Arid Plant House
This glasshouse is for plants inhabiting regions of low, irregular rainfall and for plants of dry locations such as cliffs or tree branches. The cacti and some spurges, Euphorbia spp., store water in swollen stems and so are called;stem succulents. Leaves are insignificant or non-existent, so reducing water loss by evaporation. Desert plants like agaves and aloes store water in fleshy leaves, which minimize water loss by thick waxy cuticles; these are the & leaf succulents. Other desert plants termed sclerophytes survive without storing water; being tough woody shrubs, which become dry during droughts, but are not destroyed by desiccation. Cacti and spurges are often armed with formidable arrays of spines, and spurges also have poisonous acrid latex as an additional protection against animals. Aloes have bitter sap, once used to discourage nail-biting in children, but now it is used as a tonic in face creams and shampoos. Sclerophytes are so fibrous as to be unpalatable to animals, but the most fascinating protection is the camouflage of the flowering stones; such as the Lithops ssp. and Conophytum spp., in which the succulent leaves resemble stones and pebbles.
The two most important stresses faced by desert plants are water stress and herbivory (being eaten by animals), whether the plant lives in the Old or New World. Plants have evolved adaptations to combat these stresses (e.g., thorns, spines, loss or modification of leaves, rosette growth form and camouflage). In the Old and New World deserts plants often look similar and have identical types of adaptations, but they are not related. This is called convergent evolution.
The best known succulents are the cacti. Their natural distribution was restricted to the Americas, but subsequently, they were transported to arid regions elsewhere in the world. Sometimes this had disastrous results as in the case of prickly pears, Opuntia spp., in Australia where, having no natural predators, they spread at an alarming rate ruining sheep pastures. The problem was overcome by importing moths from Argentina whose caterpillars feed exclusively on prickly pears. Stapelia spp. are known as carrion flowers, superficially like cacti, but on no way related. Their purple-chequered flowers emit a smell of rotting meat to attract bow-flies which, in passing from flower to flower, secure pollination. The agaves (century plants) are monocarpic, i.e., they build up food reserves for 15 or 20 years until ready to flower, when there arises a huge stem often 6 to 7 metres high bearing hundreds of flowers. This effort exhausts the food reserves in the leaves, which wither and the whole plant dies after the seeds have set.
Other plants of interest are the crown of thorns, Euphobia splendens, bearing red flowers all year, various Crassula, Aeonium spp. and especially the large collection of different Mammillaria, Notocatus, Cleistocactus, Rebutia, and Astrophytum spp. There are also some cacti not from arid regions such as the Epiphyllum spp. that are found in rainforests from southern Mexico to Argentina as well as the West Indies, and Schlumbergera spp. (Holiday cacti and includes zygocacti), which are bushy epiphytic or rock-dwelling cacti from south eastern Brazil. These receive more water than the other plants in this greenhouse.