98 A TEXT-BOOK OF GRASSES 
swamps grasses are rare and in thick forest only a few 
broad-bladed shade-loving species are found. 
The species of grasses are frequently gregarious, form- 
ing large masses more or less to the exclusion of other 
plants. Familiar examples are the large areas of Indian 
rice and of Phragmites in swamps, and the zones of 
Homalocenchrus oryzoides around ponds. On prairies and 
plains, grasses are usually the dominant vegetation, though 
the species may mingle more than in the swamp plants 
mentioned above. During the flowering period, a particu- 
lar species may appear to the casual observer to be the 
only species present, but close examination usually proves 
the presence of various other species, some of which may in 
their turn appear dominant at another period. 
THE ROOT AND STEM 
122. The roots of grasses are fibrous. They are usually 
found at the base of the plant, but in decumbent or pros- 
trate stems they may be produced at the nodes. Support- 
ing or prop roots are sometimes produced in erect stems 
at nodes above the surface of the soil as in Indian corn. 
Underground stems which may have the appearance of 
roots will be discussed in another paragraph. 
123. The stem of grasses, known as the culm, is made 
up of a series of nodes and internodes. The nodes are the 
more or less swollen joints at which the leaves originate. 
The internodes when young are solid, that is, filled with 
pith, but at maturity the pith usually disappears leaving 
the culm in the form of a tube with solid partitions at the 
nodes. The wheat straw is a familiar example of this 
structure. In some grasses, such as the corn and sorghum, 
the internodes retain the pithy interior. 
