Changes

==Taxonomy==

==Taxonomy==

{{seealso|Taxonomy of the Cactaceae}}

{{seealso|Taxonomy of the Cactaceae}}

The

The family of the cactaceae, with an age of a few million years, is counted as a relatively young plant family, of which no fossil finds are known. Within this short time span (short at any rate from a geological point of view) the cactuses have undergone a rapid development into extremely specialised plants.

family of the cactaceae, with an age of a few million years, is counted as a relatively young plant family, of which no fossil finds are known. Within this short time span (short at any rate from a geological point of view) the cactuses have undergone a rapid development into extremely specialised plants.

The family of the cactaceae with approx. 2,000-3,000 species and varieties in about 60 genera was originally native to the [[Americas|American]] continent, where their range extends from [[Patagonia]] to the [[Canada|Canadian]]–[[United States|American]] border. Cactuses inhabit the most diverse regions, from coastal plains to high mountain areas, from the sub-tropics to the deserts. The most dense occurrence of cactuses is to be found in the territories around north [[Mexico]] and the [[Tropic of Cancer|southern tropic]] ([[Argentina]] and [[Bolivia]]). The equivalent plants to cactuses in [[Africa]] and [[Australia]] are representatives of the so-called "other succulents". Occurrences of cactuses outside the Americas are the result of distribution by humans (or in very rare cases by migrating birds).

The family of the cactaceae with approx. 2,000-3,000 species and varieties in about 60 genera was originally native to the [[Americas|American]] continent, where their range extends from [[Patagonia]] to the [[Canada|Canadian]]–[[United States|American]] border. Cactuses inhabit the most diverse regions, from coastal plains to high mountain areas, from the sub-tropics to the deserts. The most dense occurrence of cactuses is to be found in the territories around north [[Mexico]] and the [[Tropic of Cancer|southern tropic]] ([[Argentina]] and [[Bolivia]]). The equivalent plants to cactuses in [[Africa]] and [[Australia]] are representatives of the so-called "other succulents". Occurrences of cactuses outside the Americas are the result of distribution by humans (or in very rare cases by migrating birds).

[[Image:Pereskia grandifolia3.jpg|thumb|''Pereskia grandifolia'': ''[[Pereskia]]'' is a weakly succulent genus, which also possesses leaves, and is believed to be very similar to the ancestor of all cacti.]]

[[Image:Pereskia grandifolia3.jpg|thumb|''Pereskia grandifolia'': ''[[Pereskia]]'' is a weakly succulent genus, which also possesses leaves, and is believed to be very similar to the ancestor of all cacti.]]

Cactus have never lost their leaves completely, they have only reduced its size so that they will reduce the surface area through which water can be lost by transpiration. In some species the leaves are s

Cactus have never lost their leaves completely, they have only reduced its size so that they will reduce the surface area through which water can be lost by transpiration. In some species the leaves are still remarkably large and ordinary while in other species they have became microscopic but they still contain the [[stomata]], [[xylem]] and [[phloem]]. Certain cactus species have also developed [[ephemeral]] leaves, which are leaves that last for a short period of time when the stem is still in his early stages of development. A good example of a species that have ephemeral leaves is the ''Opunta ficus indicia'' better known as the prickly pear. Cactus have also developed [[spine (biology)|spines]] which, in addition to allowing less water to evaporate through [[transpiration]] by shading the plant, as well as to defend the cactus against water-seeking animals. The spines grow from specialized structures called [[areole]]s. Very few members of the family have leaves, and when present these are usually rudimentary and soon fall off; they are typically awl-shaped and only 1-3 mm long. Two genera, ''[[Pereskia]]'' and ''[[Pereskiopsis]]'', do however retain large, non-succulent leaves 5-25 cm long, and also non-succulent stems. ''Pereskia'' has now been determined to be the ancestral genus from which all other cacti evolved.<ref>http://www.eurekalert.org/pub_releases/2006-05/uocp-hdc051206.php</ref>  

