PAPERS ON BIOLOGY AND AGRICULTURE 117 



that evidently occurred from an ancestral form while on 

 this host. 



The rusts now number 38 on 4-i hosts ; fourteen are ad- 

 ditions to the Hawaiian flora. In Hawaii there are 38 

 rusts on 999 potential hosts. In Porto Rico there are 175 

 rusts on 2250 potential hosts. In Indiana there are 172 

 rusts on 2339 potential hosts. The rust ratio of Hawaii 

 thus is .038 as against .077 for Porto Eico and .073 for 

 Indiana. The close agreement between the ratios of 

 Indiana and Porto Eico is striking, while the low ratio 

 of Hawaii shows rusts there to be approximately half as 

 common as in Porto Eico. The absence of aecial forms is 

 also striking; only one, an imported form, was found. 

 As to their continental relationship, it appears that six 

 came from America, one only from the West, and three 

 others may have come from either east or west. Con- 

 trary then to the Meliolas the rusts appear to have been 

 dominated by the east. Most of the rusts are clearly im- 

 ported and of very recent existence upon the islands. 



The six rusts on endemic hosts, too, are of veiy unusual 

 character, consisting of isolated sori and not a general 

 leaf infection such as is usual with other rusts. This 

 condition suggests to me the possibility that these rusts 

 are aU as yet ill-adapted to their hosts. All of the evi- 

 dence appears to show the rust flora to be recent as com- 

 pared with the Meliola flora. 



The Trichopeltaceae are particularly interesting mor- 

 phologically, presenting a type of thallus unique in the 

 fungi and in general resembling that of the livenvorts. 

 My collections in this group are larger than ever before 

 made and result in several new genera and some modifi- 

 cations in classilications j)reviously used. The genera 

 Trichopeltis, Trichothallus, Enthallopycnidium and Ano- 

 mothallus are of especial interest. 



Microthyriella hibisci is of interest as throwing light 

 on the possible relationship of the apple fly speck fmlgiis. 



Hexagonella is a remarkable form with solitary naked 

 asci in a tliallus of the type of the Hemisphaeriales. 



The Dothideales, which are very numerous in Porto 

 Eico, are very few in number. The genus Phyllachora so 

 common here and in most places is represented by only 



