THE STEM AND THE LEAF 



49 



of it are, as in the case of the hollow flower stalk of the garden 

 rhubarb and the dandelion. More frequently the stem is not 

 hollow, but a large mass of very light spongy pith occupies 

 the interior, as in corn, young twigs of elder (Fig. 34, A) and 

 sumach, and in the entire stem of the sunflower. 



The stiffness of the young stem may be due almost wholly 

 to collenchyma, as in the balsam (Fig. 32) and the elder 

 (Fig. 34, A), or it may depend largely on the presence of 

 wood fibers and tracheids in the bundles, as in the sunflower 

 (Fig. 30). Sometimes collenchyma and fibers cooperate, as 

 shown in the flower stalk of Eryngium (Fig. 34, .&). In the 



coll 



- -- cort 



FIG. 34. Arrangement of strengthening tissue A, I>, in stems; (7, in the root 



A, cross section of a young elder twig; B, cross section of flower stalk of Eryn- 



gium ; C, cross section of a small root ; coll, collenchyma ; cort, brittle cortex ; 



cyl, tough central cylinder ; /, fibrous cylinder around a central hollow portion ; 



p, pith ; w, woody bundles surrounding the pith. After Strasburger 



case of dicotyledonous trees the stiffness of the trunk, resisting 

 the severest storms, is mainly due to the immense number of 

 tracheids and fibers in the wood of the annual cylinders. 



The stems of woody climbers need to be at once tough and 

 flexible. Many such vines have, while young, the structure 

 shown in the cross section of Dutchman's-pipe (Fig. 29), with 

 the bundles arranged in a discontinuous series around the 

 central pith and not united into a cylinder. This makes the 

 stem flexible in the same way that a wire cable is more flex- 

 ible than a solid metal rod. 



Roots (except prop roots) do not need to possess much stiff- 

 ness ; it is necessary for them to be tough to resist lengthwise 



