DISTRIBUTION OF BASIDIOMYCETES 401 



Furthermore many of them produce new crops of uredinospores 

 in the spring before the old fern leaves die. 



Among the Pucciniaceae, the distribution of Puccinia, Uro- 

 myces, and Phragmidium is global [Cunningham ( 193 1 ) ] . Trans- 

 chel'ia pruni-spinosae occurs throughout the world wherever 

 peaches and plums are grown, and Phrag7mdhtm disciflorum is 

 found wherever cultivated roses can flourish. Other limitations 

 imposed by host are exhibited by Uromycladium, confined to 

 acacias in Australia and the East Indies, and by Phragmidium, con- 

 fined to Rosaceae. Except for a few, all species of Gymnosporan- 

 gium have Rosaceae as aecial hosts. Ravenelia typically occurs 

 on leguminous hosts, but a few of its approximately 100 species 

 attack Euphorbiaceae and Tiliaceae. 



Influence of climate on rust distribution. From the many sur- 

 veys that have been made of whether the short-cycled rusts and 

 long-cycled ones are proportionally alike in all regions of the 

 world, it appears that short-cycled species are relatively more 

 abundant in mountainous regions and in northern areas than they 

 are in lowlands and in the tropics. This fact indicates that, just as 

 temperature is a limiting factor in latitudinal and altitudinal dis- 

 tribution of seed plants, so it is similarly operative among rusts. 

 In his summary of this subject as it pertains to North America, 

 Arthur ( 1929) divided the continent into the boreal zone, in which 

 23% of the total rust population is short-cycled, the temperate 

 zone, in which 19% is short-cycled, and the tropic zone, in which 

 15% is short-cycled. 



Latitudinal zonation of rusts is strikingly indicated by Arthur 

 (1929) as shown in Table 30. Some of these genera are plainly 

 more northern than others, and Ravenelia and Uropyxis are to be 

 considered tropical and subtropical. In fact, only 3 species of 

 Ravenelia, namely, R. opaca, R. cassiaecola, and R. epiphylla, 

 range north of 40° north latitude. 



Endemism among rusts. Since so many rusts attack plants of 

 economic importance, it would be anticipated that each area into 

 which alien plants or plant parts have been introduced would con- 

 tain non-native species of rusts. That such is the case is shown 

 by the work of Arthur (1929). Of approximately 1000 species of 

 North American rusts, only about 600 are held to be endemic. 

 McAlpine (1906) regards 31 of the 161 rusts in Australia as 

 aliens. 



