50 SCIENCE PROGRESS. 



definitely determine the physiological significance of the 

 parichnos tissue. 



These two parichnos scars occur both on the ordinary 

 form of Lepidodendron and Sigillaria leaf-scar, as well as 

 in the leaf-scar of Lepidophloios. In the latter genus, 

 Potonie l has recently described the same structures ; he 

 compares them to mucilage canals in the leaves of some 

 Lycopodia, and, more especially, to "transpiration strands," 

 analogous to the tissue at the base of some fern leaf-stalks, 

 which is visible at the surface as lentical dots. 



In a Lepidodendron leaf-cushion there are occasionally 

 seen, immediately below the leaf-scar, two small elliptical 

 areas, the real nature of which has not been previously 

 recognised. Potonie shows by a longitudinal section of 

 a leaf-cushion of Lepidophloios how the parichnos tissue 

 passes from the leaf-scar into the fundamental tissue of 

 the cushion, and in the latter follows a course just below 

 the epidermal tissue ; for a short distance the epidermal 

 cells of the leaf-cushion appear to be partially or completely 

 disorganised, thus leaving the parichnos tissue exposed 

 at the surface. This surface view of the two branches of 

 the parichnos is represented on the surface of a leaf-base 

 by the two elliptical marks below the leaf-scar. With 

 regard to the Lepidophloios form of leaf-cushion, there has 

 been much difference of opinion as to whether the leaf-scar 

 represents the lower or upper surface of the leaf-cushion. 

 Williamson and others consider that the scar should be 

 regarded as the upper end ; Potonie, on the other hand, 

 places it at the lower end, and brings forward certain facts 

 in support of his view which seem to be of considerable 

 weight. 2 In addition to the markings already mentioned 



1 Bet: deutsch. bot. Ges , Jahrg. ii., Heft 5, 1893, P- 3*9- 



2 The specimen figured by Potonie was originally described by Weiss 

 as an unusually large strobilus (Solms-Laubach, Fossil Botany, p. 235). 

 In 1890 (Proc. Camb. Phil. Soc, vol. vii., pt. ii., p. 3) I published a note, 

 with rough sketches, in which the real nature of the Berlin specimen was 

 pointed out. Potonie, in confirming my opinion, maintains — and, I 

 believe, rightly- — that my fig. 4 represents the leaf-cushions and scars 

 wrong side uppermost (Zeitsch. deutsch. Geo/. Ges., Protokoll, Mai, 1893). 



