THE STEM. 



103 



the tissues (fig. 118). In some cases vascular bundles are 

 found in the cortex outside the stele, when they are known 

 as cortical bundles. 



FIG. 118. Transverse section of the stem of a ground pine (Lycopodium complana- 

 tion). The stele is enclosed by the endodermis, en; p, pericycle; x, xylem bundle; 

 ph, phloem bundle; cc', cortex, c', mechanical tissue with thickened walls; ep, epi- 

 dermis. In the cortex a branch stele passing out to a leaf on the right is cut across. 

 Magnified 100 diam. After Sachs. 



130. 3. Stele, (a) Pericycle. The pericycle is rarely 

 wanting. It is much more frequently increased from one to 

 several layers of cells. In this case it commonly differ- 

 entiates into regions of mechanical cells with thick walls and 

 small cavities and a region of thin-walled cells. These 

 mechanical cells are either aggregated in strands opposite to 

 the vascular bundles of the stele, or they constitute a com- 

 plete zone 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. 119). 



131. (b) Vascular bundles. --In any section of the stem 

 the number of vascular bundles in the central cylinder varies 

 greatly, not only in different plants, but even in different 

 parts of the same plant. The bundles are commonly arranged 



