VIII 



THE PTERIDOPHVTA—OPHIOGLOSSACEyE 



233 



cell. After the plant has reached a certain age, each leaf gives 

 rise to a sporangial spike, which becomes evident while the leaf 

 is still very small. The first indication of this is a conical 

 outgrowth upon the inner surface of the leaf, about halfway 

 between the apex and base. A longitudinal section of this 

 shows it to be made up of large cells, especially toward the 

 top ; but although there was sometimes an appearance that 

 indicated the presence of a single apical cell, this was by no 

 means certain, and if there is such an initial cell, its divisions 

 must be very irregular. 



The subsequent growth of the leaf is for a long time mainly 

 from the base, and the young 

 sporangial spike is much nearer 

 the apex in the next stage (Fig. 

 114, B). No distinct petiole 

 has yet developed, but the centre 

 of the young 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. The young 

 spike now forms a beak-shaped 

 body curving inward and up- 

 ward, and sections of slightly 

 older stages than the one 

 figured show the first indications 

 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, irregularly 

 polygonal in outline, with large stomata at intervals, about which 

 the cells are arranged concentrically, and frequently with a good 

 deal of regularity. The stomata themselves (Fig. 116), 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. The upper walls of the guard 

 cells are thickened irregularly, giving them the appearance of 



Fig. 116. — Stoma from the leaf of Ophioglossuin 

 pendtibim, x 260. 



