PBOM THE LOWER COAL-MEASURES. 93 



The innermost zone of these radicular tissues abuts upon the 

 tissue described as probably the pericycle, into which it passes 

 without any break in the organic continuity (PI. XVII. fig. 3, iz). 

 Its constituent cells, however, are much larger and very different 

 in appearance from those of the pericycle. They show, moreover, 

 at some points a slight tendency towards a radial arrangement ; 

 but this is ill-defined, and is soon lost in passing outwards. 



The Vascular Bundles are found at all distances from the 

 vascular-bundle ring, and are not restricted to a limited number 

 of radii (PI. XVI. fig. 2, db). Those near the ring are cut approxi- 

 mately at right angles, while the rest are more or less oblique. 

 But even near the periphery the obliquity is frequently very 

 small, indicating that some of them have a nearly longitudinal 

 course. Others, however, appear to run more obliquely and at 

 length pass out at the surface. As in other vascular bundles a 

 xylem part and a phloem part can be distinguished, the former 

 being turned towards the centre, and the latter towards the peri- 

 phery (PI. XVII. fig. 6,ph, #). This orientation is retained all 

 through the enveloping tissues, but the transverse sections * offer 

 • no evidence as to whether or not it changed when the bundles 

 entered the appendages. The xylem is triangular in shape and 

 consists of thick-walled tracheae, mostly of the scalariform type, 

 while the elements of the phloem have thin walls and are nowhere 

 sclerenchymatous. The bundles give no evidence of secondary 

 growth, and retain their original dimensions, so long at any rate 

 as they run in the cortex. 



Each bundle is surrounded by a special envelope of parenchyma, 

 which in most cases is thicker in those near the periphery than 

 in tho^e near the vascular-bundle ring. In those nearest the 

 ring this parenchyma is scarcely, if at all, distinct on the inside 

 from the medullary ray opposite which the bundle stands. But 

 on the outside it is quite distinct from the surrounding paren- 

 chyma, and its elements have a concentric arrangement. In 

 those farthest from the ring the bundle is surrounded by paren- 

 chyma on all sides, the volume of which has evidently increased, 

 while the concentric arrangement is still retained (fig. 6, cp). 

 In some of the latter an additional feature is met with in the 

 form of a second outer envelope of parenchyma of greater thick- 

 ness than the first, whose elements are in some instances arranged 

 in radial rows, and become elongated in the same direction 



* Vide infra, p. 95. 



