VASCULAR CYLINDER. 15 



equally conspicuous. In tangential sections they are readily discerned (Plate VII, 

 fig. 11, b"b"), sometimes limited to a single cell, at others consisting of several such 

 cells. In the majority of cases these rays consist solely of radially elongated paren- 

 chyma, but they sometimes contain small barred Tracheids, as in b" , Plate IV, fig. 

 17. I can obtain no explanation of the significance of this substitution beyond 

 what follows. 



In ' Memoirs,' Part II, p. 236, Plate xxvii, fig. 23, and in Part XI, p. 294, 

 Plate liii, fig. 21, and Plate liv, fig. 22 (' Phil. Trans.'), I showed that similar barred 

 Tracheids enter into the composition of the ordinary medullary rays of some species 

 of Lepidodendron, and that Corda found them in his Diploxylon cycadeoideum. It 

 is probable, therefore, that the presence of these barred Tracheids in the medullary 

 rays may characterise some species of Lepidodendra and Sigillarias, whilst they are 

 absent from others ; and since the vascular zone of Stigmaria is a mere down- 

 ward extension of the exogenous zone existing in the aerial stems of the above 

 plants, similar differences may be expected to exist in the roots of the same 

 species. 



That this vascular cylinder grows exogenously by means of a meristem ring, 

 equivalent to a cambium-zone, is no longer disputed. The specimens in my cabinet 

 demonstrate this fact. My largest cylinder is one from the Staffordshire coalfield, 

 for which I am indebted to Mr. Ward, of Longton, the possessor of the well-known 

 collection of the fossil Carboniferous Fishes of Staffordshire. Its mean diameter 

 is about \-^q (= 1186) of an inch. On the other hand, I have a perfect vascular 

 cylinder from Halifax, the diameter of which is only ^ (= "41 2) of an inch (Plate 

 V, fig. 16, a). A third, equally perfect, is -^ (= -187) of an inch in diameter; 

 yet smaller than either of these must have been one belonging to the section of the 

 bark and rootlets represented in Plate IX, fig. 18. The two outer cortical layers 

 being well preserved in this section we can ascertain the original mean diameter 

 of the entire root, which has been about f| (= *218). In its centre, b, but in a 

 disarranged state, are the vascular wedges that constituted the xylem cylinder, 

 and which in all probability did not exceed, if it even reached, *1 of an inch in 

 diameter. 1 Stigmarian roots which I examined in the Oldham Forest, attached to 

 the aerial stems, tapered away to very small dimensions. 



1 Specimens of Stigmaria are occasionally met with which contract suddenly from a diameter of 

 three or four inches to an obtuse point. Such examples have been quoted to show that all the roots 

 of Stigmaria terminated in this abrupt manner. Thus, in his general outline of the characteristics of 

 Stigmaria, M. Renault says : " Son extremite etait obtuse et legerement aplatie " (' Cours de Botanique 

 fossile,' premiere annee, p. 152, 1881). In the course of a prolonged life I have only met with one 

 fragmentary example of this kind. Steinhaur says (' Trans. American Phil. Society,' vol. i, 1868) : 

 "Amongst tbe vast number of specimens examined, only one was detected which appeared to 

 terminate, closing from a thickness of three inches to an obtuse point." Whilst recognising that 

 these, with some other peculiar forms, " were only monstrosities," the same author tells us that he 



