124 RHIZOME OF PTERIS. [CH. IX 



cork-cells are loose and traversed by intervening spaces, 

 through which the internal parts of the stem receive air. 

 So that the lateral lines of the rhizome, although of 

 different morphological value, would seem to have the 

 physiological character of lenticels. 



The inside of the section presents three obviously 

 distinct tissues. It has patches and dots of a dark colour 

 and hard consistence which are irregular strands and 

 plates of sclerenchyma running longitudinally. Secondly, 

 there are yellowish spots of rounded or oval outline. 

 These, the vascular bundles, are not arranged regularly, 

 although they make, with the larger sclerenchyma bands, 

 a more or less defined mass in the section. The rest of 

 the rhizome is made up of soft pith-like parenchyma. 



These various tissues must be examined in detail. 

 The cells of the sclerenchyma are many times as long as 

 broad, and fit close together without intercellular spaces. 

 The walls are lignified, and have simple oblique slit- like 

 pits. 



The soft parenchyma is made of polygonal cells 

 roughly hexagonal in transverse section, with cellulose 

 walls not fitting closely together, but leaving inter- 

 cellular spaces. It is this tissue which comes to the 

 surface at the lateral lines. 



The parenchyma cells are crowded with starch grains 

 and serve as the storehouse of the rhizome. 



Vascular bundles 1 . 



Each bundle is surrounded by a bundle-sheath or 

 endodermis consisting of a single layer of small cuticu- 

 1 See Preface on the word stele. 



