FOSSIL PLANTS. 49 



In the first place, with regard to the genus Lepidoden- 

 dron. The last memoir by Prof. Williamson 1 enables us 

 to fill in various gaps in our knowledge concerning the 

 connection between certain external features and internal 

 structure in species of Lepidodendrece. 



The well-known species Lepidodendron Harcourtii 

 Brong. represents a particular form of the genus in which, 

 as yet, no traces have been found of a development of 

 secondary xylem, such as characterises the central stele of 

 other examples of Lepidodendron. Williamson's latest con- 

 tributions to our knowledge of this species are chiefly 

 concerned with the outermost cortical tissues, the leaves 

 and leaf-traces. The leaf-traces arise as groups of tracheal 

 elements which become detached from regularly placed 

 projections on the outer side of the primary tracheal cylinder 

 (" corona" of Bertrand) ; each trace is double, and consists 

 of a small group of tracheids with hard and soft bast, of 

 which the former is frequently preserved ; the bundles were 

 probably of the collateral type. As each leaf-trace reaches 

 the outer part (" prosenchymatous zone") of the cortex of 

 the stem a new structure becomes closely associated with 

 it : a tangential section of a trace at this stage of its course 

 towards a leaf shows, immediately above the tracheids, a 

 group of fairly large and irregularly arranged cells, consti- 

 tuting a well-marked structure, to which Bertrand has 

 assigned the name " parichnos ". 



As the leaf-trace continues its course towards a leaf the 

 parichnos divides into two halves, which become slightly 

 separated laterally from the vascular element of the leaf- 

 trace, and finally appear in the surface view of a leaf-scar 

 as two dots placed one on either side of the central vascular 

 bundle scar. 



In a well-preserved Lepidodendron stem it is easy to 

 see the three characteristic marks on a leaf-scar ; the cen- 

 tral dot has for some time been referred to the leaf-trace, 

 but the nature of the lateral dots has remained a puzzle 

 until quite recently, and even now it is impossible to 



1 Phil. Trans., vol. clxxxiv., 1893, B, p. 1. 



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