VIII THE PTERIDOPHYTA—OPHIOGLOSSACE/E 243 



also found in B. teniatum and some other species, but in B. 

 simplex and the other simpler species it is perfectly smooth, as 

 in OpJiioglossuin. The young leaves in B. Virginianuvi arc 

 bent over, and the segments of the leaf are bent inward in a 

 way that recalls the vernation of the true Ferns. The spor- 

 angiophore grows out from the inner surface of the lamina, 

 and its branches are directed in the opposite direction from 

 those of the sterile part of the leaf. 



The vascular bundles of the stem are much more 

 prominent than in OpJtioglossuni, and form a hollow cylinder, 

 with small gaps only corresponding to the leaves. This 

 cylinder shows the tissues arranged in a manner that more 

 nearly resembles the structure of the stem in Gymnosperms 

 or normal Dicotyledons than anything else. Surrounding the 

 central pith (Fig. 123, P) is a ring of woody tissue (r) with 

 radiating medullary rays {111), and outside of this a ring of 

 phloem, separated from the xylem by a zone of cambium (c), 

 so that here alone among the Ferns the bundles are capable 

 of secondary thickening. The whole cylinder is enclosed by 

 a bundle -sheath (endodermis) consisting of a single layer of 

 cells. 



The cortical part of the stem is mainly composed of 

 starch-bearing parenchyma, but the outermost layers show a 

 formation of cork, which also is formed in the cortical portions 

 of the roots. 



The free surface of the stem apex is very narrow, and the 

 cells about it correspondingly compressed. The apical cell 

 (Fig. I 24, A, B), seen in longitudinal section, is very deep and 

 narrow, but as comparison of cross and longitudinal sections 

 shows, has the characteristic pyramidal form, and here there 

 is no doubt that only lateral segments are cut off from it. 

 Holle's ^ figure of Botrydiium rutczfoliuvi closely resembles B. 

 Virginianuvi, and probably the other species will show the 

 same form of apical cell. The divisions are decidedly more 

 regular in the segments of B. Virginianuvi than in OpJiio- 

 glossum, and can be more easily followed, although here, too, 

 as the division evidently proceeds very slowly, it is difficult to 

 trace the limits of the segments beyond the first complete set, 

 which in transverse section are sufficiently clear. The first 

 division divides the segment into an inner and an outer cell, 



1 nolle (I), PL IV. Fig. 32. 



