A number of plant species have been investigated as potential sources of petroleum. The shrub called the gopher plant (Euphorbia lathyrus) produces significant quantities of a milk-like sap—called latex—that is an emulsion of hydrocarbons in water. Another candidate is Pittosporum resiniferum, a native of the Philippines. The fruit of this plant, called a petroleum nut, is quite large and the oil harvested from it is frequently used for illumination. Various experiments are in progress to use vegetable and seed oils as diesel substitutes, particularly in farm machinery.