370 PLANT MORPHOLOGY 



also of fibers and parenchmya (Fig. 314.4, B). Companion cells do not 

 occur in pteridophytes and gymnosperms. As in the Filicinae and gym- 

 nosperms, leaf gaps are formed in the vascular cylinder in connection with 

 the departure of leaf traces. 



Stelar Types. The stem of most woody dicotyledons is like that of 

 gymnosperms in being an ectophloic siphonostele, the vascular tissues 



Cortex Epidermis p^,^^^ 



Voiscular rav \ ^^-^^^^^^Z .Xylem 



\^(gP^^~^^iJM>»^LiZ^^^^fc_/ fibers 

 P'+l^x J^lttz^m^YmM^l^\>. Cambium 



Fig. 315. Cross section of a >oung stem of magnolia {Magnolia grandiflora), showing 

 vascular cylinder surrounded by the cortex and enclosing the pith, X 17. 



forming a nearly continuous cylinder enclosing the pith (Fig. 315). This 

 cylinder, consisting of xylem and phloem in collateral arrangement, is 

 traversed by numerous vascular rays. As in the root, the cambium, a 

 meristematic layer of cells, arises between the primary xylem and phloem. 

 Through cambial activity, the stems of woody dicotyledons undergo a 

 great deal of secondary thickening, increasing in diameter from year to 

 year. 



The stems of herbaceous dicotyledons are like those of woody dicotyle- 

 dons except that the vascular tissues are greatly reduced in amount, 

 either as a result of diminished cambial activity, resulting in a continuous 

 but narrow vascular cylinder, or by the breaking up of the cylinder into 

 separate bundles to form a dictyostele. In such stems the vascular bun- 

 dles are at first separated by wide bands of parenchyma connecting the 

 pith with the cortex. The cambium may extend across these ''pith rays " 



