In the Canon:- Edit
No such plant or vegetation exists on Terry Pratchett's Discworld. It is solely the creation of fanfic author a.A. Pessimal.
In the Pessimal Discworld:-Edit
This hardy desert dweller must share some common genes with the Counting Pine of the Central Continent. In fact, those few hardy and intrepid research botanists who have not only visited the deserts of the Central Howondalndian Plain - and got out again - speculate that millenia ago, Counting Pine seed must have somehow got here and instantly mutated into a plant best suited for the local harsh climate.
For the Tequila Cactus, along with its close chemical relative the Mescal Cactus, has, in accordance with ancient genetic survival logic, proven to be a plant of great sustainable merit to human beings living in a desert.
The way it works is simple to describe. Its long roots draw up water from deep below the desert, via interaction with natural sugars created by photosynthesis, to where the natural action of the sun and desert heat cause it to evaporate, but in a controlled manner that allows its rich sap to fractionally distil into a lethally strong alcoholic spirit which may be tapped off. Evaporated water is diverted to separate expanding chambers within the cactus, which may with care be opened and their contents removed, and which are thought to have developed to answer the problem of freezing cold desert nights. Otherwise, freeze-expansion could occur, which would force a cactus to explode.
Any Indian with a glass and a knife to chip off some ice , therefore, may tap off a very palatable tequila on the rocks more or less on demand.
The resultant hangover is reputed to be part of the reason why the Apache Indians are so tetchy with outsiders.
Some fine specimens are said to exist in the hothouses at Unseen University's School of Extreme Horticulture. The hothouses at the Assassins' Guild's Animal Management Unit also house some fine and carefully conserved specimens, as Assassins like a good dangerous drink like anyone else. Doctor Davinia Bellamy cultivates only the finest and healthiest specimens.