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, 



