Opuntias (subfamily Opuntioideae) and “cactoids,” two groupings of “core cacti,” make up the majority of the 1,500–1,800 species of cacti (subfamily Cactoideae). The majority of these two groups’ members are unmistakably cactus. They have squishy succulent stems, which are important photosynthesis-producing organs. They lack, have tiny, or have temporary leaves. They produce blooms with ovaries that are submerged deeply into a fleshy receptacle under the sepals and petals (the part of the stem from which the flower parts grow). All cacti feature areoles, which are short, highly specialized branches with incredibly short internodes that produce blooms, spines, and regular shoots. [10]
Only two groupings of cacti remain: the relatively smaller Maihuenia and the three tree-like taxa Leuenbergeria, Pereskia, and Rhodocactus (all originally included in Pereskia). Since these two groups differ significantly from other cacti[10], they must frequently be excluded from general descriptions of cacti. The species of the first three genera resemble other tropical forest trees on the surface. When fully grown, they feature long-lasting leaves that serve as the primary source of photosynthesis, along with woody stems that may be covered in bark. Their blooms may have areoles that produce additional leaves and superior ovaries, which are located above the sites where the sepals and petals attach. The two varieties of Maihuenia have pronounced succulent leaves and succulent stems but not photosynthetic ones. [10]
Growth habit
Cacti have a wide range of development behaviors that are challenging to categorize into distinct, straightforward groups.
Cacti can resemble trees (are arborescent), which means they usually have a single, more-or-less woody trunk that is topped by a number of branches. The branches of the genera Leuenbergeria, Pereskia, and Rhodocactus are covered in leaves, making it difficult to identify the species of these genera as cacti. As with Pachycereus pringlei or the larger opuntias, most other cacti have branches that are more like regular cacti, devoid of leaves and bark and coated in spines. Some cacti, like larger specimens of Echinocactus platyacanthus, can grow to be tree-sized without having any branches. When many stems emerge from the ground or from branches that are very low to the ground, as in the case of Stenocereus thurberi, a cactus is often referred to as being shrubby. [11]
Columnar is a good word to describe smaller cacti. They are made up of upright, cylinder-shaped stems that may or may not branch, with no obvious distinction between the trunk and branches. It is challenging to draw a line between columnar forms and forms that resemble trees or shrubs. For instance, Cephalocereus senilis has smaller, younger specimens that appear columnar, and older, larger specimens may resemble trees. The “columns” might occasionally be horizontal rather than vertical. Stenocereus eruca can therefore be characterized as columnar while having stems that grow parallel to the ground and root at regular intervals. [11]
Cacti with even smaller stems could be referred as as globular (or globose). Unlike columnar cactus, they have stems that are shorter and more formed like balls. Globular cacti can grow alone, like Ferocactus latispinus, or their stems can group together to form enormous mounds. A cluster’s stems may all or some of them share a root. [11]
Other cacti look very distinct from one another. Some plants develop as forest climbers and epiphytes in tropical areas. Their stems often lack spines and are flattened, resembling practically leaves in form. Large climbing cactus are common; one Hylocereus specimen measured 100 meters (330 feet) from root to farthest stem. When growing in trees high above the ground, epiphytic cacti like the Rhipsalis or Schlumbergera species frequently hang downward and group together in dense groups. [11]
The stem is what part of the cactus?
Cacti have numerous adaptations that enable them to survive in arid climates; these adaptations enable the plant to efficiently gather water, store it for a long time, and conserve it (minimizing water loss from evaporation).
Cacti have thick, succulent stems with rigid walls that store water when it rains. The stems are fleshy, green, and photosynthetic. Either the stem’s inside is spongey or hollow (depending on the cactus). The water inside the cactus is prevented from evaporating by a thick, waxy layer.
Long, fibrous roots are common in cactus, and these roots take moisture from the earth. Some cacti, such as ball cacti, have smaller, more compact roots that can capture dew that falls from the cactus.
Most cacti feature scales or spines in place of leaves (which are modified leaves). These scales and spines do not evaporate their water (unlike regular leaves, which lose a lot of water). Predators (animals that would like to consume the cactus to gain food and/or water) are kept at bay by the spines. On a cactus, areoles are a circular collection of spines. An areole is where flowers bud, and it is also where new stems branch.
What is the name for cactus stems?
A perennial plant is a cactus. Their cylindrical or flattened stalks are covered in meat or succulents. The photosynthetic, green stems typically serve this purpose instead of the leaves, which are typically much diminished in number or entirely nonexistent in most adult cacti. Sharp bristles and spines that cover the majority of cactus species provide excellent protection and discourage most herbivores.
Cactus plants feature multiple surface areoles, which are cushion- or pit-like structures from which clusters of spines typically emerge. Areoles are typically understood in terms of developmental biology as axillary stem branches that are still in the process of developing. In reality, the spines are modified leaves. Additional defenses for the areoles include hook-like barbs called glochidia. Cacti have shallow, potentially widely dispersed soil roots.
Cacti typically have complete (bisexual) flowers that have both male reproductive organs (stamens) and female parts (a pistil). Although numerous distinct flowers may be present on a cactus at once, the flowers usually appear alone rather than in clusters. Most cacti species have huge, beautiful flowers that can be white, red, pink, orange, or yellow but seldom blue. The multiple petals and the sepal-like calyx combine to form an attractive, frequently fragrant flower that produces nectar and attracts pollinators including hawkmoths, bees, bats, and birds, particularly hummingbirds and tiny doves. The fruit is a berry with many seeds.
Cacti are xerophytic plants, which means they have evolved physiologically and morphologically to survive in extremely dry environments like deserts. The following characteristics of cacti make them suitable for xerophytic environments: (1) their succulent, water-retentive stems; (2) a thick, waxy cuticle and few or no leaves to significantly reduce water losses through transpiration; (3) stems that are photosynthetic, so leaves are not necessary to carry out this function; (4) stems that are cylindrical or spherical in shape, which lowers the surface to volume ratio and aids in moisture preservation; and, finally, (8) a periodic pattern of growth, productivity, and flowering that takes advantage of the moisture availability during the brief rainy season, while the plant remains dormant at drier times of the year. (5) tolerance of high tissue temperatures; (6) protection of the biomass and moisture reserves from herbivores by an armament of stout spines; (7) a physiological tolerance of long periods of drought; and (8) tolerance to high tissue temperatures.
As part of their so-called crassulacean-acid metabolism, cacti only absorb atmospheric carbon dioxide at night when their stomates are open. When the sun is shining during the day, the carbon dioxide is fixed into four-carbon organic acids and can then be released within the plant to be converted into sugars by photosynthesis. The crassulacean-acid metabolism, which enables stomates to remain tightly closed during the day, is an effective method of water conservation in arid settings.
In Texas’ Big Bend National Park, a prickly pear cactus. The only common eastern cactus in the United States is the prickly pear. As far north as southern Ontario, it can be found. By Robert J. Huffman, a photograph. Publications by Field Mark. Reproduction permitted.
Despite not being related to cactus, some dryland plant species look very similar to one another (at least, apart from their flowers and fruits, which are always distinctive among plant families). Convergent evolution—the similar evolutionary growth of unrelated species or families under similar types of environmental selective pressures—is what led to this. Non-botanists frequently mistake some species of spurges (family Euphorbiaceae) that grow in arid environments for cactus, despite the fact that they are actually relatively unrelated.