166 



LYCOPODIALES 



[CH. 



of the stele of Lepidodendron vasculare: it is probable that these 

 elements are vestiges of conducting tissue which in ancestral 

 forms formed a solid and not a medullated stele. 



The primary xylem is limited externally by an unequally 

 fluted surface with exarch protoxylem elements; it is, however, 



Fi<). 182. Lepidodendron IViinschianum. The stele of the stem shown in 

 fig. 181, A. (Cambridge Botany School.) 



noteworthy that there is not always a very clearly defined 

 difference between the small protoxylem and the large centri- 

 petally developed tracheae. Immediately beyond the primary 

 xylem occur numerous thin-walled parenchymatous cells with 

 spiral and reticulate pitting; beyond these is the broad zone of 



