262 MOSSES AND FERNS chap. 



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 developed 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. 143, 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 

 ( (i) PI. iv., Fig. 32) figure of Botrychium rutcefolmm closely 

 resembles B. Virginianum, and probably the other species will 

 show the same form of apical cell. The divisions are decidedly 

 more regular in the segments of B. Virginianiim than in Ophio- 

 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, the former 

 probably being directly the initial for the central cylinder. The 

 outer cell by later divisions forms the cortex, and the epidermis 

 which covers the very small exposed surface of the stem apex. 

 As in Ophioglossum, it is impossible to determine exactly the 

 method of origin of the young leaves, one of which probably 

 corresponds to each segment of the apical cell, but as soon as the 

 leaf can be recognised as such it is already a multicellular organ. 

 It grows at first by an apical cell which seems to correspond 

 closely in its growth with that of the stem. From almost the 

 very first (Fig. 143) the growth of the leaf is stronger on the 

 outer side, and in consequence it bends inward over the stem 

 apex. 



The arrangement of the tissues of the fully-developed stem 

 shows, as we have seen, a striking similarity to that in the 

 stems of many Spermatophytes. The xylem of the strictly 

 collateral bundle is made up principally of large prismatic 

 tracheids (Fig. 144), whose walls are marked with bordered 

 pits not unlike those so characteristic of the Coniferse, but some- 



