THE LYCOPSIDA AND PTEROPSIDA 245 



and to the forests of Mesozoic and Cenozoic times, as well as to 

 the herbaceous types which under modern conditions more and 

 more predominate in the plant population of our earth. 



A more detailed characterization of the Lycopsida is now desir- 

 able. Fig. 176 represents diagrammatically the organization of 

 an axis of the lycopsid type together with its appendages. The 

 latter consist of leaf, branch, and root. Root and branch are 

 intimately associated, the former usually proceeding from the base 

 of the latter. The upper end of the main axis is represented in 

 transverse section to show the anatomical relations of the organs. 

 The fibrovascular cylinder in the particular case illustrated is 

 siphonostelic, although it might equally well be protostelic, espe- 

 cially in the lower representatives of the group. The tubular 

 stele or central cylinder constituted by the fibrovascular sys- 

 tem is interrupted only at one point where the fibrovascular 

 supply of a branch takes its origin from the main cylinder. The 

 gap thus appearing is known as the branch gap or ramular 

 lacuna. On the margin of the stem appear certain projections, the 

 leaf bases, within which are included the fibrovascular strands 

 destined for the leaves or foliar traces. These subtend projections 

 on the surface of the central cylinder occupied by groups of proto- 

 xylem. The projections in question are the starting-points of the 

 foliar traces, and it is clear that in no instance is there an inter- 

 ruption in the continuity of the central cylinder corresponding to 

 the departing leaf traces. The central cylinder of the Lycopsida 

 is said on this account to be without foliar gaps. The absence of 

 leaf gaps or foliar lacunae is characteristic of all the Lycopsida and 

 is a diagnostic feature of importance for the great group or phylum 

 which they represent. Even in those Lycopsida in which the 

 leaves are, superficially at least, relatively large (for example, the 

 Sigillariae, in which the foliar organs were sometimes over a meter 

 in length) there were still no foliar gaps. The absence of foliar gaps 

 is an anciently inherited or palingenetic feature of the Lycopsida, 

 and they may therefore be technically characterized as palin- 

 genetically microphyllous (small-leaved). 



Not only has this large group universally small leaves, but it 

 also possesses another important, salient, and constant general 



