284 THE ANATOMY OF WOODY PLANTS 



of the tracheary origin of the pith is also open to question in other 

 representatives of the Lycopsida. In Phylloglossum the medulla 

 in the region of the tuberous portion of the stem is actually sur- 

 rounded by an endodermal layer, a structure peculiar to the cortex. 

 In Selaginella laevigata, also, the tubular stele is bounded internally 

 as well as externally by a well-defined endodermal layer, and the 

 tissues of the pith, further, clearly resemble those of the cortex. 

 In both Psilotum and Tmesipteris the medullary tissues are fre- 

 quently composed to a large extent of brown sclerenchymatous 

 elements resembling similar structures found in the external 

 fundamental tissues. In the genus Equisetum dark-brown scle- 

 renchyma is present in the medulla and the cortex. It is clear 

 on the basis of data derived from resemblances between the tissues 

 of the pith and the cortex, on the one hand, and the tracheary 

 tissue, on the other, that the preponderance of evidence weighs 

 heavily on the side of the fundamental or cortical origin of the 

 medulla. And heavily as the scale seems to incline on the side 

 of the fundamental derivation of the pith in the Lycopsida, it 

 seems entirely overwhelming in the case of the Filicales. 



There are two general arguments which have been invoked 

 against the origin of the pith from the fundamental tissues in 

 the Filicales. One is the denial that it is possible for the stele 

 to include the tissues of the cortex. This reasoning cannot be 

 given very serious weight in view of the fact that in certain fili- 

 cinean steles for example, that of the polypodiaceous genus 

 Onoclea and the schizeaceous genus Anemia the possibility of 

 the inclusion of fundamental tissues within the tubular stele must 

 apparently be granted, because the epidermis and chaffy ramentum 

 as well as the outside air are actually included within the medullary 

 region of the central cylinder. Moreover, the phloem, a tissue 

 primitively occurring on the outer surface of the central cylinder, 

 is frequently included in its interior. If any further evidence 

 were needed as to the possibility of the xylem including tissues 

 of another morphological category, it is furnished by the case 

 of certain filicinean foliar traces within which it is universally 

 admitted, even by the most convinced adherents of the hypothesis 

 of the tracheary origin of the pith, fundamental tissues may not 



