FIBRO VASCULAR TISSUES: WOOD 



called wood rays. The secondary wood need not further occupy 

 us in the present chapter. 



Turning our attention now to the longitudinal organization of 

 the primary wood, we find it characterized by the presence of certain 

 elements appearing in a somewhat regular sequence. The general 

 situation is represented in Fig. 13. On the left of the diagrammatic 

 illustration lies an elon- 

 gated thin-walled cell 

 marked by the presence 

 of spiral strengthening 

 bands. In its present 

 state this element is de- 

 void of protoplasmic sub- 

 stance, but in an earlier 

 phase, as indicated in the 

 next figure (Fig. 14), liv- 

 ing matter was present 

 and specially aggregated 

 in the regions of the 

 thickened spiral bands. 

 To the right lies a second 

 element in which the 

 strengthening horizontal 

 ridges, which reinforce 

 from the inside the gen- 

 erally thin walls of the 

 water-conducting cell, are 

 nearer to one another and 

 in some instances are more or less united. By accentuation of the 

 condition of approximation, fusion between the bands results and 

 we have as a consequence the presence of the scalariform or reticu- 

 late tracheid. In general, among the Pteridophyta this is the 

 extreme stage of evolution of the tracheary cells of the primary 

 wood, but occasionally among the more complicated and extinct 

 vascular cryptogams, and characteristically in all seed plants, the 

 final state of the primary wood is characterized by the presence of 



FIG. 12. Transverse section of a stem of a 

 lepidodendrid trunk from the Paleozoic (after 

 Scott), showing the strong distinction between 

 secondary (radially seriate) and primary (unseri- 

 ate) xylem characteristic of ancient forms. 



