THE PLANT SKELETON 



251 



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which are often traversed by branching canals extending out- 

 wards from the cell cavity. Stone cells lend rigidity to the shells of 

 nut fruits, and form the "pit" or stone of drupe fruits. They are 

 also found in the bark of woody plants where they afford some 

 support to this protective tissue. Both 

 collenchyma and stone cells are of little 

 economic importance. 



Fibers are sclerenchyma cells which 

 are greatly elongated, like the tracheids of 

 conifer wood; the extremities of each fiber 

 are pointed or chisel-shaped so that the 

 cells dovetail into one another and form 

 compact strands (fig. 175). Fibers lose their 

 protoplasm and are therefore dead when 

 mature. They are the most highly developed 

 of plant supporting tissues, being flexible 

 enough to bend under tension and strong 

 enough to stand considerable compression. 

 Fibers of stems are found in the pericycle 

 zone, between the vascular bundles and 

 the bark; in the food conducting region of 

 the phloem; and in the xylem or wood. 

 Fibers found in the former two regions of 

 the stem are commonly known as bast 

 FIBERS, while the xylem fibers are known as 

 WOOD FIBERS. Wood fibcrs are more com- 

 pletely lignified than most bast fibers and 

 make up a large proportion of the wood of a dicot. 



The fibers constituting the skeleton of a woody dicot stem are 

 highly specialized, and completely diflerentiated from the con- 

 ductive tissues (sieve tubes and vessels). Where the wood fibers 

 are numerous in the xylem, the wood is dense and hard as is 

 typically seen in the oaks or hickories. Many of the commercially 

 important Dicot woods are of this nature and are therefore 

 referred to as hardwoods. However some Dicots such as bass- 

 wood and balsa are light soft woods because of the preponderance 

 of thin walled skeletal elements. The grain of hardwoods, like 

 that of conifers, is determined by the annual growth of the wood 



Fig. 175.— Fibers 

 are greatly elongated 

 sclerenchyma cells 

 with pointed extremi- 

 ties; they are known 

 as bast fibers when 

 found in the phloem, 

 and as wood fibers 

 when located in the 

 xylem. 



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