486 



Oil the Structure of Roots. 



The woody axis thus formed, exhibits at its outer surface (next 

 the rind) a cambinm-ve^ion, where new development of wood 

 takes place, as in the stem, in perennial plants forming annual 

 rings, and where the buds giving rise to branches originate. But 

 when we proceed outwards from here we miss the next consti- 

 tuent of the stem, namely, the liber, or bast fibres, which are 

 absent from the root, ending at the " collar," or point of junction 

 of the root and stem. On the other hand, the cellular structure 

 of the rind or bark is mostly very much developed, and is renewed 

 on the inside by the cambium region, in proportion as its outer 

 parts are destroyed. The outer part of the rind of oldish roots 

 exhibits a corky texture, and in the roots of trees this rind 

 acquires great solidity, forming a kind of false corky bark if the 

 roots are exposed. 



Where the roots of Dicotyledons become tuberous, very dif- 

 ferent departures from tlie regular structure are met with in 

 different plants : for example, in the turnip and its allies, the 

 carrot, parsnip, &c., and the beet or mangel-wurzel. In the first 

 group the unnatural production of succulent cellular tissue takes 

 place in the medullary rays which invade and break up the 

 woody bundles, and scatter their elements so that they are found 

 distributed in irregular radiating rows in a great mass of paren- 

 chymatous tissue (fig. 13), This tissue is by no means a conti- 

 nuation of the pith of the stem, 

 although it bears some resem- 

 blance to it. There is a distinct 

 boundary of wood where the root 

 joins the stem. This is probably 

 of importance as regards the 

 " keeping" qualities of the roots. 

 In the carrot (fig. 6) there is a 

 similar development in the woody 

 region, but not so marked ; 

 while an equal, if not greater, 

 production of parenchyma takes 

 place on the outer side of the 

 cambium, forming a thick fleshy 

 rind. A thickened rind of this 



Cross slice of a young turnip, showing ihat tlie kind is found in mOSt of the 



tuberous structure exists in the wood, breaking fleshy, fibroUS rOOtS of perennial 



up and scattering the fibro-vascular structures ,,•' -,^. •■, , 



which appear in radiating lines, but are most herbaCeOUS DlCOtylCClonS SUCh 



developed near the circumference where they as ^roundsel primrose, &C. 



adjoin the rind. f^ ^j^^ ^^^J^^ ^^^ Structure both 



of the stem and root is unlike that of ordinary Dicotyledons, and 

 the changes produced by cultivation cannot be discussed here. 



The roots of Monocotyledonous plants — such as those of 

 grasses, onions, ordinary bulbous plants, &c., are temporary struc- 



Fig. 13. 



