172 Mr Seward, Notes on the Binney Collection 



enclosed by a zone of secondary xylem as it traverses the secondary 

 wood of the stem. 



The phloem, cortical tissues and reproductive organs unknown. 



Megaloxylon Scotti, gen. et spec. nov. 



In the type-specimen the primary single stele has a diameter of 

 1 to 2 cm.; it consists of a peripheral leaf-trace region and a 

 central metaxylem region; the metaxylem consists of tracheids 

 varying in shape from isodiametric and somewhat flattened to 

 more or less elongated elements, reaching a diameter — in the case 

 of the shortest tracheids — of '4 mm., with numerous bordered pits 

 on their walls. With the large isodiametric or even horizontally 

 elongated tracheids occur some much smaller short tracheids and 

 occasionally irregularly-shaped longer tracheal elements. The 

 metaxylem tracheids occur in groups of varying size and form 

 scattered through a parenchymatous ground-mass, which includes 

 small secretory cells. 



At the periphery of the primary stele numerous strands of 

 spirally pitted protoxylem tracheids occur in an exarch position ; 

 these strands of protoxylem occupy different positions in regard to 

 one another in different parts of the stem, according to the 

 position in its vertical course at which a leaf-trace is seen. A leaf- 

 trace has the form of an elliptical mass of long tracheids — with 

 bordered pits on their walls and of somewhat larger diameter than 

 the tracheids of the secondary wood, but much narrower than the 

 large metaxylem tracheids — associated with short parenchymatous 

 cells ; several groups (at least 6) of protoxylem elements occur on 

 the external edge of the trace. As a leaf-trace passes deeper into 

 the stem the tracheids become less compactly arranged and the 

 whole leaf-trace becomes wider and less well defined ; its long and 

 narrow tracheids are gradually replaced by shorter elements of 

 more irregular and variable form, and these are eventually linked 

 on to the short and large tracheids of the metaxylem region ; the 

 peripheral leaf-trace region and the axial metaxylem regions of 

 the stele are in close organic connection. 



The secondary wood of the stem is made up of regular radial 

 rows of tracheids with multiseriate bordered pits on their radial 

 walls, and of broad and deep medullary rays composed of short 

 parenchymatous cells. 



As a leaf-trace passes through the secondary xylem of the stem 

 its primary tissues become enclosed by a zone of secondary 

 tracheids and medullary rays. 



[Type-specimen from the Lower Coal-Measures of Lancashire; 

 preserved in the Binney Collection in the Geological Museum, 

 Cambridge.] 



