86 THE MORPHOLOGY OF PTERIDOPHYTES 



Species which are monosteUc throughout are more primitive 

 than the more complex species. However, much caution is 

 necessary before accepting this view. In the first place, not 

 all monosteles are directly comparable (e.g. S. selaginoides 

 and S. Braunii). In the second place, it would seem that 

 some members of the monostelic Homoeophyllum group 

 are highly advanced in other respects. Thus, S. rupestris and 

 S. oregana (in the Tetragonostachya subsection) are re- 

 markable for the possession of vessels in their xylem. 

 Among the tracheids are Hgnified elements whose transverse 

 end-walls have dissolved, leaving a single large perforation 

 plate so that, hke drain pipes placed end to end, they provide 

 long continuous tubes. While it is true that vessels are known 

 in some other pteridophytes, they are not of this advanced 

 type which occurs, elsewhere, only among the flowering 

 plants. The metaxylem tracheids have scalariform thicken- 

 ings, while the protoxylem elements are helically thickened 

 and exhibit a feature which is found elsewhere only in 

 Isoetes — viz. the helix may be wound in different directions 

 in different parts of the cell.^^ 



The phloem, composed of parenchyma and sieve cells, is 

 very similar to that of Lycopodium and is separated from 

 the xylem by a region of parenchyma one or two cells 

 thick. To the outside of it is a region of pericycle, and then 

 comes a trabecular zone which is characteristic of Selaginella. 

 This zone differs markedly in detail from species to species, 

 but is usually a space, crossed in an irregular fashion by 

 tubular cells or by chains of parenchyma cells. Endodermal 

 cells are recognizable also in this region because of their 

 Casparian bands, but it sometimes happens that a single 

 Casparian band may encircle a bunch of several tubular 

 cells. Whatever the exact constitution of the zone, however, 

 it is very deUcate, with the result that the stele usually drops 

 out of sections cut by hand. The outer regions of the stem 

 are frequently made up of thick-walled cells and the epi- 

 dermis is said to be completely without stomata. 



