Examples of plants not in botanic garden collections
Hydrostachys polymorpha is a perennial herb that cannot survive without the turbulence of water. It is found attached to rocks within waterfalls or rapids in Southern Africa. The specific ecological requirements of this aquatic species present a technical challenge and many botanic gardens will lack the horticultural capacity to grow this difficult species (More info here http://pza.sanbi.org/hydrostachys-polymorpha).
Hydrostachys polymorpha (D Gwynne-Evans)
Another species not currently in cultivation, and requiring technical expertise is the tiny parasitic plant Pilostyles thurberi. Only a few millimetres long, it is rarely seen and lives completely within the stem tissue of small desert shrubs in Mexico and the United States. Once a year, minute red flowers appear on the host stem. More research is needed to bring this unique family of endo-parasitic plants (Apodanthaceae) into ex situ collections. (More info http://waynesword.palomar.edu/ploct98.htm).
Some species are so rare, it is difficult to find them and bring them into collections. For example Rhabdodendron gardnerianum, one of only three species in the mono-generic family Rhabdodendraceae, is only known from a pressed plant collection made in 1839 from Bahia, Brazil. It may now be extinct.
Barbeuia madagascariensis is a liana found only on the island of Madagascar in coastal forests. Although widespread, this species, the only member of the family Barbeuiaceae, is not recorded in any botanic garden collections. This is likely due to the limited capacity of Malagasy botanic gardens.
Geissoloma marginata is only found on the southern slopes of the Langeberg mountain range in the Cape region of South Africa. It is a small, inconspicuous, evergreen shrub, and amongst 8,000 other species that are found in the Cape is likely to have been overlooked by botanic gardens amongst its horticulturally spectacular neighbours.
Geissoloma marginatum (Jan Thomas Johansson)