2()6 



THIRD GROUP.— VASCULAR CRYPTOGAMS. 



FIG. 242. 



quali/olia. A in the ii 

 puscles, eu under epid 

 cellular spaces, sji stor 



nsverse sect 



)ns through the leaf of Selagiiiella it 

 3 at the margin, ch the chlorophyll-c 

 upper epidermis, / air-conducting in 



unusually large granules (Fig. 242). The leaves generally have stomata only on the 

 under side, the small leaves of S. pubescens have them on both sides. In some species, 



as 6". Martensiidi^di S.stejiophylla, 

 epidermal cells may be observed 

 with their walls so thickened that 

 the lumen is occluded (Russow). 

 In most species the epidermis is 

 different on the two sides of the leaf, 

 in a few (5. Galeotii, S. Kraus- 

 siand) it is the same on both 

 sides. Thtfiindajnental tissue of 

 the stem consists as in Lycopo- 

 diiDH of elongated cells with oblique 

 transverse walls or with a dis- 

 tinctly prosenchymatous arrange- 

 ment of the cells ; but in contrast 

 to most Lycopodiaceae these cells 

 have thin walls and wide lumina, 

 thicker walls and narrower lumina 

 occurring only in the hypodermal 

 layers (Fig. 243). It would appear 

 that the cells of the fundamental 

 tissue and with them of the other 

 tissues also are capable of long 

 continued growth in length and 

 circumference ; the distance be- 

 tween the leaves in older stems and the considerable thickness of such stems both 

 point in this direction, and it would be worth while to investigate the matter 

 thoroughly in the Selaginelleae, the Lycopodiaceae, and in many Ferns. One marked 



peculiarity in the Selaginelleae, as 

 in the stems of the Mosses, is that 

 the fundamental tissue, owing 

 probably to the prosenchymatous 

 arrangement of the cells, forms 

 none of the usual small inter- 

 cellular spaces, but their place is 

 supplied by a large air-space which 

 surrounds every vascular bundle 

 in the stem and is only interrupted 

 by transverse cellular filaments, like 

 flying buttresses to support the 

 bundles (Figs. 243, 244) ; if the 

 cells in these filaments are of 

 rounded form, the bundle is sur- 

 rounded by a loose spongy paren- 

 chyma, as in Fig. 243, which is 

 clearly distinguishable from the 

 compact fundamental tissue without interstices. The fundamental tissue of the leaf 

 is a loose spongy parenchyma containing chlorophyll, and in slender species with 

 thin leaves is developed only round the single bundle which traverses the leaf, while 

 at the thin margins of the leaves the upper and lower epidermis simply lie on one 

 another (Fig. 242). 



The vascular bundles., one or more of which traverse the stem, are cauline, as in the 

 Lycopodiaceae ; they may be traced up to the rudimentary condition in the extremity of 



Fig. 243. Transverse section of the stem of Seht^^inetla deniiciilata ; 

 xylem of the central bundle not yet completely lignified ; b air-cavity 

 rounding a bundle which bends out into a leaf. 



