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Photosynthesis in cacti involves a mechanism termed crassulacean acid metabolism (CAM). They take in carbon dioxide during the night, store it as malic acid, which is then used in photosynthesis.Cacti are native to the Americas and belong to the family Cactaceae. They have some unique anatomical and physiological features that are not seen in conventional plants. The characteristic features of a cactus include their fleshy stems, which perform the functions of the leaves; and the spines, which are believed to be modified leaves. The shape and size of the spines and the stems vary from one species to another. Different types of cactus plants can be seen as trees, shrubs, and vines; and some of them are epiphytic.
It is believed that, these plants evolved in the Americas during the last 30 to 40 million years, after the continental drift, which separated the region from other continents. This explains the abundance of cactus plants in the Americas, as compared to other parts of the world. Cacti were introduced to the arid environment in other continents by migratory birds, who carried the seeds in their digestive tract.
Taxonomy
There are around 2000 species of cacti, that come in different sizes, ranging from the tallest 'Pachycereus pringlei', which has a maximum recorded height of 19 meters; to the smallest 'Blossfeldia liliputana', which has a diameter of one centimeter. Most of them belong to subfamilies 'Opuntioideae' and 'Cactoideae'. While 'Opuntioideae' has 15 genera; 'Cactoideae' is the largest cacti family with nine tribes and 108 genera. The family 'Cactaceae' has two more subfamilies, and they are 'Maihuenioideae' and 'Pereskioideae'. Both the subfamilies have a single genus each, and they are 'Pereskia' and 'Maihuenia'. While the genus 'Pereskia' has 17 genera, there are only two species in the genus 'Maihuenia'.
Anatomy
saguaro cactus
A typical cactus has succulent stems, spines, and no leaves. Though most have these features, some species do not fit into this description. Some cacti have small leaves, while others lack spines, or have hair-like modified spines. Even those with all the characteristic features vary in size and growth. You may also find variations in the basic features. The fleshy stem of a cactus can be cylindrical, elongated, ridged, or flat.
Leaves
saguaro cactus
Saguaro has all the characteristic features of a typical cactus. While almost all species of the genus Pereskia have leaves; this feature is also seen in most of the species in the subfamily Opuntioideae. Some members of the genus Maihuenia too have leaves. Among them, members of the genus Pereskia have leaves, like that of conventional plants. These cacti look like normal plants with woody stems and bark. Their leaves are non-succulent and longlasting.
Stems
flowering cactus
Cacti have succulent stems to store water. The outer layer of the stem is very thick, and has a waxy coating that prevents loss of water. Usually, the stems are green, to carry out photosynthesis. Many cacti are cylindrical or spherical, others are columnar or tube-like. Some have outgrowths, like the nipple-shaped projections in Mammillaria longimamma. Most of them are ridged, to store more water. During water shortage, the ridges become prominent.
Spine of Cactii
cactus spines
Cacti spines are modified leaves. The characteristic features of cacti include spines that are actually modified leaves. However, both the leaves and spines are entirely different, as far as their structural and functional aspects are concerned. These spines protect the plant from being eaten by animals, and minimize water loss. In some cacti, spines provide shade, to avoid water loss. The white, hair-like radial spines that cover the old man cactus (bottom right) protect the plant from frost and sun. Apart from reflecting sunlight, these spines trap moisture and hide the sharp, yellow central spines on the plant. The nature of spines may vary with different species. They can be sharp or blunt, soft or hard, long or short, and straight or hooked. They are found in groups, growing on mount-like structures called areolas. Most of the cacti belonging to the subfamily Opuntioideae have short, barbed spines, named glochids (second row, right).

Spines emerge from highly evolved structures called areolas. The spines as well as the flowers emerge from areolas located on the stem of cacti. Areolas are highly evolved structures that are said to be condensed branches. Usually, these mount-like structures have short hair on them. They are also found as hardened mounts in a different color. In case of cacti with leaves, spines are seen around the base of leaves. In cacti that lack leaves, spines occupy locations which are meant for leaf bases. The older areolas stop producing flowers or spines as the plant grows. Usually, those on the tips of the plant are found to be active. In some cacti, areaolas join to form ridges, whereas others have distinct areolas.
Cactus Flower
cactus flower
Cacti produce showy, short-lived flowers with bright colors ranging from white to yellow, orange, red, and lavender. The nature of flowers vary with the species, and their size can range between 0.2 to 30 centimeters. In general, a cactus flower has numerous stamen and petals. It has a single style with more than one stigma. In some plants, flowers bloom during nighttime.These flowers are pollinated by moths, other insects, small animals, birds and even bats.
Cactus Plant Adaptations
cactus
  • Cacti stems store water. The stems have thick outer layers and a waxy coat to prevent water loss.
  • The ridged stems expand to store water, and shrink as the water level reduces.
  • Most of these plants have shallow roots that cover large areas, and grow close to the surface of the ground. During rains, the roots collect water, which is stored in the stems. For example, a very small saguaro cactus, with a height of 12 centimeters, has a root system that covers a diameter of 2 meters. However, none of them are found to be more than 10 centimeters deep. In case of a fully grown saguaro, the root system can grow up to 15 meters in length.
  • The cylindrical shape of the stem reduces the surface area of the plant, which in turn minimizes water loss. Taproots (long, straight roots that grow vertically downward) are found in large columnar cacti. Such roots help the plant in anchoring and for collecting water and nutrients.
  • Lack of leaves is another adaptation that helps in preserving water.
  • Succulents, like cacti take in carbon dioxide during night, when the temperature is low. They store this carbon dioxide and use it during daytime, in photosynthesis. This mechanism is termed crassulacean acid metabolism (CAM).
Reproduction
butterfly pollination
The pollen in the flowers is carried to the stigma by pollinators, fertilizing the egg cell. They produce fruits; and the seeds are scattered by animals, birds, wind, and rain. In asexual reproduction, stems with weak joints fall off. Due to their water retention capacity, they can survive till the conditions become favorable. Young plants take a year to grow to a height of 2.5 centimeters, whereas older plants grow three to four inches yearly. The lifespan ranges between 25 to 300 years.
Cacti Uses
dragonfruit
The fruits of cacti belonging to the genus Hylocereus are called dragon fruits, and are widely used for consumption. Even the fruits and pads of prickly pear cacti are edible. Some cacti are of pharmaceutical importance, others use cactus wood for construction. Peyote (Lophophora williamsii) is a small cactus that has been used by the indigenous Americans. This cactus contains a hallucinatory alkaloid, called mescaline. The Native Americans use it for ceremonial as well therapeutic purposes.
Above all, some species of cacti are grown as exotic ornamental plants. You can also be the proud owner of one such beautiful plant and enhance the beauty of your garden. Numerous cacti species are grown for ornamental purposes. It is always better to have basic information about the requirements of the particular species, so that you can take care of it in the proper manner.