The genus Arceuthobium, commonly called dwarf mistletoes, is a genus of 26 species of parasitic plants that parasitize members of Pinaceae and Cupressaceae in North America, Central America, Asia, Europe, and Africa. Of the 42 species that have been recognized, 39 and 21 of these are endemic to North America and the United States, respectively. They all have very reduced shoots and leaves (mostly reduced to scales) with the bulk of the plant living under the host's bark. Recently the number of species within the genus has been reduced to 26 as a result of more detailed genetic analysis. Description They are dioecious, individual plants being either male or female. The fruit is unusual in that it builds up hydrostatic pressure internally when ripe and shoots the single sticky seed up to speeds nearly , an example of rapid plant movement. The [[Arceuthobium americanum|lodgepole pine dwarf mistletoe, Arceuthobium americanum]], has been found to explosively-disperse its seeds through thermogenesis. Dwarf mistletoe seeds are enveloped in a hygroscopic, glue-like substance called viscin. Many fail to land on a suitable host's shoot, but some succeed, and in this way they are spread through the forests as a pest front. The spread of dwarf mistletoes in forest stands is greatest from the overstory to the understory, owing to gravity. Advantageous stand conditions for the spread of the parasite include an uneven-aged stand structure with severely infected hosts in dominant and codominant crown classes, species composition dominated by the primary host, and tree densities between 175 and 500 trees/ha. There are also several species from Europe and Asia including one of the smallest in the genus, A. minutissimum that lives on its host, Pinus wallichiana in the Himalayas. Effects of parasitism In western forest ecosystems of North America, numerous dwarf mistletoe species are considered to be serious forest-borne disease agents. Severe dwarf mistletoe infection can result in a reduction in tree growth, premature tree mortality, reduced seed and cone development, and reduced wood quality, and increases the susceptibility of the host tree to pathogen and/or insect attack. Most of the commercially important conifers in western North America are parasitized by one or more dwarf mistletoes. The interaction between dwarf mistletoes and their host can be generalized as a source-to-sink relationship. Dwarf mistletoes derive the majority of their nutrition from the host’s vascular tissues. Dwarf mistletoes have a root-like endophytic system, composed of primary and secondary