ROBINSON.— GENERA OF THE EUPATORIEAE. 431 
The presence or absence of a true (though often much reduced) 
pappus becomes especially important in distinguishing Ageratum § 
Coelestina and Alomia and leads, as will be seen elsewhere in this 
paper, to some readjustment of generic lines. The character is be- 
lieved to be a good one and to lead to a real distinction between groups 
which have hitherto been very poorly delimited. 
To see clearly what is here meant it is only necessary to compare 
Ageratum micropappum, possessing a true but excessively reduced 
pappus, with Ageratum heterolepis, A. echioides, A. microcephalum, or 
A. microcarpum, all of which so far as observed by the writer have no 
real pappus but merely a sub-corollar annulus and in consequence 
are to be referred to Alomia. 
The involucre in the Eupatorium tribe takes on many forms, all 
more or less intergrading. In Eupatorium itself, though the involucre 
goes through the whole gamut of variation, the number of species is 
so great that grouping by involucral differences has never appeared 
to be sharp enough to permit any satisfactory generic segregation of 
the elements concerned. It is very easy to refer certain marked 
Species to Osmia Sch. Bip. and others to Kyrstenia Necker, but tran- 
sitions are innumerable. On the other hand in many of tbe smaller 
genera involucral characters furnish distinctions of sectional or even 
generic value. Three types of involucre may be recognized as follows: 
1) in which the chief scales are definite or subdefinite in number (four 
in Mikania and Kanimia, five or six in Stevia) often surrounded at the 
base by 1-3 considerably reduced scales i. e. calyculate. 2) the chief 
scales of indefinite number (though rarely very numerous) and sub- 
equal, appearing to be in 1-3 series and often accompanied by a 
very few much reduced outer ones. This is the common form of invo- 
lucre in Ageratum, Kuhnia, etc. 3) scales of indefinite number (usually 
humerous) conspicuously unequal and gradually diminishing outward, 
forming apparently several to many series, though of course in reality 
spirally arranged. These three types of involucre, while sometimes 
confluent, are in general pretty readily distinguished. 
There are also considerable differences in the texture of the involu- 
cral scales and to some extent these may be used, at least as supple- 
mentary aids, in distinguishing genera. Thus in Ageratum, most 
Alomias, and several other genera the scales are prevailingly of rather 
firm texture and pretty definitely 1-3-costate, while in Brickellia, 
ic, tax oa oem Oaxacania, etc. the scales are prevailingly 
, , ely striate. 
One other character of special classificatory significance in the 
