VINE TYPE OF STEM IN DICOTYLEDONS 119 



Obtain a transverse seetioii of a one-year-old stem of the 

 " Dutchman's l^ipe" [Ayistoluchia) and examine it under low 

 power. The following tissues and regions can be seen in pro- 

 gressing from the edge of the section to its center, viz. : 



(fl ) A typical uniseriate epidermis composed of rather densely 

 protoplasmic cells. Note the extremely thick cuticle which covers 

 the epidermis. 



(h) Internal to the epidermis occurs the cortex, formed of an 

 outer zone of rather thin-walled "angular" collenchyma cells and 

 an inner zone of large "isodiametric" parenchyma cells; note 

 the presence of large druses in many of the parenchyma cells of 

 the cortex. 



(c) The stele is sharply delimited by a broad cylinder of 

 closely joined pericyclic fibers, the secondary walls of which are 

 still increasing in thickness. Internal to the peric.yclic fibers oc- 

 curs the parenchynia of the pericycle which is quite similar in 

 general structure to the cortical parenchyma. 



(d) The vascular system of the stele consists of a ring of 

 typical collateral hundles in each of which the clearly distinct 

 phloem and xylem is separated by a cambial zone (the fascicular 

 cambium). Notice that an interfascicular cambium has arisen as 

 the result of the tangential division of certain of the parenchyma 

 cells of the broad medullary rays which separate the bundles. 



The pith of the stem is large and is composed of ' ' isodiametric ' ' 

 parenchyma cells in many of which druses are evident. 



Next obtain a transverse section of a two- or three-year-old 

 stem and notice the profound changes in structure which have 

 been occasioned by secondary growth, viz. : 



(a) A discontinuous and rather thick layer of phellem (cork) 

 has appeared as the result of the activity of the phellogen layer. 

 Notice particularly how the epidermis has been forcibly broken 

 away from the cortex ; strips of epidermal tissue are visible on 

 the outer surface of the corky tissue. A comparatively extensive 

 development of phelloderm can also be seen internal to the phello- 

 gen. Lenticels are w^ell-developed at certain points. 



(b) The previously continuous cylinder of collenchyma has 

 been broken as the result of the pressure of secondary growth. 



