TJie Fossil Plants of the Coal Measures. 1 1 5 



leaf-traces demonstrate that they possessed leaves in some 

 form. 



My early difficulties in the interpretation of my limited 

 series of specimens arose from a fact I was then ignorant of 

 I now know that the structure of the axile primary tracheal 

 strand is polymorphic. We appear to possess it in four con- 

 ditions. 



In type A the strand appears to consist wholly of barred 

 tracheids ; no trace of a central medulla being visible. This 

 condition is seen in my cabinet specimens of vegetative 

 twigs, 419b, No. I,* 419b 2, 419b 3, and in the strobilus 

 624c and 627. 



In type B the primary tracheal strand approaches 

 closely to A, but in its centre there is a small number of 

 thin-walled structures, which appear more like cells than 

 vessels. See the twig 419 b4. 



In type C we now find a perfectly distinct medulla, 

 enclosed within a primary tracheal cylinder, composed of 

 fully-matured and uniform barred tracheids. See twigs 624e, 

 624f, and a similar one in 6i9f, but from which latter the 

 medulla has disappeared. The Strobili 624, 624a, 624f, 

 624g and 626 have similar medullse. 



The type D is the most interesting. The periphery of 

 the primary tracheal cylinder consists, as in C, of fully- 

 developed barred tracheids. The centre is occupied by very 

 thin-walled vertically elongated cells of a strongly-marked 

 pro-cambial aspect ; but at the zone of contact of these two 

 peripheral and central tissues we have an unmistakeable 

 form of spiral Protozylem. These conditions appear in the 

 twigs 419c and 4i9e, and in the Strobilus 424b. 



Of the innermost cortex no trace remains. In most of 

 the transverse sections a thin ring of the middle cortex 

 surrounds the primary tracheal cylinder at a little distance 

 from it. In some cases (C.N. 4190 we find this zone in 



• There are four stems in this shde severally marked i, 2, 3 and 4. 



