CHAPTER XIX 

 THE LYCOPODIALES 



This group, as has been indicated in the last chapter, has spiral 

 phyllotaxy. It includes both isosporous and heterosporous 

 families. The latter are distinguished by the presence of a ligule, 

 while in the former this structure is lacking. In the genus Lyco- 

 podium the central cylinder of the stem is radial and protostelic. 

 As a consequence of the radial organization of the stele, the masses of 

 phloem lie in the intervals between the generally radially directed 

 oands of xylem. The sieve tubes are separated from the tracheids 

 by several rows of parenchyma on either side. In the vertically 

 directed reproductive axes of Lycopodium the organization of the 

 fibrovascular tissues is typically radial, while in the creeping stems 

 or rootstocks the arrangement of the xylem and phloem is somewhat 

 dorsiventral. The upright axes of Lycopodium, aside from the 

 fact that they bear leaves, are scarcely distinguishable in structure 

 from the roots. This resemblance in organization between root 

 and shoot is an indication of the antiquity of the lycopodineous 

 stock, since in the higher groups the differentiation between the 

 axial and radical organs becomes more and more marked. 



The monotypic genus Phylloglossum possesses a siphonostelic 

 stem. In this form the lower region of the axis is tuberous and 

 contains the most massive development of the fibrovascular system. 

 In the tuber also, as is shown in Fig. 178, there is both internal 

 phloem and internal endodermis. As the stele of the inferior 

 region passes upward, it gives off a branch into the peduncle of 

 the tuber which is to perpetuate the plant in a subsequent season. 

 The trace of this appendage in departing from the central cylinder 

 leaves a well-marked branch gap. The foliar organs indicated as 

 swellings on the outline of the section cause no interruptions in the 

 continuity of the central cylinder by their departure from its surface. 

 In this respect Phyllcglossum shows itself, to be a veritable repre- 

 sentative of the Lycopsida. In the higher and aerial region of the 



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