002 



SECONDARY GROWTH IN THICKNESS 



many cases arises from unlignified thin-walled xylem- parenchyma : in 

 certain instances, however, it is lignified elements associated with 

 vessels, or cells of the xylem rays, that undergo this secondary 

 growth and division. Warburg, on the other hand, describes how, in a 

 species of Bauhinia, the initial cells of the strips and wedges of 



Fig. 2S7. 



T.S. through a stem of Bauhinia sp. The irregular masses of xylem () dotted, the 

 phloem and parenchymatous tissue (6) shown in white. Two-thirds nat. size. After 

 Schleiden. 



dilatation-parenchyma originate within the phloem, and only secondarily 

 penetrate into the wood. 



Once separate strands of wood have been formed, they may each 

 grow in thickness by means of independent secondary meristems ; 

 additional strands may also be interpolated between the first-formed 

 bundles. The whole stem thus comes to consist of a tangle of inter- 

 woven strands of wood, and accordingly often presents a highly 

 complicated appearance in cross-section (Fig. 287). The resemblance 

 of such a liane-stem to a thick rope is greatest, where the plates 

 of tissue interpolated between the various strands of wood break 

 asunder and are partly replaced by periderm. In such cases, mere 

 external inspection shows the stem to be split into a number of 

 longitudinal strands enveloped in cork, and interwoven or partially 

 fused in a complicated manner. 



