VII PTERIDOPHYTA—PlLICINEAl—OPIilOGLOSSACEJE 251 



has yet developed, but the centre of the younsL^ leaf, up to the 

 point of attachment of the spike, is traversed by the thick mid- 

 rib, above which the lamina is still very small. Indeed in this 

 stage it looks as if the spike were really terminal and the lamina 

 a lateral appendage. The young spike now forms a beak- 

 shaped body curving inward and upward, and sections of 

 slightly older stages than the one figured show the first indica- 

 tions of the developing sporangia. Later still the base of the 

 leaf becomes narrowed into the petiole, and the spike also 

 becomes divided into the upper sporiferous portion and the 

 short slender pedicel. 



The anatomical structure of the leaf is extremely simple. 

 The epidermis is composed of 

 rather thick-walled cells, irreg- 

 ularly polygonal in outline, 

 with large stomata at intervals, 

 about which the cells are ar- 

 ranged concentrically, and fre- 

 quently wdth a good deal of 

 regularity. The stomata them- 

 selves (Fig. 135), seen from 

 above, have an angular outline, 

 but from below are perfectly 

 oval, and cross-sections show 

 that this appearance is due to a 

 partial overarching of the 

 guard cells of the stoma by the 

 surrounding epidermal cells. ^ ^ , , , r r ^ , • 



^ ^ Fig. 135. — Stoma from the leaf of Ophio- 



The upper walls of the guard giossum pendulum, X260. 



cells are thickened unequally, 



giving them the appearance of being folded longitudinally. 

 There is no distinct hypoderma formed, and the bulk of the leaf 

 is made up of a uniform mesophyll composed of nearly globular 

 cells with much chlorophyll, and separated by numerous inter- 

 cellular 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. 136, /), surrounded 

 by nearly uniform cells with 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 



