THE STEM. 87 



109. Stele. — The outermost part of the stele often pro- 

 duces mechanical cells with thick walls and small cavities. 

 They are either aggregated in strands opposite to the vascular 

 strands of the stele, or they constitute a complete zone 



Fig. 76.— Transverse section of the stem of Elatine, showing intercellular canals, C. 

 Magnified about 15 diam.— After Reinke. 



around it. Many of the most valuable textile fibers, such as 

 those of flax, hemp, and ramie, are obtained from this region 

 of the stem (fig. 77). 



In any section of the stem the number of vascular strands 

 in the central cylinder varies greatly, not only in different 

 plants, but even in different parts of the same plant. The 

 strands are commonly arranged in pairs, a bast strand and a 

 wood strand being placed side by side, the former occupying 

 the side next the surface of the stem, and the latter .the side 

 next the center (figs. 72, 78). The number and position of 

 these bundles is, however, subject to change. In some 

 cases pne of the strands surrounds the other. Commonly 

 it is the bast which surrounds the wood, as in the fernworts. 

 Sometimes independent bast strands are found with which 

 are associated no wood strands. In the bast certain cells 



