Manchester Memoirs, Vol. li. (1907), No. 8. 17 



directly apparent (see Text-fig. 8), from the similar pits 

 described above, in the fact that we have there only a 

 single cavity running right across the leaf cushion in the 

 form of a groove. This groove Hovelacque has represented 

 diagrammatically in fig. 41, p. 82, and though so different 

 in aspect from the lateral pits, it may possibly have served 

 the same function. Hovelacque, it is true, does not 

 mention any connection between it and the parichnos 

 strands, but in his figures on pi. IV. there seems some 

 evidence that the parichnos strands come close to .the 

 surface in this sillon, and that there were some specialised 

 cells lining the upper surface of this groove. 



If special provision is made by the parichnos strands 

 for the respiration of the stem in the Lepidodendracese, 

 we should also expect that some provision would have 

 been made for those parts of Lepidodendron and Sigillaria, 

 which were embedded in the water-logged soil of the 

 marsh in which they are supposed to have been rooted. 

 As far as the Stigmarian axis is concerned, it would no 

 doubt receive the air necessary for its respiration through 

 the lacunar tissue of the mid-cortex, which we must 

 suppose to have been in continuity with that of the 

 aerial stem, and thence through the parichnos tissue 

 with the mesophyll of the leaf, or in addition, through 

 the lateral pits more directly with the exterior. 

 But the Stigmarian appendages or rootlets would also 

 require air for respiratory purposes, and their very 

 defective mid-cortex suggests that it was through this 

 tissue that the air must have reached the rootlet from 

 the rhizom. The rootlet, as is well known, springs from 

 a rootlet cushion of apparently parenchymatous cells on 

 the outside of a dense mass of cortex, and partly of 

 periderm, through which the vascular bundle passes out 

 into the rootlet. Is this periderm layer penetrated like 



