TAXONOMY OR CLASSIFICATION 157 
economic value of included species. Keys are given to all 
genera native or commonly cultivated in the United 
States. 
198. Characters used in classification—The con- 
sensus of botanical opinion is that genetic relationships 
among phanerogams are best shown by the structure of 
the flowers. Grasses are no exception to this rule and 
hence the classification is based upon the structure of the 
spikelets. The preceding key indicates the characters 
used in classifying the tribes. The classification appears 
to be somewhat artificial, but nevertheless it brings to- 
gether in the same tribe genera that are evidently related. 
But it also in some cases, separates into different tribes 
genera that are closely related. Reference will be made 
to some of these cases again in the appropriate place. 
199. Phylogeny.—As to the phylogeny of the grasses, 
it is probable that the most primitive existing forms are 
those in which the spikelet consists of a series of flowers 
in the axils of herbaceous bracts. The simpler genera of 
Bambusex, such as Arundinaria, probably represent the 
lower or more primitive forms. It must not be understood 
that this tribe is, as a whole, less developed than the other 
tribes. Some genera are highly developed. There is good 
ground for believing, however, that the Bambusez arose 
from forms more primitive than those that gave rise to 
the other tribes. The Festucez and Hordex probably come 
next in phylogenetic development, while the Andro- 
pogonese and Panicee are highly developed. The exact 
relationship of the various tribes and the smaller groups 
is, of course, for the present largely a matter of conjecture 
and individual opinion. Phylogenetic ideas are expressed 
by the grouping of forms rather than by attempting to 
trace lines of descent. We may group allied species into 
