HISTOLOGY OF THE FERN 



113 



longer than the vertical; the two sides, furthermore, show some- 

 what flattened ledge-like surfaces which are turned upward, 

 a differentiation offering more resistance to up-rooting than 

 would be the case if the sides were smoothly rounded. The 

 cross section also shows various cellular differentiations. On 

 the outside is a layer of cells termed the cortex, consisting of an 

 outer black and lifeless layer to which the name epidermis is 

 given, while below this is a layer of hardened, almost wood-like, 



FIG. 44. Transverse section of a vascular bundle surrounded by funda- 

 mental tissue. The conducting system of the plant. (From Sedgwick and 

 Wilson.) 



cells forming the bulk of the cortex. These cortex cells are 

 hardened by the lignincation of the protoplasmic structures, 

 this lignin forming the chief element in the composition of the 

 still more hardened wood tissues within the rhizome. Within 

 the cortex is a mass of living parenchyma cells with soft and un- 

 differentiated protoplasm, well supplied with starch, and form- 

 ing the bulk of the mass of the rhizome. Scattered among these 

 parenchyma cells are two large masses and other smaller patches 

 of dark brown material formed by the lignincation of the funda- 



