THE GEOGRAPHICAL DISTRIBUTION OF THE GENUS GYMNOSPORANGIUM
This study of geographical distribution shows that species of the genus Gymnosporangium are found in the northern hemisphere only and occur most abundantly in the temperate portion. Each of the three major continents contains a distinctive Gymnosporangium flora and, with the exception of three species called the "tricontinental species", species occur naturally in one continent only. The genus contains about 48 species; 33 occur in North America, 15 in Asia and 6 in Europe (including the three tricontinental species in each case). Explanations of the types of geographical distribution of the North American species are given under four categories: (i) species that occupy all potential territory covered by the coincident ranges of their alternate hosts, (ii) species that are confined by the range of their "primary" telial host, (iii) localized species that are confined within a portion of the coincident ranges of their alternate host, and (iv) widely distributed species that are not limited in their range by either alternate host group.