THE PLANT SKELETON 



253 



fibers have become of considerable economic importance as 

 textiles. In the jute plant the sclerenchyma fibers are developed as 

 part of the phloem. In the hemp and flax plants the bast fibers 

 are found in the pericycle just underneath the bark. Hemp 

 fibers may be nine feet in length and are often lignified. Flax 

 fibers are shorter — usually one to three feet in 

 length — and are less lignified; for the latter 

 reason they are more pliable and suitable for 

 weaving into cloth. 



The arrangement of supporting tissues in a 

 Dicot is such as to adequately bear the alter- 

 nating stresses of compression and tension. If 

 the only strain imposed upon a trunk were 

 that of compression, resulting from the down- 

 ward pressure of the weight of foliage and 

 branches, the supporting tissues could be 

 distributed throughout the trunk in almost any 

 fashion. But in addition to supporting the 

 weight, the trunk is continually subjected to 

 sidewise stresses which produce a tension that 

 becomes greater in the outermost regions of the 

 trunk. For this reason the best distribution of 

 the supporting tissues involves their occurrence 

 groups or strands around the periphery of the 

 stem. The engineering design of a tree trunk 

 with its disposition of skeletal tissues is similar 

 to the construction of the I-beams used in 

 [.building. Such a beam is expanded upon the 

 upper and lower surfaces where there is 

 the greatest stress; these flanges are con- 

 nected by a narrower central web which needs to bear a lesser 

 strain. Since a plant stem has to meet lateral stresses firom all 

 directions, the groups of supporting tissues symmetrically ar- 

 ranged underneath the bark act as the flanges of a number of 

 imaginary girders whose webs — the pith or parenchyma tissues — 

 cross each other at the center of the stem (fig. 177). 



The distribution of supporting tissues in leaves is correlated 

 with the fact that the upper surface of the leaves is usually under 



Fig. 177.— Ar- 

 rangement of sup- 

 porting tissues in a 

 Dicot stem is such 

 as to meet lateral 

 stresses from all 

 directions; sup- 

 porting tissues rep- 

 resented by dot- 

 ted areas. 



