86 



THE PLANT SKELETON 



and strengthening sclerenchyma cylinder, and all are bound 

 together by thick-walled ground parenchyma, and bundles of 

 subepidermal bast fibers flank the most exterior bundles (Fig. 

 39). These facts account for the hardness and strength of the 



exterior part of many grass 

 stems, such as corn and bam- 

 boo. 



In palm stems the skeleton 

 consists of numerous strands 

 of bast fibers in the peripheral 

 ground tissue, and large masses 

 of these fibers of extraordinary 

 hardness and strength sur- 

 rounding each vascular bundle 

 wherever located (Fig. 40). 

 There being no cambium, wood 

 fibers are not produced as in the 

 Dicotyledons, and the xylem 

 part of the vascular bundle is 

 occupied by tracheal tubes, 

 tracheids, and xylem paren- 

 chyma. 



In leaves the skeleton is in 

 the form of bast fibers attend- 

 ing the main ramifications and 

 anastomoses of the vascular 

 bundles; and sometimes sub- 

 epidermal bast strands occur at the edges of leaves or at other 

 places without direct connection with the vascular bundles. 

 Collenchyma is also sometimes employed to strengthen the 

 leaf borders. 



Fig. 41 indicates diagrammatically the progress in the devel- 

 opment of a woody plant. At and near the growing apex there 

 are no skeletal tissues. Some distance back from the apex 

 the collenchyma appears, and further back bast fibers and 

 stone cells reenforce the collenchyma. Finally, in older parts 



Fig. 41. — Diagram showing the progress- 

 ive development of the skeletal tissues 

 from the apex toward the base of the stem. 



