The Classification And Characteristics Of The Cactus Plant Species


In the following article, I have written about the classification of the cactus into various types depending upon their physical appearance and other factors. The different parts of the cactus plant, such as leaves, stem and spines are also detailed. In addition, I have mentioned some of the uses of cacti.

Distinguished by their fleshy and sometimes spiky appearance, cacti are members of the plant family Cactaceae. Most species of cacti grow in regions that experience some amount of drought, while many others are found in extremely dry environments.

Distribution of various species of cactus


The earliest cacti were shrubs that had reduced succulent properties. They were small trees, wherein the leaves carried out photosynthesis. They grew in tropical regions that experienced drought from time to time. As for the evolution of the cactus, there were no fossilized remains found anywhere that suggested where exactly the cactus came from. However, based on their current geographical locations, they are plants of South America and southern parts of North America. While early cactus plants may have originated around 25 million years ago, the current cacti species are believed to have developed in the last 10-15 million years.

Today cacti are found in regions that are diverse – from coastal plains to high mountain areas. Cacti adapted to drought climates are found mainly in Mexico and the southwestern US, the southwestern Andes – in Peru, Bolivia, Chile and Argentina and Eastern Brazil. Cacti that live in and climb trees require environments that are more moist and hence are found in southeastern Brazil – in the coastal mountains and Atlantic; in Bolivia and in forest areas of Central America. Rhipsalis baccifera is the exception that is found in the Americas as well as Africa, Madagascar and Sri Lanka. It was probably widespread because of the transport of the seeds through the digestive tract of migrating birds. In the 19th century, Opuntia - a species of cactus was introduced in Australia to serve as a natural fence.

While most cacti are well adapted to hotter climates and deserts, many grow in the cooler mountainous regions. If over watered, cactus plants start to rot and collapse. Cactus plants must be allowed to dry completely between watering to grow successfully.

Classification of cactus species


There are between 1500-1800 different cactus species, which fall into two main groups or categories –

1. Opuntias - of the subfamily Opuntioideae

2. Cactoids - of subfamily Cactoideae

Common features to both these groups include fleshy stems which carry out photosynthesis; small or absent leaves and flowers with ovaries that are buried in a fleshy receptacle (the part of the stem from which the flowers grow). All cacti possess areoles - tiny shoots from which the spines, flowers and normal shoots are produced.

The other cacti, not characterized by qualities of the abovementioned species, fall into two other groups –

1. Pereskia

2. Maihuenia

Mature plants of these species have woody barks that resemble trees; leaves which carry out the photosynthesis process; flowers, whose ovaries are not sunken into the fleshy receptacle and finally, areoles, from where more leaves are produced. The two species of maihuenia have small globular bodies with leaves at the top. Some members of the species, particularly Pereskia look a lot like other trees – they have barks and permanent leaves. But the one feature that makes them cacti is the presence of areoles. Pereskia cacti are considered the closest relative to the ancestors from which all other cacti evolved. Forest-climbing cacti exist in tropical regions – they have flattened stems that look more like leaves and very few or even no spines, like the well-known Christmas cactus and Thanksgiving cactus species.

Cacti grow in a variety of ways (shapes and sizes). They can be like the Pereskia species mentioned earlier, which look so much like other tropical trees and shrubs, that they can be mistaken for trees, rather than cacti; have no leaves or bark, fully covered with spines (Opuntias); as high as trees, but without branches and those that resemble shrubs, whose many stems come from the ground or lower branches. Smaller cacti have erect cylinder like stems, with or without branches. Some have globular bodies – small globe-shaped stems.

The main elements of a cactus plant


Stems


Cactus stems are generally succulent, that is, they can store water. The stem can be smooth or covered with tubercles - small bumps. They can be ribbed in shape; the ribs are visible when the cactus is short of water and invisible when the cactus is full – containing up to 90 % of water. Stems are mostly some shade of green and contain chlorophyll, which enables them to carry out photosynthesis. They also have stomata – which open and close to allow the passage of gases. Some cactus stems appear waxy.

Areoles


These are the hair or fur-like areas on the stem, from which the spines and flowers grow. Areoles may be circular, oval, entirely separate or separate, but connected visibly to the other. The multicellular hairs of the areoles give them the yellowish or brownish woolly/fur-like appearance. Among most cactus species, the areoles produce spines or flowers only for a few years, and then become inactive.

Leaves


Most cacti don't have any leaves, in which case the stems carry out photosynthesis. Some cacti have tree-like leaves which may be permanent or just short-lived.

Spines


Spines are not thorns; spines are modified leaves, thorns are modified branches. Spines always grow from the areoles and are present in those species which have leaves as well. Some species of cacti, particularly tree-living ones, have spines only when they are young. Spines differ in thickness, and may appear hair-like or bristle-like. They can be straight, slightly curved or hooked. Some species of the Opuntioideae have very small spines, which easily shed and if these enter our skin, can be difficult to remove and cause irritation.

Roots


Ground-living cacti have fine roots that spread out around the base of the plant, closer to the surface. Some species have taproots that are much larger than the body and help to support the taller column-like cacti. Tree-climbers have roots that are produced along the stem when they come across something they can cling/root onto.

Flowers


Like cactus spines, flowers differ from species to species. They can either grow in clusters or as single flowers and range from white or yellow to red and magenta.

How the cactus stores water


The early cactus species were found in areas of drought and so already prepared the descendant species to adapt to climates with less moisture.

1. Cacti with thin leaves lose water at a rate higher than those with fleshy stems, because the loss of water depends on the surface area of the plant, while the water present is dependent on the volume of the plant.

2. The spines not only help to protect the plant from herbivores, but also provide some shade for the plant as well as reducing air flow close to the plant. The latter two uses also ensure no water is lost. During fog and mist, the spines condense the moisture in the air and then drop it onto the ground where it is then absorbed by the roots.

3. Stems with a waxy appearance reduce water loss and hence are grayish or bluish in colour.

4. Many cactus roots spread far and wide, but not very deep into the earth. When rain falls after a period of drought, cacti roots grow very fast. The root cells also contain a high concentration of salt.

5. Most cacti use a method called Crassulacean Acid Metabolism (CAM) during photosynthesis. While photosynthesis takes place during the day, transpiration, whereby carbon dioxide enters the plant happens only at night, when the climate is cooler and more humid. Since the method of converting carbon dioxide into oxygen takes place in the coolness of night, water loss is significantly reduced.

Cactus plant reproduction


Some cacti grow long, tube-like flowers, which allow for certain species of moths to reach the nectar and hence pollinate the plant. Some species are capable of self pollination – a process called cleistogamy. Cactus fruit are pleasant to taste and differently coloured, thereby attracting goats, mice, birds, ants and bats to help with the spreading of the seeds.

Uses of cacti


Cacti have a variety of uses – they can be used as decorative plants, particularly the flowering ones; as fodder for cattle and their fruit can also be consumed.

1. Opuntias or Prickly Pears were used in many ways by the Aztecs whose horticulture system was very complex.

2. The coat of arms of Mexico shows an eagle holding a snake while perched on a cactus.

3. The Indian fig cactus has been used as food for a long time.

4. Cactus fences serve the purpose when real fences are not financially possible. They also help keep locations safe – their sharp spines can deter trespassers.


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