CHAPTER VI 



THE FIBRO VASCULAR TISSUES: SECONDARY WOOD RAYS 



In the preceding chapter the subject of the origin of longitudinal 

 storage cells has been discussed and to elements of this type the 

 general appellation of wood parenchyma has been given. In the 

 case of the ligneous tissues of vascular plants a much older type 

 of storage device 

 exists in the form of 

 radially directed 

 bands of masses of 

 cells which often, with 

 a high degree of im- 

 propriety, are desig- 

 nated medullary rays. 

 This denomination is 

 erroneous from the 

 historical and evolu- 

 tionary standpoint, 

 since it is clear that 

 in the first instance 

 and in the earlier and 

 primitive forms rays 

 had no relation what- 

 ever to the pith or 

 medulla and conse- 

 quently cannot with 

 any degree of pro- 

 priety be designated 

 as medullary. Fig. 44 illustrates the conditions obtaining in the 

 case of the so-called medullary rays of the ancient genus Lepido- 

 dendron. The region of the pith (p) is represented largely by an 

 empty space, which is in turn surrounded by the primary wood. 

 This is distinguished by the non-seriate and irregular arrangement 



61 



FIG. 44. Transverse section of the stem of a 

 lepidodendrid, showing well-developed primary and 

 secondary wood, the latter being radially seriate and 

 provided with storage rays (after Scott). 



