VASCULAR CYLINDER. 17 



terminal undivided roots, thus formed, may be identical with what led M. Renault 

 to doubt their being roots and to regard them as rhizomes. 



Exogenous Growth of the Vascular Cylinder. 



As already observed, the existence of this growth, alike in the aerial stems of 

 Sigillariae, of most Lepidodendra, and in their common Stigmarian roots, is no 

 longer disputed. The great differences in the diameters of their several vascular 

 cylinders can be accounted for in no other way. But independently of the argu- 

 ment derived from the necessities of the case, our specimens demonstrate its 

 occurrence. Thus in the section Plate VII, fig. 14, we find at b" b" a thin peri- 

 pheral zone of vascular laminae, sharply distinguished from those which it encloses, 

 by the much smaller size and less regular distribution of its component vessels. 

 A portion of this peripheral zone, enlarged 39 diameters, is shown in Plate IV, 

 fig. 19. At b we have the outer margin of the matured wedges of the older 

 portions of the cylinder, whilst at b' is the layer of new growths. Though the 

 vessels of the latter are of smaller, though unequal, sizes, they are obviously 

 prolongations of the older lamina?. They correspond, in these respects, with 

 similar young growths occurring in various other Carboniferous plants in which 

 this exogenous development occurs. 



The existence of such a process of exogenous growth demands the pre-existence 

 of some meristemic equivalent of a cambium. Plate IV, fig. 20, represents such a 

 layer. Like fig. 19, this is part of a transverse section of a vascular cylinder, 

 enlarged 75 diameters, the outermost vessels of which are seen at b separated by 

 the secondary medullary rays, b". At c we have a thin investing zone of what, in 

 this section, appears to be an extremely delicate but otherwise ordinary form of 

 parenchyma, the cells of which tend more or less to arrange themselves in radial 

 rows with parallel tangential divisions. Plate VII, fig. 10, is a radial, longitudinal 

 section through the same specimen as fig. 20, in which b again represents the 

 outermost vessels of the xylem cylinder. But we now see that the layer c of fig. 

 20 consists of narrow, vertically elongated cells with square ends, and which may 

 fairly be regarded as cambiform products of a cambial layer, the meristemic activity 

 of which may have manifested itself irregularly rather than periodically. The 

 elements composing this cambiform layer being so very different from any which 

 enter into the composition of the true cortex, we can scarcely doubt that their 

 function has been as specialised as their structure and position, and that they 

 represent the zone within which the exogenous growths, successively added to 

 the exterior of the vascular cylinder, originated. 



four primary roots, each of these has subdivided much more frequently, apart from the so-called Tap- 

 roots, than is the case with our ordinary examples of Stigmaria ficoides.. 



