V] IMPERFECT CASTS. 103 



decay of the delicate cells surrounding each leaf-trace bundle on 

 its way through the cortex of the stem. The surface g exhibits 

 a somewhat different appearance, owing to the fact that we 

 have the cast of the stem taken at a slightly different level. 

 The surface of the thick layer of coal at a shows very clearly 

 the outlines of the leaf-cushions; on the somewhat deeper 

 surfaces 6, c and d the leaf-cushions are but faintly indicated, 

 and the long narrow lines on the coal at c represent the leaf- 

 traces in the immediate neighbourhood of the leaf-cushions. 

 It is not uncommon among the older plant-bearing rocks to 

 find a piece of sandstone or shale of which the surface exhibits 

 a somewhat irregular reticulate pattern, the long and oval 

 meshes having the form of slightly raised bosses. The size of 

 such a reticulum may vary from one in which the pattern is 

 barely visible to the unaided eye to one with meshes more than an 

 inch in length. The generic name Lyginodendron^ was proposed 

 several years ago (1848) for a specimen having such a pattern 

 on its surface, but without any clue having been found as to the 

 meaning of the elongated raised areas separated from one another 

 by a narrow groove. At a later date Williamson investigated 

 the anatomy of some petrified fragments of a Carboniferous 

 plant which suggested a possible explanation of the surface 

 features in the structureless specimens. The name Lyginoden- 

 dron was applied to this newly discovered plant, of which one 

 characteristic was found to be the occurrence of a hypodermal 

 band of strong thick-walled tissue arranged in the form of a 

 network with the meshes occupied by thin-walled parenchyma. 

 If such a stem were undergoing gradual decay, the more 

 delicate tissue of the meshes would be destroyed first and the 

 harder framework left. A cast of such a partially decayed 

 stem would take the form, therefore, of projecting areas, 

 corresponding to the hollowed out areas of decayed tissue, and 

 intervening depressions corresponding to the projecting frame- 

 work of the more resistant fibrous tissue. A precisely similar 

 arrangement of hypodermal strengthening tissue occurs in 

 various Palaeozoic and other plants, and casts presenting a 



1 Williamson (73) p. 393, PI. xxvii. Described in detail in vol. n. See also 

 Solms-Laubaoh (91) p. 7, fig. 1. 



