380 PLANT STUDIES 
It is the combination of characters which determines a 
group. 
250. Monocotyledons. In the Monocotyledons about forty 
families are recognized, containing numerous genera, and 
among these genera the twenty thousand species are dis- 
tributed. It is evident that it will be impossible to con- 
sider such a vast array of forms, even the families being too 
numerous to mention. 
Prominent among the families are the aquatic pond- 
weeds of various kinds, the marshy ground cat-tails, the 
grasses and sedges, the tropical palms, the aroids, the lilies, 
and the orchids. Of these, the grasses form one of the 
largest and one of the most useful groups of plants. It is 
world-wide in its distribution, and is remarkable in its dis- 
play of individuals, often growing so densely over large 
areas as to form a close turf. If the grass-like sedges 
be associated with them there are about six thousand 
species, representing nearly one third of the Monocotyle- 
dons. Here belong the various cereals, sugar-canes, bam- 
boos, and pasture grasses, all of them immensely useful 
plants. 
The palms and the aroids each number about one thou- 
sand species, and are conspicuous members of tropical vege- 
tation. 
In temperate regions, however, the lilies and their allies 
stand as the best representatives of Monocotyledons, with 
their usually conspicuous and well-organized flowers. 
In number of species the orchids form the greatest 
family among the Monocotyledons, the species being vari- 
ously estimated from six thousand to ten thousand. In 
display of individuals, however, the orchids are not to be 
compared with the grasses, or even with the lilies, for in 
general they are what are called "rare plants." Orchids 
are the most highly developed of Monocotyledons, and their 
brilliant coloration and bizarre forms are associated with 
marvellous adaptations for insect visitation. 
