40 LYCOPODIALES [<^H. 



for twigs of conifers. As Dr Bommer^ has pointed out in his 

 interesting paper on "Les causes d'erreur dans I'^tude des 

 empreintes vegetales'' some dicotyledonous plants may also 

 simulate the habit of Lycopods : he cites Phyllachne clavigera 

 Hook (Candolleaceae), Tafalla graveolens Wedd (Compositae) 

 and Lavoisier a lycopodioides Gard. (Melastomaceae). Another 

 point illustrated by fig. 121 is the close agreement in habit and 

 in the form of the leaves and leaf-cushions between the recent 

 plants and the Palaeozoic Lepidodendreae. 



In his masterly essay " On the vegetation of the Carboni- 

 ferous Period, as compared with that of the present day" Sir 

 Joseph Hooker called attention to the variation in the shape 

 and arrangement of the leaves in the same species of Lycopo- 

 dium. The three woodcuts which he publishes of Lycopodium 

 densum, a New Zealand species, afford striking examples of the 

 diversity in habit and leaf- form and justify his warning "that 

 if the species of Lepidodendron were as prone to vary in the 

 foliage as are those of Lycopodium, our available means for 

 distinguishing them are wholly insufficient''." 



As we have already noticed, there is a considerable 

 diversity among recent species, both as regards habitat and 

 habit ; in the anatomy of the stem also corresponding variations 

 occur within the limits of a well-defined generic type of stele. 

 In species with creeping stems, such as L. clavatum^, the 

 stele exhibits an arrangement of vascular tissue characteristic 

 of the plagiotropic forms. The xylem consists of more or less 

 horizontal plates of scalariform tracheae, each surrounded by 

 small-celled parenchyma, alternating with bands or groups of 

 somewhat ill-defined phloem. The protoxylem and proto- 

 phloem elements occupy an external position (exarch), pointing 

 to a centripetal development of the metaxj'lem. This centri- 

 petal or root-like character of the primary xylem is an important 

 feature in recent as in fossil Lycopods. The close agreement 

 between the roots and stems of recent species in the disposition 

 of the vascular elements also denotes a simpler type of anatomy 



1 Bommer (03) PI. ix. figs. 140, 141. 



2 Hooker (48) p. 423, figs. 12—14. 

 » Jones (05). 