till remarkably large and ordinary while in other species they have became microscopic but they still contain the [[stomata]], [[xylem]] and [[phloem]]. Certain cactus species have also developed [[ephemeral]] leaves, which are leaves that last for a short period of time when the stem is still in his early stages of development. A good example of a species that have ephemeral leaves is the ''Opunta ficus indicia'' better known as the prickly pear. Cactus have also developed [[spine (biology)|spines]] which, in addition to allowing less water to evaporate through [[transpiration]] by shading the plant, as well as to defend the cactus against water-seeking animals. The spines grow from specialized structures called [[areole]]s. Very few members of the family have leaves, and when present these are usually rudimentary and soon fall off; they are typically awl-shaped and only 1-3 mm long. Two genera, ''[[Pereskia]]'' and ''[[Pereskiopsis]]'', do however retain large, non-succulent leaves 5-25 cm long, and also non-succulent stems. ''Pereskia'' has now been determined to be the ancestral genus from which all other cacti evolved.<ref>http://www.eurekalert.org/pub_releases/2006-05/uocp-hdc051206.php</ref>  

[[Image:Cactus1web.jpg|thumb|left|Many species of cactus have long, sharp [[spine (botany)|spines]].]]

[[Image:Cactus1web.jpg|thumb|left|Many species of cactus have long, sharp [[spine (botany)|spines]].]]

Most cacti have very shallow roots that can spread out widely close to the surface of the ground to collect water, an adaptation to infrequent rains; in one examination, a young [[Saguaro]] only 12 cm tall had a root system covering an area 2 meters in diameter, but with no roots more than 10 cm deep.<ref name="dalhousie">[http://cactus.biology.dal.ca/biology.html Dalhousie University: Biology of Cacti]</ref> The larger columnar cacti also develop a [[taproot]], primarily for anchoring but also to reach deeper water supplies and mineral nutrients.<ref name="dalhousie"/>

Most cacti have very shallow roots that can spread out widely close to the surface of the ground to collect water, an adaptation to infrequent rains; in one examination, a young [[Saguaro]] only 12 cm tall had a root system covering an area 2 meters in diameter, but with no roots more than 10 cm deep.<ref name="dalhousie">[http://cactus.biology.dal.ca/biology.html Dalhousie University: Biology of Cacti]</ref> The larger columnar cacti also develop a [[taproot]], primarily for anchoring but also to reach deeper water supplies and mineral nutrients.<ref name="dalhousie"/>

One feature distinguishes the cactuses from all other plants: cactuses possess ''[[areole]]s'', as they are known. The areole appears like a cushion with a diameter of up to 15 mm and is formed by two opposing buds in the angles of a leaf. From the upper bud develops either a blossom or a side shoot, from the lower bud

One feature distinguishes the cactuses from all other plants: cactuses possess ''[[areole]]s'', as they are known. The areole appears like a cushion with a diameter of up to 15 mm and is formed by two opposing buds in the angles of a leaf. From the upper bud develops either a blossom or a side shoot, from the lower bud develop thorns. The two buds of the areoles can lie very close together, but they can also sometimes be separated by several centimetres.

develop thorns. The two buds of the areoles can lie very close together, but they can also sometimes be separated by several centimetres.

Like other succulents in the families of the [[Crassulaceae]], [[Agavaceae]] (agaves), [[Euphorbiaceae]] (euphorbias), [[Liliaceae]] (lilies), [[Orchidaceae]] (orchids) and [[Vitaceae]] (vines), cacti reduce water loss through transpiration by [[Crassulacean acid metabolism]].<ref name="dalhousie"/> Here, transpiration does not take place during the day at the same time as [[photosynthesis]], but at night. The plant stores the [[carbon dioxide]] chemically linked to [[malic acid]] until the daytime. During the day the [[stoma]]ta are closed and the plant releases the stored CO₂ and uses it for photosynthesis. Because transpiration takes place during the cool humid night hours, water loss through transpiration is significantly reduced.

Like other succulents in the families of the [[Crassulaceae]], [[Agavaceae]] (agaves), [[Euphorbiaceae]] (euphorbias), [[Liliaceae]] (lilies), [[Orchidaceae]] (orchids) and [[Vitaceae]] (vines), cacti reduce water loss through transpiration by [[Crassulacean acid metabolism]].<ref name="dalhousie"/> Here, transpiration does not take place during the day at the same time as [[photosynthesis]], but at night. The plant stores the [[carbon dioxide]] chemically linked to [[malic acid]] until the daytime. During the day the [[stoma]]ta are closed and the plant releases the stored CO₂ and uses it for photosynthesis. Because transpiration takes place during the cool humid night hours, water loss through transpiration is significantly reduced.