188 EVOLUTION OF PLANTS 
tropics, which seem to be to a certain extent interme- 
diate between the two. 
Another very sharply defined order of Monocotyle- 
dons is the Graminesx, or Grasses. These are cosmo- 
politan in their distribution, and in the temperate regions 
they form one of the most important elements of the 
vegetation, especially over open, exposed areas. Eco- 
nomically they are the most important of all plants, as 
they include all the cereals, as well as sugar-cane and 
bamboo, and are the most important food plants for 
herbivorous animals. The number of grasses exceeds 
that of any other group of Monocotyledons except the 
orchids. They are, however, very uniform in the struct- 
ure of the stem and leaves as well as of the flowers. 
The stems are jointed and usually hollow, sometimes 
of gigantic size, 80-40 metres in some of the bamboos. 
The narrow, two-ranked leaves, with their sheathing 
bases, and the chaffy scales about the simple flowers, 
are constant characters of this very natural family. 
Certain peculiarities of the ovule and the gameto- 
phyte indicate that the grasses belong near the bottom 
of the series of Monocotyledons, but their great num- 
bers and wide distribution show that they have become 
sufficiently modified to adapt them very perfectly to 
existing conditions. 
Owing to the absence of any forms intermediate be- 
tween the grasses and the other Monocotyledons, their 
exact position in the system is still very uncertain. 
Closely resembling the grasses externally, but differing 
from them in many important particulars, are the Sedges 
(Cyperacee), which are usually associated with the 
grasses in a single order, Glumacew. It seems proba- 
