THE FILICALES 279 



departing strands of any organs. The root is not responsible for 

 the appearance of a gap in the wall of the stelar tube unless it 

 happens, as is sometimes the case, to be closely related to a foliar 

 organ. Under these conditions the apparent gap is naturally 

 foliar and not radical. 



Before passing to the discussion of modifications of the siphon- 

 ostelic central cylinder as presented by the stem in the Filicales 

 it will be well to consider the general topography of the fibrovascular 

 system in the various organs. The account of the stelar system 



FIG. 200. Transverse section of the stem of Adiantum pedatum 



of the stem outlined in the preceding paragraph will suffice for 

 the cauline organ, so that it is possible to turn at once to the 

 discussion of the leaf and the root. As has been indicated above, 

 the foliar trace departs from the tubular central cylinder of the 

 axis in topographical relation to a lacuna in the stelar wall 

 which is known as the foliar gap. The outgoing foliar trace in 

 the lower region where it runs in the stipe or rachis or even 

 sometimes in the subdivisions in- the main veins of the flattened 

 region or lamina, is concentric or bicollateral in its organization. 

 This characterization means that the xylem is either completely 

 surrounded by phloem or at least has phloem on its two 

 opposite sides. As the fibrovascular strands which innervate 

 the blade of the leaf become more finely divided they lose 



