LEPIDODENDRON 157 



probably L. Hickii, Watson, this lacunar tissue lies at 

 the base of the depressions which constitute the external 

 prints. There appear to have been numerous stomata 

 in the overlying epidermis. 



The parichnos-strands die out in the leaf itself, 

 losing themselves in the mesophyll. Thus the parichnos- 

 tissue kept up communication between the delicate 

 parenchyma of the inner cortex and the assimilating 

 mesophyll of the leaves, as well as with the patches of 

 aerenchyma in the persistent leaf-bases. The function 

 may probably have been to facilitate respiration. 1 Mr. 

 T. G. Hill has lately shown that strands comparable to 

 the parichnos occur in the leaves of Isoetes Hystrix and 

 various species of Lycopodium ; in these cases the function 

 of the strands is secretory, as may sometimes have been 

 the case in fossil Lycopods. 2 



The little triangular print on the upper part of the 

 cushion, immediately above the leaf-scar, is of special 

 interest, for we now know that this represents the ligule, 

 an organ which is characteristic of Selaginella and 

 Iso'etes, though absent from the other recent genera 

 of Lycopods. Stur was the first to identify the ligule 

 in Lepidodendron, but until it was demonstrated in 

 specimens with structure preserved, there was no proof 

 of the correctness of his interpretation. The presence 

 of a ligule was proved almost simultaneously by Count 

 Solms-Laubach 3 in specimens from the Lower Carboni- 

 ferous of Silesia, probably referable to the L. brevifolium 



1 F. E. Weiss, "The Parichnos in the Lepidodendraceae," Mem. and 

 Proc. Manchester Lit. and Phil. Soc. vol. li. 1907. 



2 T. G. Hill, " On the Presence of a Parichnos in Recent Plants," Ann. 

 of Pot. vol. xx. 1906. 



3 Bot. Zeitung, 1892, p. 1 10, Plate ii. Figs. 2 and 4. 



