INTERNAL ANATOMY OF ORGANS. ROOTS. 537 



of each of the primitive liber-bundles. This secondary cambium grows 

 both on its outer and its inner surface, forming ultimately on the outer 

 side liber-cells, on the inner side vessels exactly as in the case of the Dico- 

 tyledonous woody bundles. In general terms, then, it may be said that 

 tne Dicotyledonous root consists of a cellular mass encircled by cortex. 

 In the central cellular mass are formed two or more sets of vessels and 

 of liber-cells, each distinct from the other and alternating one with the 

 other (fig. 583). After this primary stage of growth is completed, a 

 secondary development of fibro-vascular bundles, with the liber outside 

 and the vessels inside, takes place on the interior of the primitive liber- 

 bundles, which latter are therefore pushed outwards. Ultimately, then, 

 there are two or more radiating plates of primitive vascular tissue sepa- 

 rated by cellular tissue from a series of radiating plates, consisting of 

 liber and vessels. 



Great variations occur in different plants in the number and exact dis- 

 position of the bundles &c., for an account of which the original memoirs 

 of the authors above cited must be consulted. 



In the adult state the axial root of Dicotyledons, being a direct con- 

 tinuation of the stem, displays a circular group of fibro-vascular bundles 

 as in the ascending axis ; but these mostly converge at the point of 

 junction of stem and root (collar}, so that the central axis of parenchyma, 

 the pith, is usually absent, the medullary rays remaining as in the stem. 

 The roots of Dicotyledons increase in diameter by annual layers of wood 

 formed in the fibro-vascular bundles, these, however, being less regular 

 in their arrangement than those of the stem on account of the tortuous 

 course of the roots j hence while the wood of the roots is often useful for 

 ornamental purposes, it is comparatively valueless for carpenters' uses. 

 The branches of the axial root are originally growths from the apex of 

 the root, thrown off to the side, as it were, and their woody axis is 

 derived from a division of that of the main root. 



The radicle of a germinating Dicotyledon has its root-cap, and grows 

 in the same way as that of the Monocotyledons, by development of cells 

 just behind the apex (figs. 582, 583). 



Root-hairs. Young roots are covered by a delicate epidermis ; 

 and the cells of this are abundantly produced into hairs in many 

 plants (fig. 584), especially in those growing in light soils ; these 

 fibrils are deciduous, the delicate epidermis (which is always desti- 

 tute of stomata) being gradually converted into a corky layer. 



Adventitious Roots are very common in Dicotyledons, especially the 

 herbaceous perennial kinds, and they alone can exist on plants raised 

 from cuttings &c. of stems. The roots originate much in the same way 

 as those of the Monocotyledons, appearing first as cellular cones in the 

 region adjacent to the cambium -layer, with which the fibro-vascular 

 structure soon becomes confluent. They break through the rind, with a 

 coleorhiza, and protected by a pileorhiza, just as in Monocotyledons ; but 

 when once formed, they appear to branch in the same manner as the 

 axial root, and not by the formation of secondary adventitious roots. 



Trecul states that the structure of adventitious roots differs according 

 to the part of the stem whence they emerge. If, for instance, they 

 originate opposite a fibro-vascular bundle, as in Nupha,r, the centre of the 



