There are small “bumps” to prominent, nipple-like shapes in Mammillaria species and outgrowths almost like leaves in Ariocarpus species. There may also be ribs or flutes on the stem. A cactus’s ribs may be almost invisible on its swollen stem when full (up to 90 percent of its mass may be water), but when water is low and the stems shrink, the ribs are very visible.
Most cacti have green stems, often bluish or brownish green. Those stems contain chlorophyll and can carry out photosynthesis; they also have stomata (small openings that let gases pass through). Waxy stems are common on cacti.
Leaves
A majority of cacti have no visible leaves, but their stems (which may be flattened and leaf-like in some species) perform photosynthesis. There are exceptions in three (taxonomically, four) groups. All species of Leuenbergeria, Pereskia, and Rhodocactus have numerous leaves with a midrib and flattened blades (laminas) on either side. Taxonomic clades of this group are paraphyletic. Some Opuntia species have longer-lasting leaves (as in Pereskiopsis species) or produce them only during the growing season and then drop them (as in many Opuntia species). Maihuenia is also a leaf-based photosynthesis genus. There is some variation in leaf structure between these groups. The leaves of Opuntioids and Maihuenia seem to consist only of a midrib.
Almost all species of cacti studied have leaves less than 0.5 mm (0.02 in) long in about half of the cases and less than 1.5 mm (0.06 in) long in almost all cases. It has been suggested that these leaves function in the production of plant hormones, including auxin, and in the definition of axillary buds.
Spines
A spine is a modified leaf, whereas a thorn is a modified branch. The spines of cacti are always produced from areoles. Spines appear even in cacti with leaves, such as Pereskia, Pereskiopsis, and Maihuenia, so they evolved before leaflessness. Cacti usually only have spines when young, possibly when seedlings. Especially true of Rhipsalis and Schlumbergera, but also of Ariocarpus, a ground-living cacti.
Cacti spines can be useful in species identification since they vary greatly in number, color, size, shape, hardness, and whether they are all the same or of distinct kinds. There are four types of spines: hairlike, bristle-like, needle-like, and awl-like, depending on their length and thickness. Cacti with flattened spines include Sclerocactus papyracanthus. Some cacti have hooked spines. Some spines are hooked, while others are straight (e.g., Mammillaria rekoi).
The spines in the subfamily Opuntioideae are barbed along their length and easily shed, as well as normal-length spines. Due to their fine size and easy breakability, they are difficult to remove from the skin.
Roots
The roots of most ground-living cacti are fine, spreading out over a small area near the surface. Among genera such as Ariocarpus, taproots are considerably larger and have a greater volume than the body. A taproot may help stabilize larger columnar cacti. Several cacti have adventitious roots, produced along stems when they contact a rooting medium.
Flowers
The flowers of cactus differ from their spines. In most cases, stem and receptacle tissue surrounds the ovary, creating a structure called a pericarpel. It is technically only the part furthest from the base of the tube that is floral in origin, as it is composed of tissue derived from the petals and sepals. The outer surface of the tubular structure contains areoles that produce wool. Tubes usually have small scale-like bracts, which change into sepals, then petals, making it hard to distinguish sepals and petals (and hence referred to as tepals). [11] Some cacti produce floral tubes without wool or spines (e.g. Gymnocalycium)[17] or completely lacking external structures (e.g. Mammillaria). Unlike most other cacti, Pereskia flowers are clustered.
There are many stamens in cactus flowers, but only one style, which may branch into more than one stigma. Some cacti produce stamens in distinct “series” in specific areas of the interior surface of the floral tube, while others produce stamens all over the inner surface of the floral tube.
Some species are bilaterally symmetrical (zygomorphic) in addition to being radially symmetric. The colors of the flowers range from white to yellow to magenta.
Ornamental plants
When they first arrived from the New World, cacti were cultivated as ornamental plants. Collectors in Europe grew large collections (often including other succulents alongside cacti). The price of rare plants was very high. Plants are obtained from the wild, as well as grown, by suppliers of cacti and succulents. By the late 1800s, collectors turned to orchids, and cacti became less popular, although they were still cultivated.
