102 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



thickenings (scalariform) or spiral striae, some tracheides having 

 one kind of thickening, some the other, and occasionally both 

 forms of marking occur on the same tracheide. The smaller 

 tracheides are placed externally, and contain spiral fibres. 



The exogenous zone is formed of scalariform tracheides in 

 radiating series, separated by medullary rays. Those in the 

 inner portion of the exogenous zone are small, but increase in 

 size as they proceed outwards, and become almost as large at the 

 periphery of the bundle as the large tracheides of the primary 

 bundle. In transverse section they are roughly hexagonal in form. 



The smallest vessels of the exogenous zone are in contact with 

 the smaller tracheides of the primary bundles, and form at their 

 point of contact a distinct line of demarcation between the 

 primary and exogenous portions of the xylem. 



In tangential section the medullary rays are seen to be com- 

 posed of plates of cellular tissue of little vertical extent, generally 

 of one series of cells, but occasionally they are of more than one 

 cell layer in thickness. 



Outside of the exogenous zone, and situated close to it, are 

 small isolated lenticular or circular bundles, composed of small 

 uniform tissue disposed without order. These are the foliar 

 bundles, which spring from the outer surface of the primary 

 xylem, and are entirely composed of scalariform tracheides. 



The cortical envelope is composed of two distinct layers, which 

 are intimately connected and pass almost insensibly into each 

 other. The inner layer or zone is formed of elongated prosenchy- 

 matous fibres, very dense, and termiuated by oblique extremities; 

 and as many of the contiguous cells are of about the same length, 

 their terminations form zigzag lines. They are placed in regular 

 uniform radiating series, and their walls are destitute of 

 markings. 1 The outer portion is composed of parenchymatous 

 tissue, more or less regular; the smooth cells are closely packed 

 without lacunae, and are not arranged in radiating series, nor are 

 they parallel to the surface. In the epidermal region their walls 

 appear to be thickened, and form the surface of the leaf-cushion. 



In this type of Sigillarian stem structure the wedges of the 

 vascular system, though contiguous, are perfectly distinct, and, 



1 This is probably developed from a phellogen zone, as in Lepidodendron . 



