THE FOSSIL FLORA OF THE YORKSHIRE COAL FIELD. 35 



Examining the structure of these various parts more in detail, the primary bundle is 

 composed of very long tubular utricules, irregular in size, of which the smaller arc 

 external. These utricules or vessels,* disposed without order, differ among themselves, 

 not only by their size but by their length. Their walls are marked by transverse bars 

 or spiral striae, numerous and fine, but very variable, some vessels having one kind of 

 marking, some the other, and occasionally the two forms of marking occur on the same 

 vessel. In the vessels of smaller size the bars are generally more oblique, and form 

 spirals. Earely the thickenings form hexagonal areolations, but all these various forms 

 of cell-thickening appear to occur on the same vessel. The smallest vessels are situated 

 on the exterior surface, and contain spiral fibres. 



In the exogenous zone the bundles are composed of a uniform tissue, consisting of 

 tubular radiating vessels disposed in regular parallel radiating series, which are some- 

 times contiguous or separated by narrow medullary rays. The vessels next the primary 

 bundles are smallest, and increase in size as one proceeds outwards, and become almost as 

 large as the large vessels of the primary bundles. They are hexagonal in form, but 

 irregular in section. Their walls are marked by transverse bars (scalariform), perfectly 

 regular, and form a series on each face of the vessel on all sides. In tangential section 

 the medullary rays are seen to be composed of plates of cellular tissue of little vertical 

 height, generally of one series of cells, but occasionally the medullary rays are of more 

 than one layer in thickness. 



The foliar bundles which spring from the outer surface of the primary zone are 

 entirely composed of scalariform vessels, smaller than the external vessels of the 

 secondary bundles, and do not show any regular arrangement. 



The cortical envelope is composed of two different layers, which are intimately 

 eonnected, and pass almost insensibly into each other. The inner layer is 

 formed of elongated prosenchymatous cells, very dense, terminated by oblique ex- 

 tremities, and of which many contiguous cells have the same height, so that their 

 terminations form zigzag lines. They are placed in regular uniform radiating series, the 

 walls being without punctations. The outer layer is formed of parenchymatous 

 tissue, more or less regular, the smooth cells being closely placed without lacunae, and 

 are not arranged in radiating series, nor are they parallel to the surface. In the most 

 external zone the walls appear to be thickened, and form the surface of the leaf- 

 cushion. 



After describing Sigillaria elegans, Anabathera pulcherrima, and Stigmariajicoides, 

 Brongniart says : — " These fossil stems ally themselves then, on the one part to the 

 Conifer se and Cycadacese, by the disposition and uniformity of their ligneous or vascular 

 tissue, and on the other part to the vascular cryptogams by the constancy of the 

 structure of the walls of the vessels." t 



* These, like those in ferns and lycopods, are not true vessels. They communicate with each other by lateral 

 openings, and are not continuous as in true vessels, but the septse forming the individual cells or utricules remain 

 intact. 



+ Broxgniart, Observations sur la Structure inter du S. elegans, p. 426. 



