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 Graminese, 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, 30-40 metres in some of the bamboos. 

 The narrow, two-ranked leaves, with their sheathing 

 bases, and the chaffy scale.s 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 

 (Cyperacese), which are usually associated with the 

 grasses in a single order, Glumaceae. It seems proba- 



