REVISION OF THE GENUS TITHONIA. 
By 8. F. BuaKke. 
INTRODUCTION. 
The genus Tithonia, originally described in 1789 in Jussieu’s 
Genera! without citation of species, was adopted by J. F. Gmelin? 
two years later, and the single known species was given the binomial 
T. uniflora, a name which has been universally displaced by the later 
Tithonia tagetiflora, published by Desfontaines in 1802 with a full 
description and plate. The same plant, grown by Philip Miller in 
his Chelsea garden from seed sent presumably from Veracruz by 
William Houstoun, had been described in the eighth edition of the 
Gardeners’ Dictionary in 1768 as Tagetes rotundifolia, and as this is 
the earliest binomial given the species it must now be known as 
Tithonia rotundifolia. It is a showy annual with large, orange or 
golden-yellow heads, much like the common sunflower in appearance 
except for the yellow disk, and seems worthy the attention of 
horticulturists. 
As here recognized, the genus Tithonia includes ten species, native 
from northern Mexico to Panama. One species, T. rotundifolia, 
occurs also in the Greater and Lesser Antilles, and in Venezuela 
(where certainly introduced), and another, 7’. diversifolia, has become 
a weed in Ceylon and Burma and at Singapore. As the relationships 
of the genus to Helianthus and Viguiera have already been con- 
sidered in some detail by the writer in another publication,’ only 
brief notice of them is necessary here. The typical pappus-bearing 
members of the genus are separated from Helianthus by their per- 
sistent pappus of awns and squamellae, and from Viguiera chiefly 
by their fistulose peduncles and by certain details of involucre. The 
four species in which the achene is always glabrous and the pappus 
wanting, together with 7. brachypappa, in which glabrous epappose 
achenes occur in the same head with pubescent pappiferous ones, are 
to be distinguished from the species of Viguiera in which the pappus 
is likewise absent chiefly by their fistulose peduncles. 
Tithonia, being originally based on a species with pappiferous 
achenes, has by Bentham and Gray been restricted to such species, 
the plants of similar habit but with epappose achenes being referred 
to Gymnolomia. Schultz Bipontinus, describing Tithonia calva in 
1 Desf.; Juss. Gen. Pl. 189. 1789. 
2 Syst. Nat. 1259. 1791. 
3 Contr. Gray Herb. n. ser. 54: 8-10, 19, 21. 1918. 
10322—20——4 423 
