134 ANOMALOUS TYPES OF SECONDARY THICKENING 



towards the inside, the small number cut off on the outside 

 simply forming an addition to the cortex (C.). Some of the 

 inner segments divide and undergo gradual differentiation to 

 form secondary bundles (S.b.), whilst the remainder, retaining a 

 somewhat radial arrangement, become thickened and lignified. 

 The secondary bundles embedded in this thick-walled tissue are 

 concentric with central phloem (P. ) , but the latter is often very 

 scanty in amount ; the xylem contains no vessels consisting 

 of fibrous tracheids only. 



Very complicated types of anomalous thickening are exhibited 

 by the woody climbers (lianes) of tropical forests. In many of 

 these the old trunks develop cambial rings about several centres, 

 each such cambium producing a separate xylem -core, so that a 

 rope-like structure composed of several intertwined woody strands 

 results. 



