071 the Struditre of Lei^idodendvon selaginoides. 101 



and then into a thick walled parenchyma, which constitutes 

 both the superficial portion of the epidermis and the entire 

 surface of the petioles, or bases of the leaves. 



" I have here referred the tubular bast layer partly to the 

 outer bark and partly to the epiderm, because when the latter 

 becomes detached, the line of separation usually passes 

 through the middle of the layer; but it may perhaps be 

 more correct to regard the whole of these bast tissues as one 

 sub-epidermal layer." 



Further on, in the same memoir, is appended a note, in 

 which he says : " I may premise that my more recent investi- 

 gations have compelled me to alter some of the terms applied 

 in this memoir to the several parts of the bark, in order to 

 bring them into harmony with what I find in recent Lt/co- 

 podiacece; consequently in a third memoir, recently laid before 

 the Eoyal Society, I have designated the middle bark of this 

 paper the parenchymatous layer. The outer bark I have 

 termed the prosenchymatous layer ; and what I have called 

 the epidermal, I now designate the sub-epidermal layer." * 



In the specimen which we are considering this evening, the 

 central axis (Pis. II. and Il.a, Figs. 1 and 2, a) consists of 

 parenchyma, some of its component cells being much larger 

 than others. About half of the cells which enter into this part 

 of the structure are reticulated, a character, independent of 

 their general outline, sufficient to distinguish them from the 

 scalariform vessels, when examined in longitudinal section 

 (PI. IL, Fig. 4). 



It is impossible, in this case, to say whether originally all 

 the cells of this parenchyma were reticulated or not ; but I 

 would be inclined to believe they were, for when examined 

 in longitudinal section it is found in the same pile of cells, 

 one may be reticulated and the one immediately above or 

 below it may not be so marked. The changes which have 

 taken place during mineralisation seem to me sufficient to 

 account for the difference. 



Surrounding this central portion we have a circle of scala- 

 riform vessels (PL IL, Figs. 1, 2, and 4, a, a, and PL ll.a, 



* " On the Organisation of the Fossil Plants of the Coal ]\[easures. " Pro- 

 fessor W. C. Williamson. Phil. Trans,, June 1871. 



