16 



MORPHOLOGY, OB COMPARATIVE ANATOMY. 



of Monocotyledonous plants, such as grains of Oats, Wheat, &c. ; but 

 their essential character may be still more clearly distinguished in plants 

 which form adventitious roots on well-developed stems -p,. ~ 

 and bud-like structures. The fibrils which sprout from 12P * 



the joints of the stems of numerous creeping plants 

 (Ground-Ivy, Mint, Sand-Sedge, &c.), the clamping roots 

 of Ivy-stems, the roots of an Onion-bulb, as well as those 

 formed from slips or cuttings, &c., afford familiar examples 

 of adventitious roots. 



Ramification. Where the branches of the root 

 are comparatively small and the central axis is both 

 thick and considerably elongated, the root is called 

 a tap-root (fig. 6) ; where the branches are deve- 

 loped so that the principal axis is lost as it were in 

 its own ramifications, the root is called fibrous 

 (fig. 5). The branches issue from the main root 

 in succession from above downwards (not from 

 below upwards as in stems), and are, in the 

 first instance, regularly arranged in rows one 

 above another. The number of rows varies in Fusiform tap-r 

 different cases ; and the regularity of disposition is of the Car 

 soon lost. 



root 



TOt 



ucus Carota). 



Fig. 7. 



When the tap-root exists in herbaceous plants, it often exhibits 

 a more or less succulent character, and becomes a tuberous root, as in 

 the biennial Turnip, Carrot, Beet, &c., where this organ is peculiarly 

 developed in the first season of growth, to serve as a reservoir 

 of nutriment. The tendency of such 

 plants to exhibit this character in excess 

 under the influence of stimuli renders 

 them extremely valuable for economic 

 purposes. The fibrous rootlets upon the 

 surface of tuberous tap-roots, like the 

 Carrot, Parsnep,&c., appear to be mostly 

 true roots. A distinction is made, in 

 describing tuberous roots, between those 

 which are fusiform, as in the Carrot, and 

 those which are napiform, as in the 

 Turnip. A woody tap-root is found in 

 many forest-trees, as, for example, in 

 the Oak ; but here the branches share 

 more extensively in the increase in size, 

 and their direction tends more to the 

 horizontal. Fibrous roots are particu- 

 larly characteristic of plants growing 

 in light and sandy soils or in water; 

 the perennial, woody forms are especially characteristic of shrubby Di- 

 cotyledons. 



Tuft of fibrous adventitious roots of a 

 Grass. 



