148 



PART II. ANATOMY AND HISTOLOGY. 



35 



Papayacese, where the wood consists largely of parenchyma), abundant in 

 the bast. 



In Monocotyledons there is no primary cambium-layer, the bundles 

 being all closed. In some cases, however, secondary growth in thickness is 

 effected by a ring of meristem quite external to the primary bundles; this 

 occurs in the stems and roots of some arborescent Liliaceae, such as 



Yucca and Dracaena, where 

 a rin S of meristem is usually 

 developed in the pericycle, 

 but in the roots of Dracaena 

 it is formed partly from the 

 pericycle and partly from the 

 cortex. This meristem-ring 

 is not termed a cambium- 

 ring, because it does not form 

 wood on one side and bast 

 on the other, but it forms, 

 centrifugal ly, entire closed 

 concentric (with external 



^j/ wood) bundles, together with 

 intervening ground-tissue 

 (Fig. 117). 



The development of 

 secondary vascular tissue 

 takes place almost exclu- 

 sively in such stems as are 

 monostelic and in which 

 the primary bundles are 

 common. It is clear that 

 the additions to the prim- 

 ary bundles in the older 

 internodes of the stem, as 

 well as any secondary 

 bundles which may have 

 been formed from the cam- 

 bium are not common, but cauline ; they are, however, in communi- 

 cation with the primary common bundles of the young unthickened 

 internodes which are bearing leaves ; in fact, the newly-formed 

 secondary vascular tissue of the lower internodes of the stem is in 

 communication, on the one hand with the root, and on the other 

 with the leaves ; and the channels of communication between root 

 and leaf are maintained year by year by the annual formation of 

 young conductingrtissue, both wood and bast, in the older parts of 

 the stem and of the root. 



F:o. 117. Portion of a transverse section of the 

 stem of a Dracaena: e epidermis; k periderm ; r 

 primary cortex, with a leaf-trace-bundle b; x 

 merismatic zone in which new bundles g-g are in 

 course of development ; m primary, and st second- 

 ary, ground tissue. (Magnified : after Sachs.) 



