and the Exfoliation of the Cellular Coats of their Extremities. 187 



roots to form its wood ; so that here also we find the woody axis 

 at first in the form of distinct bundles, separated from each ether 

 by cellular tissue {medullary rays), but crowded closely together 

 in the centre, so that there is no pith. In the young root we 

 find the bundles belonging to the cotyledons largest, between 

 these the bundles belonging to a number of successive leaves. 

 As the stem has its leaves developed, the number of these bun- 

 dles is increased, until at length a complete circle is formed. 

 When the stem has its joints elongated, the number of bundles 

 extending down into the root is apparently more restricted than 

 when the root is crowned by a tuft of leaves. The bundles be- 

 longing to the leaves, formed at a certain height from the root, 

 have their origin at the points where some of the lower ones run 

 out into the leaves, so that they take the place of the latter in 

 the circle surrounding the pith. 



" When the root is not tuberous, the woody bundles grow by 

 the conversion of their cambial tissue into wood and ducts, and 

 soon form a solid mass of wood, the wedge-shaped parts of which 

 are more or less distinguishable in different cases. Sometimes 

 the medullary rays separating them remain tolerably large ; in 

 other cases these are lost sight of, and the separate bundles are 

 then often only roughly traceable by the arrangement of then- 

 larger ducts. 



" The woody axis thus formed exhibits at its outer surface 

 (next the rind) a cambium-region, 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 re- 

 newed 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 

 different departures from the 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. This tissue is bv no means a continuation of 



