162 THE GEOLOGICAL HISTORY OF PLANTS. 



and Cardiocarpa in the soil of the Sigillaria forests, as I have studied 

 this at the South Joggins. The other is that the rings of fruit-scars 

 on the branches of Sigillaria are homologous with leaf-scars, not 

 with branches, and therefore should have borne single carpels and 

 not cones or spikes of inflorescence. These are merely suggestions, 

 but I have no doubt they will be vindicated by future discoveries, 

 which will, I have no doubt, show that in the family SigillariacecB 

 we have really two families, one possibly of gymnospermous rank, 

 or at least approaching to this, the other allied to the Lepidodendra. 



CRYPTOGAMIA. 



(Acrogenes.) 



Family LEPIDODENDRE^: ; Genus LEPIDODENDRON, Sternberg. 



These are arboreal Lycopods having linear one-nerved leaves, 

 stems branching dichotomously, and with ovate or rhombic leaf -bases 

 bearing rhombic leaf -scars, often very prominent. The fruit is in 

 scaly strobiles, terminal or lateral, and there are usually, if not 

 always, macrospores and microspores in each strobile. The young 

 branches and stems have a central pith, a cylinder of scalariform 

 tubes sending out ascending bundles to the leaves through a thick 

 cellular and fibrous inner bark, and externally a dense cortex conflu- 

 ent with or consisting of the leaf-bases. Older stems have a second or 

 outer layer of scalariform fibres in wedges with medullary rays, and 

 strengthening the stem by a true exogenous growth, much as in the 

 Diploxylon type of Sigillaria. The development of this exogenous 

 cylinder is different in amount and rate in different species.* This 

 different development of the exogenous axis is accompanied with 

 appropriate external appearances in the stems, and the changes 

 which take place in their markings. These are of three kinds. In 

 some species the areoles, at first close together, become, in the pro- 

 cess of the expansion of the stem, separated by intervening spaces of 

 bark in a perfectly regular manner ; so that in old stems, while widely 

 separated, they still retain their arrangement, while in young stems 

 they are quite close to one another. This is the case in L. corruga- 

 tum. In other species the leaf -scars or bases increase in size in the 

 old stems, still retaining their forms and their contiguity to each 

 other. This is the case in L. undulatum, and generally in those 

 Lepidodendra which have large leaf-bases. In these species the 



* See " Memoirs of Dr. Williamson," in "Philosophical Transactions," 

 for ample details. 



