XI] SECONDARY STRUCTURE. 395 



these branch in passing through the cortex on their way to 

 the verticils of leaves. The space b in the diagrammatic 

 section of fig. 104 was originally occupied by the phloem and 

 inner cortex. In some species of Sphenophyllum the apex of 

 each arm of the xylem strand, as seen in transverse section, is 

 occupied by a longitudinal canal surrounded by spiral tracheids, 

 as in the primary xylem of the old stem shown in fig. 105, G. 



ii. Secondary structure. 



With the exception of very young twigs the petrified Spheno- 

 phyllum stems usually show a greater or less development of 

 secondary wood. In the xylem-strand of fig. 105, B, the broad 

 concave bays of the primary wood have been filled in by the 

 development of two rows of large secondary tracheids, x, but 

 opposite the protoxylem groups, px, there are no signs of 

 cambial activitj'. In the unusually large stem represented by 

 a rough sketch in fig. 105, G, the triangular primary xylem 

 lies in the centre of a thick mass of secondary vascular tissue. 

 The secondary and primary wood together have a diameter of 

 about 5 mm. 



After the bays between each protoxylem corner have been 

 filled in, the formation of secondary wood proceeds uniformly 

 along the stem radii, but the rows of tracheids and medullary 

 rays which are developed opposite the corners of the primary 

 strand, c, differ in certain characters from the broader masses 

 of wood opposite the bays. For convenience, the secondary 

 wood, c, opposite the protoxylem groups has been spoken of as 

 fascicular wood, and the rest, d, as interfascicular wood. 



The secondary xylem consists either of tracheae with 

 numerous bordered pits on their radial walls (fig. 105, D), or of 

 tracheae with broad and bordered scalariform pits (fig. 105, E). 

 The suggestion of concentric rings of growth in the wood in 

 fig. 105, Q, is rather deceptive ; there are no well-marked regular 

 rings in Sphenophyllum stems, but irregular bands of smaller 

 elements occasionally interrupt the uniformity of the secondary 

 xylem. In some stems the medullary rays have the form of 

 rows of parenchymatous cells, which in tangential longitudinal 



