348 FERNS LESS. 



The cells of the hypodermis (E) are proportionally longer 

 than those of the parenchyma, and are pointed at each end : 

 they contain no starch. Their walls are greatly thickened, 

 and are composed not of cellulose but of lignin, a carbo- 

 hydrate allied in composition to cellulose, but containing a 

 larger proportion of carbon Schulze's solution, which, as 

 we have seen, stains cellulose blue, imparts a yellow colour 

 to lignin. 



Outside the hypodermis is a single layer of cells (c, ep) 

 not distinguishable by the naked eye and forming the actual 

 external layer of the stem : the cells have slightly thickened, 

 yellowish-brown walls, and constitute the epidermis. From 

 many of them are given off delicate filamentous processes con- 

 sisting each of a single row of cells : these are called hairs. 



In the sclerenchyma the cells (H) are greatly elongated' 

 and pointed at both ends, so as to have the character rather 

 of fibres than of cells. Their walls are immensely thickened 

 and lignified, and present at intervals oblique markings due to 

 narrow but deep clefts : these are produced by the deposition 

 of lignin from the surface of the protoplasm (see p. 32) being 

 interrupted here and there, instead of going on continuously 

 as in the case of a cell-wall of uniform thickness. 



The vascular bundles have in transverse section (B) the 

 appearance of a very complicated network, with meshes of 

 varying diameter. In longitudinal sections (c) and in dis- 

 sociated specimens they are found to be partly composed of 

 cells, but to contain besides structures which cannot be 

 called cells at all. 



In the centre of the bundle are a few narrow cylindrical 

 tubes (B and c, sp. v.) characterized at once by a spiral 

 marking, and hence called spiral vessels. Accurate exam- 

 ination shows that their walls (G) are for the most part thin, 

 but are thickened by a spiral fibre, just as a paper tube 



