234 



MOSSES AND FERNS 



CHAP. 



being folded longitudinally. There is no distinct hypoderma 

 formed, and the bulk of the leaf is made up of a uniform meso- 

 phyll composed of nearly globular cells with much chlorophyll, 

 and separated by numerous intercellular spaces. In the petiole 

 the tissues are similar, but more compact, and the walls of the 

 ground tissue are all deeply pitted. The vascular bundles are 

 nearly circular in section and show a compact mass of tracheary 

 tissue (Fig. i 1 7, /), surrounded by nearly uniform cells with 



Fig. 117. — Vascular bundle of the petiole of O. pendiihim, X 260 ; t, t, the xjdem of the bundle. 



moderately thick colourless walls. The limits of the bundle 

 are not, as in the higher Ferns, marked by a distinct bundle- 

 sheath, but are indicated simply by the somewhat smaller size 

 of the cells of the bundle itself — indeed it is not always easy 

 to say exactly where the ground tissue begins. The xylem is 

 composed of pointed tracheids whose walls are marked with 

 thick reticulate bands. This mass of tracheary tissue is situated 

 near the inner side of the bundle, which like that of the stem is 

 collateral. The rest of the bundle is composed of sieve-tubes 



