352 MR. G. BENTHAM ON COMPOSITE. 
4. Differences in the Fruit, i. e. in the Achene and its Pappus. 
The fruit, which in many large and natural orders, such as Le- 
guminose, Umbellifere, Rubiaceew, Myrtacee, &c., has furnished 
many valuable characters for the distinction of genera or tribes, 
has naturally been looked to for similar purposes in Composite ; 
and Schultz Bipontinus, for instance, has prided himself on esta- 
blishing his genera on carpological principles. But throughout 
this vast order, so far as the achene itself is concerned, the 
structure is absolutely the same; there remain only outward form 
and consistence, which, however useful in the case of many genera, 
are, if too closely relied upon, apt in Composite, as in Leguminose 
and others, to break up very natural genera, especially when the 
modifications are the result of development in the course of growth 
from the ovary, and not discernible at the period of fertilization of 
the flower. The achene (always without its pappus) is very rarely 
of any use in determining the tribe of a Composite plant, beyond a 
few vague forms appreciable perhaps to the eye, though difficult to 
describe, which are only to be found in some one or two of the 
thirteen tribes; and, moreover, there are so large a number of 
species in which the ripe fully formed achene is as yet unknown, 
that its absolute value in large genera where it is usually so uni- 
form, such as Vernonia, Aster, Senecio, &c., is as yet very doubtful. 
Outward form, however, when ripe achenes are obtainable, is so 
prominent a character that much use has been made of it, and often 
to great advantage, in the definition of genera otherwise natural, 
and sometimes for that of subtribes or smaller groups of genera. 
The principal modifications are:—(1) the shape acquired in lateral 
development, (2) in longitudinal development, (3) the development 
of the ribs, (4) the consistence, (5) the surface and indumentum. 
In lateral development the achene as it ripens becomes angular, 
terete, or flattened, with a considerable degree of constancy in 
genera or even in subtribes, although in some cases, where it has 
been the most relied upon for the separation of large nearly allied 
genera (as in Asteroide:, for instance), there are usually a few ex- 
ceptional or intermediate species which forbid any absolute reliance 
upon it. When flattened it becomes important, as in Umbellifere, 
to consider whether the flattening is lateral (that is, in the direc- 
tion of the radius of the flower-head) or dorsal (that is, at right 
angles to the radius); this forms a good distinction, for instance, 
between the subtribes Verbesinew and Coreopside of Helian- 
thoidem. This character, however, applies only to the acheues of 
the disk ; those of the outer row, even when the others are laterally 