Growing cacti in greenhouses is common in areas where cacti cannot be grown outdoors, such as northern Europe and North America. They can be grown in pots or in the ground. Several cacti are grown as houseplants, tolerant of the often dry climate. You can keep cacti in pots under cover during the winter or place them outside in the summer. Many house plants, such as epiphyllum hybrids, Schlumbergera (Turkey day cactus) and Hatiora (Easter cactus), are not drought-resistant.
Climate-suitable regions can also grow cacti outdoors. Gardens that require less watering (xeriscaping) have been promoted in arid regions. California’s East Bay Municipal Utility District sponsored a book on plants and landscapes for summer-dry climates. The cacti are one group of drought-resistant plants.
Other uses
The cactus has many uses. Usually after burning off their spines, they serve as food and fodder. Some cacti are used in herbal medicine as well as psychoactive agents. From the Americas to other regions where Opuntia grow, such as India, various species of Opuntia have been used in this way.
It is produced by a scale insect that lives on Opuntia species. Since European manufacturers began producing synthetic dyes in the middle of the 19th century, demand for natural dyes fell rapidly. The demand for natural dyes has led to an increase in commercial production.
Cacti are used as building materials. To prevent people from breaking into buildings, living cactus fences are used. As well as corralling animals. Cacti, such as Cereus repandus and Echinopsis atacamensis, have woody parts used in furniture and buildings. Some Seri houses are made with saguaro wood (Carnegiea gigantea) frames. Some cacti were used to fill pillows and weave textiles with their fine spines and hairs (trichomes).
Cultivation
There are many books on cacti cultivation because they are popular. In many countries and climates, cacti are grown naturally in a wide range of habitats, so reproducing the conditions in which they normally grow is rarely possible. It is better to grow semidesert cacti separately from epiphytic cacti, which require different conditions. We are primarily concerned with cultivating semidesert cacti in containers and under protection, such as in a greenhouse or at home, rather than outside in the ground in climates that permit it.
Watering
Desert cacti require careful watering. Depending on the location, the nature of the growing medium, and the original habitat of the cacti, it is hard to give general advice. According to Brown, more cacti are lost due to “untimely application of water” and that cacti require water even during their dormant winter months. It is also possible to withhold water during winter (November to March). It is also recommended to re-pot regularly when using hard water to prevent salt accumulation. Generally, it is recommended to allow cacti to dry out between thorough waterings during their growing season. Soil moisture can be determined with a water meter.
Light and temperature
When subjected to high light levels and temperatures in a greenhouse in summer, semi-desert cacti may still need some shading. The minimum winter temperature depends very much on the species of cactus involved. It is not recommended to allow the temperature to rise above 32°C (90°F). Unless a plant is cold-sensitive such as Melocactus or Discocactus, a minimum temperature of 5°C (41°F) to 10°C (50°F) is recommended for mixed collections. Cold-hardy cacti, such as Rebutia minuscula, can survive temperatures as low as 9 °C (16 °F) in cultivation and flower better in cold weather.
Pest
Cacti are attacked by many pests. Some of the pests that feed on sap are mealybugs, which live both on stems and roots; scale insects; whiteflies; the very small red spider mite, which can occur in large numbers and build a fine web around themselves and badly mark the cactus via sap sucking, even if they don’t kill it; and thrips. Insecticides are not effective against many of these pests, but biological controls exist. Fungus gnats and sciarid flies can consume the roots of cacti. Also slugs and snails eat them.
Cacti are attacked by fungi, bacteria, and viruses, especially when overwatered. Red-violet mold develops when fusarium rot gains entry through a wound. It is caused by Bipolaris cactivora. Helminosporium cactivorum Phytophthora species cause similar rot. Several viruses have been found in cacti, including cactus virus X, which may be resistant to fungicides. The only visible symptoms are chlorotic spots and mosaic effects. However, cactus virus X has been demonstrated to reduce growth in an Agave species, particularly when the roots are dry. Virus diseases do not have treatments.
