STRUCTURE OF THE STEM 



89 



102. Mechanical Importance of Distribution of Material 

 in the Dicotyledonous Stem. It is easy to see that those 

 tissues which are tough, like hard bast, and those which 

 are both tough and stiff, like wood fibers, are arranged in 

 a tubular fashion in young dicotyledonous stems as they 

 are in some monocotyledonous ones (Fig. 53). Sometimes 

 the interior of the stem is quite hollow, as, for example, 



B 



e ck 



c w d m 



FIG. 58. Stem of Box-Elder One Year Old. (Much magnified.) 

 A, lengthwise (radial) section ; B, cross-section ; e, epidermis ; ck, cork ; 6, hard 

 bast ; s, sieve-cells ; c, cambium ; w, wood-cells ; m, medullary rays ; d, 

 ducts ; p, pith. 



in the stems of balsams, melons, cucumbers, and squashes, 

 and in the flower-stalks of the dandelion. In older stems, 

 such as the trunks of trees, the wood forms a pretty nearly 

 solid cylinder. 



Stiffness in dicotyledonous stems is secured mainly in 

 two ways:*(l) by hard bast fibers, (2) by wood fibers. 

 Which of these types does the stem (Fig. 55) represent? 

 Which does the' flax-stem (Fig. 60) represent? 



