THE NATURAL HISTORY OF BRITISH GRASSES. 23 
surface are not always true roots, such, for instance, as the 
runners in the common couch (Triticwm repens). These 
receive the name of Rhizome, or underground stems, and 
it is by means of these that the couch tribe of grasses so 
quickly spread from a common and small centre into large 
patches ; as, though they creep for a considerable distance, 
yet their points ultimately rise to the surface and then 
expand new leaves, and, in fact, form distinct and perfect 
individuals, which, if separated from the parent, all the 
more rapidly give rise to independent colonies, and indeed 
these scions do as their parent did before them. 
Several species of grasses have this tendency, and conse- 
quently when it occurs it forms a good distinctive character. 
Hence though the Triticum repens has a rhizome, the 7. 
caninum is only furnished with'a fibrous root; some of the 
Poas, as Poa pratensis and P. compressa, have rhizomes, 
whilst Poa annua and P. trivialis are without any tendency 
to a creeping habit of growth. 
Agriculturally it is necessary to distinguish the different 
forms of couch, as the species of one district may be absent 
from another; and as even the rhizome will vary in being 
large or small, so will its eradication much depend upon its 
difference in form and habit. However, we shall hereafter 
see that several species of grass become useful from this 
very structure in keeping together banks of sea-coast, canals 
and the like; and it is a matter worthy of serious consider- 
ation and careful experiment whether they could not be 
made available in consolidating the slopes of railway cut- 
tings, which give so much trouble and cause such constant 
yearly outlay on some lines. 
Culm—Stem.—The stems of grasses are usually hollow 
(fistular), to which, however, the Molinia cerulea (purple 
molinia) of wet places offers an exception in its solid stem. 
It is rounded, except in Poa compressa (fat-stemmed mea- 
dow-grass), in which the trivial name has been given from 
the oval form on a transverse section, as though it had been 
subject to compression. 
