138 ANCIENT PLANTS 



which greatly exceeds the primary mass in thickness. 

 This zone of secondary wood became very extensive in 

 old stems, for, as will be imagined, the primary wood 

 was not sufficient to supply the large trunks. The 

 method of its development from a normal _ cambium 

 in radiating rows of uniform tracheides is quite similar 

 to that which is found in the pines to-day. This is the 



Fig. 96. — A, Lepi- 

 dodendron Stem with 

 Hollow Ring of 

 Wood w and Zone 

 of Secondary Wood 

 S. B, Longitudinal 

 View of the Narrow 

 Pits of the Wood 

 Elements. 



most important difference between the living and the 

 fossil stems of the family, for no living plants of the 

 family have such "secondary wood. On the other hand, 

 the individual elements of this wood are different from 

 those of the higher families hitherto considered, and 

 have narrow slit-like pits separated by bands of thicken- 

 ing on the longitudinal walls. Such tracheides are found 

 commonly in the Pteridophytes, both living and fossil. 

 Their type is seen in fig. 96, b, which should be com- 

 pared with that in figs. 78, a and 62, b to see the con- 

 trast with the higher groups. 



