164 The Natural Historij of British Grasses. 



expand in moisture, and so push the plants out of place, — or crack 

 in drought, in which case the rootlets, or active parts in life and 

 increase, are broken away just at the period when they are most 

 required. 'Roots are without buds, from which it will be seen 

 that all the parts of a grass which grow beneath the surface are 

 not always true roots, such, for instance, as the runners in the 

 common couch (Triticum 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 con- 

 siderable 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 consequently 

 when it occurs it forms a good distinctive character. Hence 

 though the Triticum repens has a rhizome, the T. caninum is onl}^ 

 furnished with a fibrous root ; some of the Peas, as Poa pratensis 

 and P. compressa, have rhizomes, whilst Poa anmia 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 consideration and careful experiment 

 whether they could not be made available in consolidating the 

 slopes of railway cuttings, which give so much trouble and cause 

 such constant yearly outlay on some lines. 



Culm — Stem (B). — The stems of grasses are usually hollow (y?5- 

 tular), to which, however, the Molinia cceridea (purple molinia) of 

 wet places offers an exception in its solid stem. It is rounded, 

 except in Poa compressa (flat-stemmed meadow-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. 



The stem is separated into long or short lengths, c^tWed joints, 

 by the intervention of nodes (C) (knots), which are solid and tend 

 much to strengthen the structure of the plant, to which end they 

 will be found to be closer at the base, where the strain would be 

 greatest on account of these light plants swaying forwards and 

 backwards in the wind, and more remote upwards in the culm, from 

 which are suspended the newer and more active leaves. 



Stems may vary in being quite smooth, ribbed, armed with hairs 



