THE FILICALES 



289 



dermis has never been observed. In O. claytoniana the pith 

 never resembles the cortical tissues, and neither internal phloem 

 nor endodermis is known to exist even in the most vigorous speci- 

 mens. In the three living species cited in the present chapter 

 we find a reduction series in which O. cinnamomea represents the 

 most primitive condition and O. claytoniana the most aberrant. 



A quite different interpretation is put on the anatomical facts 

 cited in the two preceding paragraphs by those who adhere to 



FIG. 206. Photographs of the fibrovascular region of the stem in Osmnnda 

 cinnamomea, showing internal endodermis and medullary sclerenchyma. 



the hypothesis of the tracheary origin of the pith. To those who 

 adopt the view that the medulla is derived from the substance of 

 the stele, the type of Osmunda without either internal phloem or 

 endodermis is the more primitive, while that in which both these 

 structures occur is more modern. Those who regard the anatomi- 

 cal facts from this standpoint are in a position that offers many 

 difficulties. First of all, they have to assume that an old type 

 like Osmunda skidegatensis has gained a high degree of development 

 and that the modern forms without open foliar gaps, sclerotic 

 pith, internal endodermis, and internal phloem represent in reality 

 a more primitive condition of organization than does the Mesozoic 

 type cited above. This view is not only not in harmony with 



