PTERIDOPHYTA. 223 



whorl of tabular air-passages is found in the cortex of the stems, 

 opposite the furrows (vallecular canals). There is also a similar 

 air-passage (carinal canals) in each of the vascular bundles, which 

 are placed in a ring, one opposite each ridge, and therefore alter- 

 nating with the vallecular canals. The vascular bundles are col- 

 lateral as in the majority of Flowering-plants, but poorly deve- 

 loped. The xylem of each bundle consists of two groups of 

 annular or spiral vessels, close to the outer border of the carinal 

 canal, and two groups of scalariform tracheids, each placed on a 

 radius passing through a group of spiral vessels. The phloem is 

 placed between these four groups, each of which has only a few 

 vessels. The stiffness of the stems is mainly due to the large 

 amount of silica in the cell-walls of the epidermis, and to the 

 sclerenchymatous cells of the ridges. 



All LEAVES are situated in whorls. The VEGETATIVE are simple, 

 undivided, 1-nerved, and are united into toothed sheaths (Fig. 

 224 a, 1). The branching of the stems in some species (E. arvense) 

 is very abundant. The branches break through the base of the 

 leaf-sheaths (Fig. 224 b), and generally alternate with the teeth 

 (leaves). 



The FERTILE LEAVES (sporopJu/lls) are different from the barren 

 ones. They are free, shield-like, each one having a short stalk 

 bearing usually an hexagonal plate (Fig. 224 d), and closely com- 

 pressed into an ear or cone (Fig. 224 a, c). The Equinetums thus 

 present an advance in development distinctly beyond that of the 

 Ferns, which is further emphasized by the circumstance that a 

 transition from the sheath- leaves to the fertile-leaves is found in 

 the involucre or annulus, a " collar " of specially modified leaves 

 situated at the base of the cone (Fig. 224 a and c). The cone 

 may be considered as a very rudimentary flower, and the annulus 

 may be regarded as a very early stage in the formation of a, flower 

 (perianth). See page 235. 



The SPORANGIA are situated on the underside of the sporophylls, 

 one at each angle ; they are sac- like, and open inwardly by a 

 longitudinal cleft (Fig. 224 d). An annulus is wanting; but in 

 the wall of the sporangium, as in the pollen-sacs of the Flower- 

 ing-plants, a layer of cells, with annular or spiral thickenings, is 

 developed, which assists in the dehiscence of the sporangium. 



The SPORES are green; the walls composed of four distinct 

 layers, of which the outer is gradually separated, except at one 

 point, and becomes split into four long bands (elaters) (Fig. 225). 



