FEOM THE LOWER COAL-MEASURES. 05 



1 



i. Two or three rows of relatively large cells arranged some- 

 what tangentially immediately round the vascular bundle (PI. 

 XVII. fig. 7, a). 



ii. A zone, 8 or 9 cells thick, whose elements are polygonal in 

 shape and have no definite arrangement (fig. 7, b). The outer- 

 most layer of this zone has some resemblance to a bundle-sheath 

 (fig. 7, s) y and the innermost layers have black contents. 



iii. A zone of thick-walled, apparently sclerenchymatous, 

 elements arranged in some degree tangentially (fig. 7, c). 



iv. A thin-walled tissue, with irregularly arranged elements, 

 passing into the thick-walled tissue on the inside and into the 

 parent tissues on the outside (fig. 7, d). 



How many of these layers properly belong to the cortex of the 

 roots cannot be definitely stated ; but I am inclined to think the 

 separation would ultimately take place somewhere in the layer 

 last described. 



In two of the tangential longitudinal sections of the specimens 

 the xylem and phloem of the roots of this type are seen to be 

 placed transversely to the long axis of the parent stem — a fact 

 which suggests that in them a torsion of the vascular bundle to 

 the extent of 90° has taken place *. 



A comparison of the structure of this type of root with that 

 of the others shows some marked differences in the cortical 

 tissues. In spite of this, it is probable that the two forms 

 represent different stages of development only, though no 

 intermediate stages between the one and the other have been 

 found. 



General and Comparative Review. 



From the preceding description it is obvious that, in this fossil, 

 we have the remains of a plant which was characterized by 

 several peculiarities whose rational explanation is by no means 



easy. The first question that naturally arises is as to the nature 

 of the part preserved ; but even on this a final and direct answer 

 can scarcely be given. It may be taken as certain, however, 

 that it is either a root, aerial stem, or rhizome. 



Against the view that it is a root must be set the important 

 fact that there is no trace of the centripetally developed and 

 alternating strands of xylem and phloem so generally character- 



Vide supra, p. 93. 



